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The colour of climate: changes in peat decomposition as a proxy for climate change – a study of raised bogs in south-central Sweden Anders Borgmark Avhandling i Kvartärgeologi Thesis in Quaternary Geology No. 4 Department of Physical Geography and Quaternary Geology Stockholm University 2005 Cover artwork by Jens Heimdahl ´Decomposing Sphagnum leaf´ ISBN 91-7155-101-8 ISSN 1651-3940 Akademitryck AB The colour of climate: changes in peat decomposition as a proxy for climate change – a study of raised bogs in south-central Sweden Anders Borgmark This doctoral thesis consists of four papers and a synthesis. The four papers are listed below and are referred to as Paper I-IV in the text: Paper I: Gunnarson, B. E., Borgmark, A. and Wastegård, S. 2003: Holocene humidity fluctuations in Sweden inferred from dendrochronology and peat stratigraphy. Boreas 32, 347-360. Reprinted with permission by Taylor & Francis AS. Paper II: Borgmark, A. 2005: Holocene climate variability and periodicities in south-central Sweden, as interpreted from peat humification analysis. The Holocene 15, 387-395. Reprinted with permission by Arnold Journals. Paper III: Borgmark, A. and Schoning, K. in press: A comparative study of peat proxies from two eastern central Swedish bogs and their relation to meteorological data. Journal of Quaternary Science. Copyright ©2005, John Wiley & Sons, Ltd. Paper IV: Borgmark, A. and Wastegård S.: Regional and local patterns of peat humification in three raised peat bogs in Värmland, south-central Sweden. Manuscript. Abstract This thesis focuses on responses in raised bogs to changes in the effective humidity during the Holocene. Raised bogs are terrestrial deposits that can provide contiguous records of past climate changes. Information on and knowledge about past changes in climate is crucial for our understanding of natural climate variability. Analyses on different spatial and temporal scales have been conducted on a number of raised bogs in southcentral Sweden in order to gain more knowledge about Holocene climate variability. Peatlands are useful as palaeoenvironmental archives because they develop over the course of millennia and provide a multi-faceted contiguous outlook on the past. Peat humification, a proxy for bog surface wetness, has been used to reconstruct palaeoclimate. In addition measurements of carbon and nitrogen on sub-recent peat from two bogs have been performed. The chronologies have been constrained by AMS radiocarbon dates and tephrochronology and by SCPs for the sub-recent peat. A comparison between a peat humification record from Värmland, south-central Sweden, and a dendrochronological record from Jämtland, north-central Sweden, indicates several synchronous changes between drier and wetter climate. This implies that changes in hydrology operate on a regional scale. In a high resolution study of two bogs in Uppland, south-central Sweden, C, N and peat humification have been compared to bog water tables inferred from testate amoebae and with meteorological data covering the last 150 years. The results indicate that peat can be subjected to secondary decomposition, resulting in an apparent lead in peat humification and C/N compared to biological proxies and meteorological data. Several periods of wetter conditions are indicated from the analysis of five peat sequences from three bogs in Värmland. Wetter conditions around especially c. 4500, 3500, 2800 and 1700-1000 cal yr BP can be correlated to several other climate records across the North Atlantic region and Scandinavia, indicating wetter and/or cooler climatic conditions at these times. Frequency analyses of two bogs indicate periodicities between 200 and 400 years that may be caused by cycles in solar activity. Keywords: Holocene, south-central Sweden, peat humification, bog hydrology, climate variation, spectral analysis Anders Borgmark, Department of Physical Geography and Quaternary Geology. Stockholm University, SE-10691 Stockholm, Sweden E-mail: [email protected] The colour of climate: changes in peat decomposition as a proxy for climate change Introduction The geographic position of Scandinavia with the North Atlantic Ocean to the west and the Eurasian continent to the east, with its cold air masses, causes high climate variability and large and often rapid weather changes (Alexandersson and Andersson, 1995; Vedin, 2004). The proximity to the ocean is a major factor affecting climate (Bradley, 1999) and the North Atlantic climate system with strong westerly winds and warm sea water controls much of the ��� Scandinavian weather and climate. Understanding the systems controlling climate conditions around the North Atlantic is central in climatic research (Marshall et al., 2001). In order to improve the understanding of climate variability and its underlying causes, reconstructions of palaeoclimate are fundamental (e.g., Alverson et al., 2001; Bradley et al., 2003a; Oldfield, 2004; Moberg et al., 2005). Variations in climate are recorded in natural archives such as ice sheets and ��� ��� ��� ��� ��� ������������ � �������� �� ��������������� ����������� � � ��������������� ���������������� � � �������������� � ��� N � ��� ������ ��� Figure 1. Map of Scandinavia with investigated and analysed bogs in this study. Studied sites are marked by filled circles, open circles represent sites that have been excluded; M: Mosstakanmossen, K: Klaxsjömossen and B: Brårudsmossen. The position of Lake Håckren, the study area for dendrochronology (Paper I) is shown as a square. Approximate location of provinces mentioned in the text is shown; Värmland (V), Uppland (U), Småland (S), Östergötland (Ö) and Jämtland (J). 5 Anders Borgmark ���������� ����������� �� ������� ���� ��������� ���� Figure 2. The Blytt-Sernander Scheme of postglacial climate periods in continental north-western Europe, 14C ages from Mangerud et al. (1974). �������� ���� ������ ��������� ���� ����� glaciers, tree rings, deep sea- and lake sediments, corals, speleothems and peatlands (e.g., Barber, 1982; Berglund, 1986; Lowe and Walker, 1997; Bianchi and McCave, 1999; Cronin, 1999; Holmgren et al., 1999; Lauritzen and Lundberg, 1999b; Briffa, 2000). These archives contain proxies of different types; biological, physical, chemical and lithological (Lowe and Walker, 1997) and analyses of these make it possible for the scientific community to interpret past climate changes and contribute to the understanding of Earth’s climate system. Although peatlands have long been used as records of past climate change, research on peatlands still receives relatively little attention within the palaeoclimatology community (e.g., Lowe and Walker, 1997; Bradley, 1999; Cronin, 1999; Bradley et al., 2003b). It is noteworthy, however, that indices of considerable climatic changes during the Holocene were first registered in peat stratigraphical records (e.g., Blytt, 1876; Sernander, 1908; Weber, 1926). It was not possible to discern such changes in the early ice and marine core records and were therefore not acknowledged until recently when high resolution records (e.g., O´Brien et al., 1995; Bond et al., 1997) confirmed the peat based indices (Chambers and Charman, 2004). This thesis focuses on Holocene climatic changes in Sweden and how such changes are recorded in peat sequences of raised bogs (Figure 1). The main objectives are to: • Study changes in peat decomposition to infer past changes in bog hydrology on different spatial and temporal scales. • Compare and correlate changes in bog hydrology with other proxies of climate change in order to evaluate on which scale, both spatial and temporal, the peat-proxies from south-central Swedish bogs could be a useful tool for palaeoclimatic reconstructions. • Assess the causes of climate change leading to changes in bog surface wetness and the extent to which precipitation and temperature control peat decomposition processes. In this thesis palaeohydrological changes in bogs have been determined primarily through measure- 6 ment of the degree of peat decomposition. As a supplement to this method, measurements of organic carbon and nitrogen have been used together with analyses of testate amoebae assemblages on two sub-recent peat sequences. The chronologies of the analysed peat sequences have been established by a combination of mainly AMS-radiocarbon dates and tephrochronology. Peat as a palaeoclimatic archive Peat has been used as an archive for climate variations for over a hundred years. Scandinavian peatlands provided the basis for the first Holocene climatostratigraphy (Chambers and Charman, 2004). The climate periods of the Holocene (Blytt, 1876; Sernander, 1908), which have become known as the Blytt-Sernander scheme (Figure 2) are still included in the Scandinavian chronostratigraphy (Mangerud et al., 1974) and less formally also in use in other parts of Europe (Chambers and Charman, 2004). The pioneer work of e.g., Blytt (1876), Sernander (1908, 1909, 1912) and Weber (1926) was followed by several investigations in Sweden (e.g., von Post and Sernander, 1910; von Post and Granlund, 1926; Granlund, 1932), which established peatland stratigraphy as a tool for the interpretation of past climate changes. At the Geological Congress in Stockholm in 1910 the hypothesis of a cyclic regeneration of peat growth i.e., from hollow to hummock and back to hollow, was presented (von Post and Sernander, 1910). This and other hypothesises, such as the one on autogenic succession of peat (Osvald, 1923), led to a misconception on how ombrotrophic peatlands develop (Backéus, 1990). For many years the predominating view was that peatlands grow primarily through autogenic processes and not under the control of allogenic processes (i.e., climate). The focus on autogenic processes led to a decline in peat-based palaeoclimatic science (Chambers and Charman, 2004). The view of cyclic regeneration as the primary process for peat accumulation in ombrotrophic mires was challenged in the early 1980’s (Barber, 1981; Frenzel, 1983). Barber (1981) rejected the hypothesis by extensive plant macrofossil analyses of peat stratigraphies along transects, showing that the hummock/hollow complexes could be stationary for long periods of time, and he concluded that the growth of a bog is to a large extent controlled by climate. Barber (1981) concluded that different thresholds could influence regional as well as local variations in bog growth, but factors such as hydrology, drainage, plants life cycles and pool-size are all The colour of climate: changes in peat decomposition as a proxy for climate change Table 1. Compilation of Granlund´s (1932) recurrence surfaces, with their relative occurrence throughout his investigation area (1 least number of surfaces found and 5 most number found). Recurrence surfaces VI, VII were described by Sandegren and Magnusson (1937). Recurrence surface I II III IV V VI* VII* Age AD/BC AD 1200 AD 400 600 BC 1200 BC 2300 BC 2900 BC 3700 BC Cal yr BP 750 1550 2550 3150 4250 4850 5650 Relative occurrence 1 2 5 3 4 N/A N/A subordinate to climate. Today a wide range of methods are used to reconstruct past climate from peat stratigraphies (Chambers and Charman, 2004). Quantitative macro- and microfossil analysis (e.g., Charman and Warner, 1997; Barber et al., 1998, 1999; Charman et al., 2000; Hughes et al., 2000; Mauquoy et al., 2004a) together with peat humification (e.g., Nilssen and Vorren, 1991; Blackford and Chambers, 1993; Caseldine et al., 2000; Langdon and Barber, 2004; Roos-Barraclough et al., 2004; Blundell and Barber, 2005) are the most widely used methods. The range of biological, physical and chemical methods that are now available make multiproxy investigations possible. Although some of the proxies may not be independent of each other e.g., a plant-species signal within the degree of humification, they can nevertheless be used as high resolution proxies with excellent possibilities for interpretation (Chambers and Charman, 2004). As mentioned above, most of the early peat studies were performed in Scandinavia. Blytt (1876) and Sernander (1908, 1909, 1912; von Post and Sernander, 1910) conducted pioneer research, which was followed by von Post and Granlund with the inventory of peatlands in southern Sweden (von Post, 1921; von Post and Granlund, 1926; Granlund, 1932). These extensive spatial and stratigraphical studies led to Granlund´s thesis (1932) on the geology of raised bogs in Sweden. In his thesis he studied the relationship between the dome-shape of bogs and the amount of precipitation in different areas of southern Sweden, investigated the capillarity of different types of Sphagnum peat and formulated the concept of recurrence surfaces (Swedish: rekurrensyta, RY). A recurrence surface is a distinct change from dark well humified peat, representing drier conditions on the bog, to light less humified peat, representing wetter conditions (Granlund, 1932). Granlund identi- N � ������ Figure 3. Southern Sweden with relative occurrence of bogs with recognised recurrence surfaces (RY) during the peat inventory of southern Sweden (1917-1923), compiled from data in Granlund (1932). White represents areas with occasional bogs with recognised RY’s. Light grey denotes moderate occurrence of bogs with RY’s and dark grey represent areas with an high occurrence of bogs with recognised RY’s. The peat inventory was restricted to south and south-central Sweden with the exception of the islands Öland and Gotland in the Baltic Sea. The northern limit of the inventory is marked by a thick line. fied five different surfaces in southern Sweden (Table 1), but the number varied throughout the investigated area (Figure 3). Most recurrence surfaces were found in Småland and further northwest towards Värmland (Granlund, 1932). Two older recurrence surfaces were theoretically deduced by G. Lundqvist (1932) and later described by e.g., Sandegren and Magnusson (1937). Detailed stratigraphical work on peatlands continued during the regional mapping of Quaternary deposits (e.g., J. Lundqvist, 1958a). During the late 1940´s and 1950´s the radiocarbon dating technique was developed and tested on peat. However, this novel dating method also introduced a problem: the radiocarbon ages did not exactly fit into the previously established chronology (e.g., J. Lundqvist, 1957; Möller and Stålhös, 1964). Subsequently, the ages of known recurrence surfaces were interpreted as being asynchronous (J. Lundqvist, 1957). Therefore, the concept of recurrence surfaces as indicators of regional climate changes was questioned 7 Anders Borgmark Bog Oxic Boun ic Anox su rf ace ACROTELM ne dary zo High de comp . Fluctuating water table CATOTELM Low dec omp. Figure 4. Schematic picture showing the position of the acrotelm, where the active decomposition takes place, and the catotelm, with very a low decay rate, in relation to the fluctuating groundwater table near the bog surface. The fluctuations of the groundwater table determine the depth of the acrotelm. (G. Lundqvist, 1962). The scientific community in Finland has been sceptical to the notion of climatic influence on the development of recurrence surfaces since the theory was introduced (e.g., Aario, 1932; Tolonen et al., 1985; Tolonen, 1987) and is, according to Korhola (1992), still placing major emphasis on autogenous succession. Due to the national and international scepticism, recurrence surfaces became considered to be of minor use as indicators of palaeohydrological changes and as exact stratigraphical markers in Sweden (Fries, 1951; J. Lundqvist, 1957; G. Lundqvist, 1962; Nilsson, 1964). Despite the general decline in peat-based studies a number of investigations have been conducted in Fennoscandia regarding different aspects of peat growth, peat accumulation rates, bog development, local environment and degree of humification (e.g., Nilsson, 1964; Tolonen, 1973, 1987; Aaby and Tauber, 1974; Aaby, 1986; Foster et al., 1988; Svensson, 1988). Aaby (1976) and Aaby and Tauber (1974) made attempts to identify cyclic variations in the climatic signal from raised bogs in Denmark and this important work has been followed by a number peatbased studies on palaeoclimate in Scandinavia (e.g., Thelaus, 1989; Nilssen and Vorren, 1991; Korhola, 1992, 1995; Oldfield et al., 1997; Mauquoy et al., 2002; Barber et al., 2004; Björck and Clemmensen, 2004; Bergman, 2005; Schoning et al., 2005). Bar- ber et al. (2004) stated that there is a need for more peat-based palaeoclimatic research from this part of Europe and several bogs in Sweden seem to provide valuable records for palaeoclimatic reconstructions (Svensson, 1988; Björck and Clemmensen, 2004; Bergman, 2005; Schoning et al., 2005). Bog hydrology and decay Peatlands are useful as palaeoenvironmental archives because they develop over the course of millennia and provide a multi-faceted continuous outlook on the past. In order to accumulate peat an excess of water is necessary to prevent complete decay of the biomass produced by the growth of plants (Charman, 2002). Because of the importance of water in peatlands their hydrology has become an important area of study, and early studies on this subject were conducted in Russia by e.g., Romanov (1968), whose ideas have been incorporated in the present standard work on peatland hydrology (Ingram, 1983). Ingram set up what is known as the “groundwater mound hypothesis” (Ingram, 1982, 1983) which seeks to explain the size and shape of raised mires. Until recently it was thought that capillary action was responsible for holding the water table above the surrounding landscape and that the increasing peat column moves the groundwater table up through the bog (Charman, 2002). However, this hypothesis was considered already in the 1930´s when Granlund (1932) conducted an investigation on the capillarity of low humified Sphagnum peat and concluded that the capillary rise of water was insufficient to create a groundwater mound. Today the domed profile is thought to be mainly a result of low hydraulic conductivity in the catotelm i.e., the lower anoxic part of the peat column (Ingram, 1982, 1983). In order to improve models of peatland groundwater hydrology the rather static model of Ingram (1982, 1983) has to be developed into more dynamic models, e.g., incorporating models of decay and different values for conductivity (Charman, 2002). The decay of plant material mainly takes place in the upper aerated layer of the peat (Figure 4, Table 2) i.e., the acrotelm (Ingram, 1978, 1983), being largely the result of microbial decomposition and larger soil Table 2. Main properties of the acrotelm and catotelm in peatlands. Modified from Charman (2002). Property Water content and movement Oxygen supply Microbial activity Decay rate 8 Acrotelm Variable, rapid movement Aerobic (periodically) High Rapid Catotelm Constant, very slow movement Anaerobic Low Slow The colour of climate: changes in peat decomposition as a proxy for climate change animals (e.g., protozoa and nematodes) (Charman, 2002). The decay rate in the acrotelm varies considerably between sites and conditions, and the major factors controlling the decay rates are water content, temperature, oxygen supply, plant material and microbial and soil animal populations (Clymo, 1983; Charman, 2002). In the catotelm (Figure 4, Table 2) these decay processes are practically absent with a decay rate estimated to be only c. 0.1 % of that in the acrotelm (Ingram, 1978; Belyea and Clymo, 2001). The roles and interactions of the different factors affecting the decay rate is complex and are not yet fully understood (see Charman [2002] for a review). The boundary between the acrotelm and catotelm is not sharp but corresponds roughly to the lowest level of the groundwater table in the summer (Clymo, 1984). The aerated acrotelm is the contributor of organic material to the catotelm and in the process of transferring material from the aerobic layer to the anaerobic layer about 80-95% of the total organic production is lost (Clymo, 1984; Warner et al., 1993). Since plant decay is almost entirely confined to the acrotelm, the thickness of this zone largely determines how decomposed the peat will become. The thickness of the acrotelm is considered to be equal to the degree of surface wetness and the decomposition of peat is therefore primarily a function of the degree of surface wetness (Caseldine et al., 2000). Surface wetness is in turn mainly a function of precipitation and evapotranspiration, but the precise relationship is not known (Charman, 2002). There are several factors, both autogenic and allogenic, affecting the surface wetness and moisture of raised bogs at different scales (Figure 5), of which climate acts on the Microtopography ���������� ������ Microclimate Vegetation Succesion Mire expansion ��������� ���������� Human impact ��� ������������� ��������� Climate (precipitation evapotranspiration) ���������� ������� Regional groundwater Figure 5. Model of factors affecting bog surface wetness at different spatial scales. Microscale processes are principally affected by autogenic factors, at larger scales is surface wetness more likely affected by allogenic factors. Redrawn from Charman (2002). © John Wiley and Sons Ltd, reproduced with permission. broadest spatiotemporal scale implying that replicate changes across sites can normally be attributed to changes in climate (Charman, 2002). Holocene climatic changes During the Holocene there have been a number of rapid changes in climate in the Northern Hemisphere (e.g., Mayewski et al., 2004; Snowball et al., 2004). The early Holocene (c. 11,500-7500 cal yr BP) was characterised by a quite variable climate; large ice sheets still covered parts of the Northern Hemisphere and changes in ice sheet extent, mass balance and large temperature differences between the warm Atlantic waters and the cooler air masses circulating over the Northern Hemisphere caused this climatic instability (Mayewski et al., 2004; Snowball et al., 2004). Despite this instability, a general warming trend occurred during the early Holocene. Sea surface- and atmospheric temperatures may have reached their Holocene maximum during the early Holocene, indicated as e.g., c. 3ºC higher temperature at the summit of the Greenland ice sheet (compared to the last 500 yr) (Dahl-Jensen et al., 1998) and c. 1.5-2.1ºC warmer than today in northern Scandinavia (Barnekow, 2000; Larocque and Hall, 2004). However, three cooling events were superimposed on the general warming trend, of which the “8200 yr” event (Alley et al., 1997; Bond et al., 1997) probably was the most striking (Snowball et al., 2004). The “Holocene Thermal Maximum”, which experienced c. 2ºC higher summer temperatures compared to today in Fennoscandia, occurred between c. 7500 and 5000 cal yr BP (Snowball et al., 2004). Different palaeoclimate records indicate that this was mainly a dry period in Fennoscandia (e.g., Digerfeldt, 1988; Eronen et al., 1999; Seppä et al., 2002). Towards the end of the thermal maximum a cooling was commenced at c. 6000 cal yr BP (Mayewski et al., 2004); this cooling is evident in e.g., the GISP2 record as an increase in Na+ and sea salt-dust (O´Brien et al., 1995; Mayewski et al., 1997), ice rafted debris (IRD) in the North Atlantic (Bond et al., 1997; Bond et al., 2001) and glacier advances in Scandinavia (Karlén and Matthews, 1992; Karlén and Kuylenstierna, 1996; Nesje et al., 2001) (Figure 6). Around 4000 cal yr BP a number of wet-shifts are observed in many raised bogs across northwestern Europe (Anderson, 1998; Anderson et al., 1998; Hughes et al., 2000; Barber et al., 2004) (Figure 6) and several other records indicate a shift towards more variable conditions (Bond et al., 1997; Barnekow, 2000; Mayewski et al., 2004; Snowball 9 Anders Borgmark ���� ���� ���� ���� ���� ���� ���� ���� ���� ����� � ���� ���� ���� ���� ���� ���� ���� ���� ���� ����� ��� ��� � ��� ����� ���������� � � � � ��� �� � ������� ���������� �������� ��� ���� ��� �������� ������� � ��� ��� �������� ��������� �� ����������� ���� ��� ��� � �� �� ��� ��� ��� ���� ��������������� Figure 6. Examples of palaeoclimate series from the Northern Hemisphere for the last 10,000 calendar years. a) Timing of global Rapid Climate Change (RCC) (Mayewski et al., 2004) b) 200 yr smoothed GISP2 Na+ ion proxy for the Icelandic Low (Mayewski et al., 1997) c) Periods of increased levels of sea-salts and dust in the Greenland ice-cores (expansion of northern polar vortex/increased meridional airflow)(O’Brien et al., 1995) d) IRD (Ice Rafted Debris) events in North Atlantic marine sediments, indicative of cooling events (Bond et al., 1997) e) Periods of glacier advances in Scandinavia (Denton and Karlén, 1973) f) Periods of increased number of wet-shifts found in peat bogs in western Europe (Hughes et al., 2000; Barber et al., 2004) g) 200 yr smoothed Δ14C residuals (Stuiver et al., 1998) a global proxy for solar variability. h) Composite record of peat humification from Värmland, south-central Sweden (average of humification index; Paper IV). et al., 2004; Jessen et al., 2005). In Finland a period of more active peat initiation, and subsequent spread, has been recognized 4300-3000 cal yr BP (Korhola, 1992, 1995). This period of cooler and possibly wetter conditions is followed by a period of climate variability between 3500 and 2500 cal yr BP (Figure 6). This period includes a number of wetshifts and periods of wet conditions recorded in peat 10 records (Hughes et al., 2000; Langdon et al., 2003; Barber et al., 2004) and a generally colder climate has been reconstructed in Finnish dendrochronological records (Eronen et al., 1999) and from chironomid assemblages in northern Sweden (Larocque and Hall, 2004). Between c. 1500 and 1000 cal yr BP many records indicate a transition to cooler climate (e.g., Denton The colour of climate: changes in peat decomposition as a proxy for climate change and Karlén, 1973; Karlén and Matthews, 1992; Bond et al., 1997; Mayewski et al., 1997, 2004; Hughes et al., 2000; Langdon et al., 2003; Barber et al., 2004)(Figure 6). During the “Medieval Warm Period” (c. 1000-750 cal yr BP), however, temperatures could have been as much as 0.5-0.8ºC higher than those today (Bond et al., 2001; Snowball et al., 2004). Tree ring records indicate that summer temperatures in high Northern Hemisphere latitudes were higher than the 20th century mean (Briffa, 2000) and European documentary records contain evidence of mild winters during portions of this time (Pfister et al., 1998). There are also indications of this being a period of lower precipitation which may be of greater significance than the temperature as an indicator of a “warm” Medieval period (Bradley et al., 2003b). The last 500 years have been characterized by a cooler climate (Mayewski et al., 2004) with advancing glaciers in Scandinavia and elsewhere (Karlén and Kuylenstierna, 1996) and increasing levels of sea salt-dust in the Greenland ice sheet (O´Brien et al., 1995). However, there is some differences in the timing and magnitude of regional variations in Scandinavia during the last 2000 years and there is only a limited synthesis of high-latitude records available (Bradley et al., 2003b; Snowball et al., 2004). Changes in climate have many underlying causes, of which many remain unknown and the effects on climate are often poorly constrained. Both external and internal forcing causes changes in an extremely complex climatic system (Emeis and Dawson, 2003). Lately solar variation has emerged as an important external forcing mechanism for climatic variability on decadal to millennial timescales (Chambers et al., 1999; Bond et al., 2001) and changes in Δ14C have been attributed to the changes in solar activity (e.g., Karlén and Kuylenstierna, 1996; Stuiver and Braziunas, 1998; Chambers et al., 1999; van Geel et al., 1999; Beer et al., 2002). A number of correlations have been made between climate proxy records and solar variability in terms of Δ14C and 10 Be values (e.g., Denton and Karlén, 1973; Reid, 1987; O´Brien et al., 1995; Stuiver et al., 1998b; van Geel et al., 1999, 2000; Bard et al., 2000; Björck et al., 2001; Blaauw et al., 2004; Mauquoy et al., 2004b; Mayewski et al., 2004). Exactly how the relatively small changes in solar irradiance can affect the climate is not clear, possible forcing mechanisms have been discussed e.g., changes in ocean circulation (Bond et al., 2001), variations in UV irradiance leading to altered production of ozone and absorption of heat in the atmosphere and as a consequence shifts in the atmospheric circulation (van Geel et al., 1999; Blaauw et al., 2004) and influence of cosmic rays on cloud formation (Carslaw et al., 2002). In this study, peat-based palaeorecords are compared to the reconstructed solar variation (Stuiver et al., 1998b) in order to find a possible cause for changes in surface wetness on bogs (Figure 6g). Geographical setting The main study area is Värmland (Figure 1) located in south-central Sweden (Paper I, II and IV). The region is heavily forested with mainly conifers (i.e., Scots pine and Norway spruce) and is intersected by north-south fault-lines with long and often narrow river valleys and elongated lake systems. The bedrock consists mainly of granites and gneisses and the Quaternary deposits are dominated by till, glaciofluvial deposits (eskers and deltas), numerous peatlands and in lower areas clays (J. Lundqvist, 1958a). The area was deglaciated between 10,100-9200 14C yr BP (c. 11,000-9800 cal yr BP) and the highest shoreline is situated between 180-200 m a.s.l. and increases in elevation towards the north (J. Lundqvist, 1958a, 1994; Wastegård, 1998). In Värmland a number of distinct shifts in peat humification – recurrence surfaces – have been reported (Granlund, 1932; J. Lundqvist, 1957, 1958a, 1958b; Persson, 1966) and the geographical/climatological setting is thought to be favourable for palaeoclimatic investigations based on peat stratigraphy and the occurrence of several tephra horizons, which make detailed comparison between sites possible (Persson, 1966; Boygle, 1998; Zillén et al., 2002). In order to facilitate selection of suitable peatlands the majority of the sites were chosen from the extensive survey of Quaternary deposits made by J. Lundqvist (1958a). Since some of these bogs have since been drained, cut or disturbed in other ways other bogs were also cored and analysed. In paper I, II and IV humification data obtained from Stömyren, Kortlandamossen and Fågelmossen have been used (Figure 1). Paper III describes the bogs Ältabergsmossen and Gullbergbymossen in Uppland, south-central Sweden (Figure 1), where the climate during the last 150 years was investigated. In Östergötland, sites described by Granlund (1932) were initially chosen for study together with bogs chosen directly from the map of Quaternary deposits (Svantesson, 1981). Many bogs have, however, been disturbed by anthropogenic activities in this region and only one sequence was analysed. Ängstugsmossen (N 58° 50.8’, E 14° 16.5’; 130 m a.s.l.; Figure 1) is a relatively small bog, about 500x300 m, with an undulating irregular border to the surrounding forest. There are some indications 11 Anders Borgmark 15 10 60 5 40 0 20 0 -5 Jan Feb Mar Apr MayJun Jul AugSep Oct NovDec 100 80 100 5 40 0 20 0 -5 Jan Feb Mar Apr MayJun Jul AugSep Oct NovDec -10 Precipitation (mm) 60 0 20 20 10 10 5 0 15 80 -5 Jan Feb Mar Apr MayJun Jul AugSep Oct NovDec c) Herrberga -10 20 T: +6.0ºC P: 522 mm 15 10 60 5 40 0 20 0 -5 Jan Feb Mar Apr MayJun Jul AugSep Oct NovDec of anthropogenic disturbance around the bog, but no evidence of large drainage was observed. Two peat sequences from the Faroe Islands were also analysed but were excluded from the study due to a problematic stratigraphy in one case and low temporal resolution for most of the Holocene in the other (Wastegård, 2002). The climate varies somewhat between the study areas, according to the 1961-1990 climate normals (Alexandersson et al., 1991). Värmland is characterised by relatively maritime climate in the south and slightly more continental conditions in the north (Vedin, 2004). In the investigated area, the mean annual temperature is c. +4.6°C and total precipitation is c. 700-800 mm/yr (Figure 7 a, b). In the area of Ängstugsmossen the mean annual temperature is c. +6.0° C, and a precipitation of c. 520 mm/ yr (Figure 7 c). In the area where Ältabergsmossen and Gullbergbymossen are located, the mean annual temperature is c. +5.1° C and total precipitation is c. 555 mm/yr (Figure 7 d). Methods Field work A large number of sites were selected for initial investigation. Preferably bogs would be geographically well defined and contain distinct changes in peat humification. The stratigraphy should cover as much as possible of the Holocene. The majority of the possible sites were discarded directly because they lacked sufficient peat records, were severely drained/cut or displayed indications of problematic stratigraphies (e.g., hiatuses). For those bogs that were further investigated general stratigraphies were described. A number of borings along transects were made using a Russian peat corer to determine the stratigraphy for each bog. In some cases a more detailed stratigraphy was established (Paper IV). Sampling locations were selected with respect to the established stratigraphies 12 15 40 -10 Temperature (C) Precipitation (mm) 80 T: +4.8ºC P: 794 mm 20 T: +5.1ºC P: 555 mm 60 b) Djurskog 100 d) Svanberga Temperature (C) 20 Figure 7. Precipitation (bars) and temperature (line) averages 1961-1990 (Alexandersson et al., 1991) for Torsby, Djurskog (Värmland), Svanberga (Uppland) and Herrberga (Östergötland) the meteorological stations situated closest to the sites in this study. Annual mean temperature (T) and total annual precipitation (P) is also shown. Temperature (C) T: +4.6ºC P: 700 mm Precipitation (mm) 80 a) Torsby Temperature (C) Precipitation (mm) 100 -10 and were in some cases also based on earlier investigations (J. Lundqvist, 1957, 1958a; Boygle, 1998). A Russian peat corer of 1 m length and 5 cm diameter was used for coring. In some cases the uppermost part of the bogs was sampled with a half-cylindrical PVC tube (50x10 cm; monoliths). All cores were wrapped in plastic and transported back to Stockholm University for further analyses. Laboratory analyses Sample preparation Peat cores and monoliths were stored in a cold room (c. 5°C) until subsampling was conducted. During subsampling contiguous samples were cut out from the peat cores and dried at 105°C overnight. All samples were ground and stored in 5 ml plastic containers. Humification The method to chemically analyse and determine the decomposition of peat was originally developed by Overbeck (according to Bahnson, 1968) and later modified by Bahnson (1968). The method is based on colorimetric determination of an alkaline extract of the peat, where the alkaline absorbance is proportional to the amount of dissolved humic substances (Aaby, 1976, 1986). Highly decomposed peat is usually dark brown/blackish whereas less decomposed peat is light brown/yellowish in colour corresponding to the amount of extractable humic substances. The methodology has been modified and improved by Blackford and Chambers (1993), and this method is now the standard used for determining peat decomposition. In this study a slightly modified version has been used in order to better suite a large number of samples. For the present study 0.100±0.01 g dry peat was dissolved in 25 ml 8% NaOH in 50 ml plastic tubes, and boiled in a water bath at 95°C for 1.5 hr. The colour of climate: changes in peat decomposition as a proxy for climate change was performed using a transfer function based on modern testate amoebae assemblages (Woodland et al., 1998; Schoning et al., 2005; Paper III). � ����������������� � � � � Tephra � � � � ���� ���� ���� ���� ���� ���� ���� ���� ���� ��� ���� ���� ���� ���� ���� ���� ���� ���� ���� ���������������� Figure 8. Distribution of 53 measurements of absorbance on a standard sample consisting of Sphagnum peat, the average value is 1.30 and standard deviation 0.04. For the sequence from Stömyren (Paper I) the humic extracts were vacuum-filtered and diluted at the dilution rate determined by Blackford and Chambers (1993). In subsequent analyses, the boiled samples were centrifuged and filtered and, in order to maintain the same dilution rate, 12.5 ml of the resulting extract was diluted with distilled water to 100 ml. Each sample was then measured three times in a UNICAM spectrophotometer at 540 nm and the average was calculated and corrected to the normal-weight of 0.1 g (Paper II). The reliability of the method was tested by analysing a number of samples consisting of one large batch of dried and ground Sphagnum peat. The frequency of the measurements (Figure 8) gives a distribution of 1.3±0.1 for the standard sample and approximates a normal distribution (Davis, 1986). C/N For carbon and nitrogen analyses 0.5-1 mg of dried and ground peat was enclosed in metal capsules. The analyses were performed with a Carlo Erba NC2500 analyzer connected, via a split interface to reduce the gas volume, to a Finnigan MAT Delta plus mass spectrometer at the Department of Geology and Geochemistry, Stockholm University. The results are shown as weight percentage and C/N ratio (Paper III). Testate amoebae Analyses of testate amoebae assemblages were performed by Kristian Schoning (Gotland University) following the sample preparation and taxonomy of Charman et al. (2000). Contiguous 0.5-1 cm sub samples were obtained and thereafter mounted on slides. A sum of 150-200 testate amoebae were counted in each sample under a light microscope at a magnification of 400X. The reconstruction of the water tables All cores except the lower parts of the sequences from Kortlandamossen (c. 5000 to 10,000 cal yr BP) were searched systematically for microscopic tephra shards by Stefan Wastegård (Stockholm University) using the methods outlined by Pilcher et al. (1995). Contiguous peat samples of 5-10 cm thickness were combusted at 550°C for 4 hr, washed in 10% HCl and mounted for microscopy in Canada Balsam. For those samples in which tephra shards were detected, the equivalent sediment interval was re-sampled using 1-2 cm slices, in order to determine more precisely the concentration and distribution of tephra particles in each sample. The tephra shards were prepared for geochemical analysis using Wavelength Dispersive Spectrometry (WDS) at the Department of Geology and Geophysics, Edinburgh University. Samples were acid digested, following the procedure outlined by Dugmore (1989). Results of the geochemical analyses are also presented in Wastegård (2005). Radiocarbon dating All accelerator mass spectrometer (AMS) radiocarbon dates were performed at Ångström Laboratory, Uppsala University. Dating of the samples from the Stömyren cores were performed on bulk peat, whereas remaining dates were performed on plant material >125 µm and usually only leaves and stems of Sphagnum were selected. If insufficient Sphagnum was present or if the peat was to highly decomposed, bulk material >125 µm was used. Samples were dried at 50-75°C temperatures overnight and at 105°C for c. 3 hr. Radiocarbon dates were calibrated with Calib 4.0 (Stuiver et al., 1998a) and OxCal 3.5 (Bronk Ramsey, 1995) using the INTCAL98 calibration curve. All calibrated ages are shown with 2σ confidence intervals using the mid-point age (Table 3). Chronology The chronology for each peat bog was established with the aid of the calibrated radiocarbon ages and tephra horizons. For the sub-recent peat from Ältabergsmossen and Gullbergbymossen (Paper III) spherical carbonaceous particles were also used (Schoning et al., 2005). The ages and geochemistry of the 13 Anders Borgmark Table 3. AMS radiocarbon dates used in this study. Site Stömyren Stömyren Stömyren Stömyren Stömyren Stömyren Stömyren Stömyren Kortlandamossen Kortlandamossen Kortlandamossen Kortlandamossen Kortlandamossen Kortlandamossen Kortlandamossen Kortlandamossen Kortlandamossen Kortlandamossen Kortlandamossen Fågelmossen Fågelmossen Fågelmossen Fågelmossen Fågelmossen Fågelmossen Fågelmossen Fågelmossen Fågelmossen Fågelmossen Fågelmossen Ängstugsmossen Ängstugsmossen Core/depth (cm) Stm-20 Stm-38 Stm-54 Stm-60 Stm-76 Stm-99.5 Stm-150 Stm-187 Kort1-60 Kort1-140 Kort1-440 Kort1-540 Kort1-640 Kort2-110 Kort2-175 Kort2-320 Kort2-495 Kort2-550 Kort2-619 Fgm1-104 Fgm1-170 Fgm1-210 Fgm1-245 Fgm1-280 Fgm1-324 Fgm2-101 Fgm2-198 Fgm2-290 Fgm2-410 Fgm2-518 Ang-99 Ang-253 Laboratory code Ua-16209 Ua-16210 Ua-16211 Ua-16212 Ua-16213 Ua-16214 Ua-16215 Ua-16216 Ua-19026 Ua-19027 Ua-19028 Ua-19029 Ua-19030 Ua-22494 Ua-22495 Ua-23251 Ua-22496 Ua-23252 Ua-22497 Ua-22488 Ua-22489 Ua-23247 Ua-23679 Ua-23248 Ua-22490 Ua-22491 Ua-22492 Ua-23249 Ua-23250 Ua-22493 Ua-17865 Ua-17866 Peat component dated Bulk peat Bulk peat Wood Bulk peat Bulk peat Bulk peat Bulk peat Bulk peat Sphagnum Sphagnum Sphagnum Sphagnum Bulk>125μm Sphagnum Sphagnum Sphagnum Sphagnum Bulk>125μm Bulk>125μm Sphagnum Sphagnum Sphagnum Sphagnum Sphagnum Sphagnum Sphagnum Sphagnum Sphagnum Sphagnum Sphagnum Sphagnum Bulk>125μm mid Holocene tephra layers are well constrained from several other investigations (e.g., Dugmore et al., 1995; Pilcher et al., 1995; Larsen et al., 1999, 2001; Wastegård et al., 2001; van den Bogaard et al., 2002; van den Bogaard and Schmincke, 2002; Bergman et al., 2004; Boygle, 2004) and provided that the geochemical identification of the tephras is robust, they can be used as fixed age-depth points in the age depth models. Age-depth relationships were therefore defined using linear interpolation between dated points, except at Ältabergsmossen where a second order regression was used by Schoning et al. (2005). The chronological uncertainty increases when comparing sites and therefore full use of time parallel markers (i.e., tephras) should be made (Telford et al., 2004). A linear interpolation age-depth curve implies instantaneous shifts in the accumulation rate at each radiocarbon date or tephra horizon, which is extremely unlikely. However, whilst the curve is forced to pass through the dates, it cannot deviate too far from reality. Errors given by the uncertainties 14 14 C age (yr BP) 755±55 1340±55 2445±100 2485±60 2475±55 3485±55 5190±75 7195±65 865±50 1420±50 4465±65 6105±70 8790±85 855±40 1280±40 1905±45 4785±45 6985±55 8685±70 850±35 1265±35 2075±45 2900±40 3295±45 3810±40 605±40 1175±40 1920±45 2960±45 4240±45 2005±65 7740±100 2 σ calibrated age (cal yr BP) 666±97 1254±82 2483±272 2551±199 2549±197 3743±149 5961±149 8017±149 800±120 1335±85 5090±220 6980±230 9875±325 790±120 1185±105 1830±120 5470±140 7940±130 9720±190 790 ±120 1185±105 2040±120 3050±160 3520±120 4220±140 600±60 1100±130 1850±120 3140±180 4740±130 1971±150 8647±281 of radiocarbon dates i.e., calibrated 14C ages are unlikely to be the true date of that point, will be incorporated in the age-depth relationship when the curve is forced to pass through every date (Telford et al., 2004). The associated errors of calibrated ages are not incorporated in linear interpolation age-depth relationships either. The linear interpolation curve has been used despite the incorporated errors in order to keep the age-depth relation of synchronous tephra horizons fixed. Data analyses In order to facilitate interpretation of the humification records, the data sets were subjected to standardisation and detrending (Davis, 1986; Chambers et al., 1997; Chambers and Blackford, 2001). For standardisation an average of 0 and a standard deviation of 1 were used in the equation: Y= X −µ σ The colour of climate: changes in peat decomposition as a proxy for climate change Normalized abs Raw humification 4 3 X 2 1 Step 1: Y=(X-µ)/σ 2 1 Y 0 -1 Step 2: Ŷ=0.0042Y-1.4044 Detrended abs -2 Step 3: Y`=Y-Ŷ 2 Y` 1 0 -1 0 50 100 150 200 250 300 350 400 450 500 550 600 650 Depth (cm) Figure 9. Procedure for normalizing and detrending humification data. Step 1, normalization: From each absorbance value (X) is the average (μ) subtracted and the residual is divided with the standard deviation (σ). Step 2: The equation of the linear regression is determined. Step 3, detrending: The results from the linear regression (Ŷ) are subtracted from the normalized values (Y). where X is the raw value, Y is the standardised value, µ is the average and σ is the standard deviation of the data set (Figure 9). Detrending was performed to minimize influence from the decay that occurs in the catotelm, which is extremely slow compared to decomposition in the acrotelm (Clymo, 1984; Mauquoy and Barber, 1999; Belyea and Clymo, 2001), and thus the subsequent trend is usually removed (Mauquoy and Barber, 1999; Chiverrell, 2001). To remove the trend a linear regression equation was used (Figure 9), where the resulting value from the equation at every data point (Ŷ) is subtracted from each corresponding standardised value (Y) resulting in a detrended value (Y`=Y-Ŷ). Spectral analysis has become an important tool for interpretation of palaeo-climatic data, particularly for identifying traces of external factors such as orbitally induced climatic oscillations (e.g., Milankovich cycles) or solar forcing. The so called ‘quasiperiodic’ climate processes have also gained more attention lately (Burroughs, 2001). These processes are not strictly periodic and often are the result of complicated and only partially understood interactions within Earth’s climate system e.g., the North Atlantic Oscillation (NAO) and El Niño/ENSO (Burroughs, 2001; Marshall et al., 2001; Lindeberg, 2002). Spectral analysis in Paper II was conducted with the aid of a Lomb-Scargle Fourier transform, which can handle unevenly spaced data without using interpolation (Schulz and Stattegger, 1997). The analyses were performed with the program SPECTRUM (Schulz and Stattegger, 1997) and the estimations of the red-noise (i.e., noise that has more power at lower frequencies; Lindeberg, 2002) spectra were done with the program REDFIT (Schulz and Mudelsee, 2002), which constructs an estimation of the first-order autoregressive parameter directly from an unevenly spaced time series. This makes it possible to test the null hypothesis that the spectrum obtained from the time series originates from a first-order autoregressive process i.e., noise. A composite record was established by compiling data from five sequences covering the last 4000 years. This time frame was divided into even 50-yr intervals where the closest humification values from each separate sequence were positioned. At those levels where three or more standardised humification values were present an average was calculated. 15 Anders Borgmark If changes in humification are at random, then successive averages should not have the same value, nor should they display a trend. When applying this relatively crude methodology a number of errors could affect the outcome. The composite record does, however, provide a broad picture of regional changes in bog hydrology. Results Summary of paper I Gunnarson, B. E., Borgmark, A. and Wastegård, S. 2003: Holocene humidity fluctuations in Sweden inferred from dendrochronology and peat stratigraphy. Boreas 32, 347-360. This paper tests the hypothesis that past hydrological changes are not only locally induced but can be found also over larger regions such as central Sweden. Tree ring and humification data covering the last 7 000 years were analysed in order to identify and compare humidity changes in Fennoscandia. Subfossil woods of Scots pine (Pinus sylvestris) found in lakes around Lake Håckren, Jämtland (Figure 1), were used to provide a dendrochronological data set. The dendrochronology was standardised, using a RCA method, and filtered to show low frequency variations that could be compared to the peat humification record from Stömyren, Värmland (Figure 1). The chronology of the peat sequence was established with aid of six AMS radiocarbon dates and three tephra horizons. One additional tephra that was previously not described was also found. For peat humification measurements, a modified standard method was used. Interpretation of the regeneration of pine, peat humification and tree ring growth tentatively indicate synchronous periods of drier climate c. 6850-6750, 4350-4150, 4050-3750, 3450-3050, 1900-1750, 1550-1350 and 600-450 cal yr BP. Possible wetter periods were inferred at 5550-5350, 5150-4850, 4150-4050, 3650-3450, 3050-2850, 2050-1900, 1750-1550, 1200-1050 and 400-250 cal yr BP. Note that in the paper all ages are shown as BC/AD. The wet and dry periods revealed by the tree ring and peat stratigraphy data indicate considerable humidity changes during the Holocene. Björn Gunnarson conducted the dendrochronological analyses and interpretations, Anders Borgmark performed peat humification analyses and interpretations and Stefan Wastegård analysed the cores for tephra. All authors are equally responsible for the discussion and conclusions. 16 Summary of paper II Borgmark, A. 2005: Holocene climate variability and periodicities in south-central Sweden, as interpreted from peat humification analysis. The Holocene 15, 387-395. This paper deals with analyses of peat sequences from Stömyren and Kortlandamossen in Värmland (Figure 1). The focus of the study was to identify synchronous shifts between low and high humified peat in two ombrotrophic peatlands and to evaluate the potential of spectral analysis as a tool for analysing climate data derived from peat humification variations. Both bogs were probably formed by paludification in the early Holocene. Chronologies were established with AMS radiocarbon dating and tephra horizons. Stömyren covers the last 8000 years and Kortlandamossen was initiated nearly 10,000 years ago according to the radiocarbon dates. The humification data was interpreted in terms of wet and dry periods and the data sets were also subjected to spectral analysis, using the Lomb-Scargle Fourier transform. One advantage of this method is that the transform function can analyse unevenly spaced data, a common characteristic of palaeoclimatic time series. The resulting spectra have to be interpreted with caution, but application of an AR1 model, which gives a null hypothesis of the possibility that the spectra only show red noise, strengthens the interpretation that there are significant periodicities in the data. Frequencies that have a wavelength less than twice the spacing between sample points cannot be detected, since the highest frequency that can be estimated is the Nyquist frequency, which is the wavelength exactly twice the distance between successive observations. Each sample represents an age of c. 50 years in Kortlandamossen and c. 75 years in Stömyren, and thus frequencies shorter than 100-150 years are impossible to identify. In the low frequency area, the influence of red noise increases, indicated by a rise in the AR1 model. Therefore, frequencies longer than c. 500 years are difficult to identify, leaving a window of c. 100-500 years where periodicities can be recognized. The results indicate that there are a number of more or less synchronous shifts in hydrology in the two bogs. The most pronounced shifts occur at c. 7500 cal yr BP (wet-shift), 4500 cal yr BP (dry-shift), 4300-4200 cal yr BP (wet-shift), 3700-3500 cal yr BP (wet-shift), 3250 cal yr BP (dry-shift) and 2250 cal yr BP (wet-shift). A periodicity of around 250 yr is evident in the spectral analysis of both cores, and periodicities of the same magnitude have been reported from other proxies as well as from other peat based studies. A The colour of climate: changes in peat decomposition as a proxy for climate change periodicity of c. 200 yr, possibly corresponding to the Suess solar cycle, is found in the spectral data from Stömyren and a periodicity of around 400 yr that could be of solar origin is found at Kortlandamossen. The results of spectral analysis have to be interpreted with caution because of the level of noise, particularly frequencies longer than c. 500 yr should be interpreted carefully when data of this type and resolution are at hand. Anders Borgmark performed all analyses and interpretations and is responsible for discussions and conclusions. The tephrochronological work was performed by Stefan Wastegård and geochemical data is presented in Wastegård (2005). Summary of paper III Borgmark, A. and Schoning, K. in press: A comparative study of peat proxies from two eastern central Swedish bogs and their relation to meteorological data. Journal of Quaternary Science. Peat humification, C/N and testate amoebae assemblages were analysed on a high resolution peat sequence from Ältabergsmossen spanning the last 150 yr, and on a shorter sequence from Gullbergbymossen spanning the last 60 yr, testate amoebae and peat humification were analysed (Figure “karta”). The physical and chemical proxies were correlated to each other and compared to the testate amoebae assemblage inferred water table and to meteorological data from Uppsala. On the sequence from Gullbergbymossen only the uppermost part could be used (corresponding to ca 1940-2000) because of a possible hiatus in the peat sequence. The chronologies were established by a combination of SCPs, radiocarbon dates and the Askja AD 1875 tephra (Schoning et al., 2005). The age-depth relationships imply a peat accumulation at a rate of 2.5-5 mm/yr at both sites, providing an excellent opportunity to compare the peat proxies with available temperature and precipitation data for the last 150 years. Peat humification was measured using the modified standard method, and carbon and nitrogen content were analysed on the same samples. The testate amoebae were counted according to standard methodology. Peat humification and the inferred water table show significant changes with the same trends in both records. This indicates that changes in water table are driven by external forcing. It was not possible to measure to a statistically significant level the responses or the lag of the physical/chemical parameters (peat humification and concentration of carbon and nitrogen) versus the meteorological/biological (testate amoebae assemblages) parameters in this study. The results indicate, however, a lag between the biological parameter and the physical/chemical parameters. This lag is likely due to the decay of deeper layers in the peat during drier periods. The term secondary decomposition is used for the process of further decay of previously deposited peat due to a lowering of the water table. This process has implications for the interpretation of multi- and single-proxy data from ombrotrophic mires, especially when interpreting recent and/or high-resolution data. Biological proxies e.g., plant macrofossils and testate amoebae, reflect the moisture conditions at the time when the plants and amoebae lived. Peat humification and C/N ratio on the other hand reflect conditions during the decay-process of the organic material i.e., processes that influence previously deposited material. We conclude that it is necessary to perform highresolution multiproxy studies of peat sequences with good temporal resolution to be able to interpret relationships and leads/lags between biological and physical/chemical proxies. Anders Borgmark prepared the samples for C/N analysis and did the peat humification analyses and the interpretations of these two proxies. Kristian Schoning analysed and interpreted the testate amoebae assemblages and provided the chronologies. Both authors are equally responsible for the discussion and conclusions. Summary of paper IV Borgmark, A. and Wastegård S.: Regional and local patterns of peat humification in three raised peat bogs in Värmland, south-central Sweden. Manuscript. Peat humification data from three bogs in Värmland, south-central Sweden, is presented, with replicate sequences from two of the bogs. The degree of peat decomposition was used to infer past bog surface wetness. A comparison between the results from the peat humification analyses and a number of different records of Holocene climate change was conducted, in order to interpret similarities and dissimilarities and their possible causes. Tephra horizons and a total of 30 AMS radiocarbon dates constrained the chronologies (Table 3). Tephra from five different eruptions were found in the cores and geochemically fingerprinted. The Kebister and Hekla-3 and 4 tephras were found in several cores, whereas Askja 1875 and the Stömyren tephra were 17 Anders Borgmark Stefan Wastegård searched the peat sequences for tephra horizons, conducted the tephra analysis and interpreted the geochemical data. Anders Borgmark performed peat humification analyses and interpretations of the humification records. The interpretation of climatic changes has been performed by both authors. 18 Additional results Cores were collected from three additional bogs in Värmland (Figure 1) and partly analysed for humification and tephrochronology. However, the data has not been used because of indications of anthropogenic disturbances at the sites. Brårudsmossen had earlier been cut and an initial analysis of the peat humification revealed a disturbed record. Mosstakanmossen was cored, but when visited for resampling it was drained and peat-cutting had commenced. The bog was therefore no longer suitable for the replicate core research it was intended for. Klaxsjömossen had also been extensively cut and, based on the disturbed peat humification record shown at Brårudsmossen, the bog was excluded. The peat humification record (Figure 10) from Ängstugsmossen, Östergötland (Figure 1), supports the conclusion of Granlund (1932) that very few shifts in peat humification i.e., recurrence surfaces, are found in this part of Sweden. Two radiocarbon dates from the profile (Table 3) indicate a total age of c. 8700 cal yr for the peat. Transitions to less humified peat can be found at c. 3000 and c. 600 cal yr BP, with the former transition being gradual and the latter relatively abrupt. �� 0 ������� 10 20 30 40 50 60 70 80 90 ������� 100 Depth (cm) only found in one core, respectively. The tephras are time-synchronous markers, providing excellent opportunities for core correlation, and to evaluate if changes in surface wetness were synchronous within a single peatland. The replicate cores from Fågelmossen display similar patterns of changes in surface wetness at times, which is also the case at Kortlandamossen where the sequences, covering almost 10,000 years, seem to be somewhat offset to one another. The mid Holocene cooling (6000-5000 cal yr BP) is not directly evident in the data from Stömyren. In Kortlandamossen a change to more unstable conditions starts at ca 5500-5000 cal yr BP, with a wet-shift present in both sequences. Both sequences from Kortlandamossen display wet-shifts within this period, but they do also have indications of drier conditions and dry-shifts. The humification record of Stömyren indicates high to medium water table. This period has, however, been reported as a dry period in Fennoscandia which in combination with a general cooling possibly could result in this type of humification record. All three bogs indicate a change to wetter conditions between 4500 and 4000 cal yr BP, as many other records do across the North Atlantic region. Between 3700 and 3200 cal yr BP, wet-shifts occurred in the three Värmland bogs as well as in many other European bogs, simultaneous with decreasing temperatures, registered in e.g., the Greenland ice cores (GISP2) and Norwegian speleothems. The 2800 cal yr BP climatic change is evident in all bogs. At Kortlandamossen, this cooling may have caused a hydrological threshold to be crossed, leading to generally lower decomposition of the peat i.e., prevailing wet surface conditions. A large number of wet-shifts are present during the last 2500 yrs at European bogs; around 1600 cal yr BP, several records indicate a climate change that is visible as wet-shifts in three of the analysed sequences. The composite record, covering the last 4000 yr, shows distinct wet-shifts at c. 3800-3300, 31002800 and 1700-900 cal yr BP, which correlate well with many other climate records. The composite record could be a powerful tool in interpreting regional changes in peat hydrology caused by climatic variation. 110 120 130 140 ������� 150 160 170 180 190 200 210 220 230 240 250 �������� 260 1 2 3 4 5 6 Absorbance Figure 10. Raw humification (absorbance) values from Ängstugsmossen, Östergötland. The positions of the two AMS-radiocarbon dates (Table 3) and the Hekla-4 tephra horizon are also shown. The colour of climate: changes in peat decomposition as a proxy for climate change Discussion Local correlation of humification records Due to the nature of raised bogs one could not expect to obtain stratigraphical concordance over the entire bog, since natural processes at different scales influence peat formation, accumulation and decomposition (Figure 5). There are several reports of differing results from replicate records (e.g., Caseldine et al., 1998; Mauquoy et al., 2002), which emphasize the need for replicate records. However, it has also been noted that replicability improves over time, and that this improvement could be explained by bog expansion (laterally and vertically) leading to surface homogeneity, which in turn decreases the influence of local hydrological changes on the bog surface (Charman et al., 1999). At the sites Kortlandamossen and Fågelmossen replicate peat sequences were analysed, providing an opportunity to evaluate at which scales changes occur and which processes affect surface wetness at meso and macroscale. Since the replicate records of bog hydrological changes were obtained at distances shorter than 1000 m but more that 100 m, mesoscale (Figure 5) autogenic processes cannot be ruled out. However, mesoscale changes in surface wetness are quite likely to be linked to climate (Charman, 2002), and therefore changes that occur more or less simultaneously are likely to be of climatic origin. At both Kortlandamossen and Fågelmossen a number of changes fall within this criterion, but usually the magnitude and/or the timing differ to some degree. The differences could presumably be attributed to microscale and some mesoscale processes, such as microtopography, microclimate, succession and vegetation and the hydrological thresholds that result from them (e.g., Barber, 1981; Charman, 2002). However, the presence of similar trends in the replicate cores implies that external forcing caused ��� ���� ����� ���� ��� ��� ��� ��� ���� � ��� ���� ���� ���� ���� ���� ���� ���� ���� ���� cal yr BP Figure 11. The five recurrence surfaces from Granlund (1932) plotted together with the peat composite record from Värmland (average of humification index; Paper IV). Age intervals of recurrence surfaces are Granlund´s age (transformed to cal yr BP) ± 100 yr. much of the changes in bog surface wetness. The four records also imply that replicability increases over time in both Fågelmossen and Kortlandamossen (cf., Charman et al., 1999). Regional correlation of humification records It is evident in this study that periods of mainly wetshifts are temporally clustered in bogs on a regional scale. This has also been shown in several other studies (e.g., Anderson, 1998; Blackford, 1998; Barber et al., 2000, 2004; Hughes et al., 2000; Mauquoy and Barber, 2002; Langdon et al., 2003). In paper IV, the regional pattern of changes in peat humification and the composite record (Paper IV: Figure 6) indicates periods of wetter surface conditions between c. 3700-3200 and 1700-900 cal yr BP and periods of relatively dry conditions between 2600-1900 and 800-300 cal yr BP. Additionally, a very distinct wet-shift is evident at c. 3000-2800 cal yr BP. Between c. 500 cal yr BP and the present, the composite record seems to indicate increasing surface wetness on the investigated bogs. These periods of relatively good concordance give an indication of the resolution at which regional patterns can be recognized, given the constraining factors of the composite record (Paper IV; nearest 50-yr sample, time resolution and age-depth relationships). By improving the techniques used for compiling records, it is likely that more information could be obtained and this type of study might then be a complement to multi-proxy studies of one or two sequences. Despite the uncertain ages of Granlund´s (1932) recurrence surfaces (Table 1) there seems to be a fair agreement between the main shifts in peat humification in Värmland and Granlund´s recurrence surfaces (Paper I; Paper II). A diagram with Granlund´s recurrence surfaces I-V superimposed on the regional humification record from Värmland (Figure 11) shows that there are indications of wet conditions close to the four recurrence surfaces covered by the time period of the composite record. This implies that the scheme set up by Granlund is principally correct, however, since Granlund´s ages of these shifts are very uncertain should the term recurrence surface perhaps be used only in its conceptual sense and focus should not be on Granlund´s interpreted ages of the recurrence surfaces. Comparison to other climate data As outlined in Paper I several synchronous periods of drier/wetter conditions can be identified when comparing peat humification data and lake-level 19 Anders Borgmark fluctuations inferred from the temporal distribution of Scots pine, even though the records originate from different regions in Sweden. Synchronous patterns of hydrological change between different types of archives located close to each other have been found by e.g., Baker et al. (1999) and Charman et al. (2001). Many climate archives in the North Atlantic region display a pattern of coherent climatic events (Figure 6), and on a broad scale many shifts in bog surface wetness appear at these times of climatic change. Three periods of inferred changes in bog surface wetness are detectable in the composite record at c. 3700-3200, 3000-2800 and 1700-900 cal yr BP (Figure 6 h) and their correspondence to other palaeoclimate records on different spatial scales are discussed further here. The period of wetter conditions between 3700 and 3200 cal yr BP can be found in all analysed sequences presented in Paper IV. A transition to wetter conditions during this period has been found in lake records from southern Sweden (e.g., Digerfeldt, 1988; Hammarlund et al., 2003; Jessen et al., 2005), and a cooling and an increase of winter storminess during this period are inferred from many other Scandinavian records (Lauritzen and Lundberg, 1999a; Snowball et al., 1999; Björck and Clemmensen, 2004). In north-western Europe many wet-shifts are registered in bogs (Figure 6 f) at this time (Nilssen and Vorren, 1991; Anderson et al., 1998; Hughes et al., 2000; Barber et al., 2004) and a period of increased lake-levels has also been reported in central Europe (Magny, 2004). On a larger scale has an increase in Na+ in the GISP2 record been detected (Figure 6 b), indicating a deeper Icelandic Low (Mayewski et al., 1997), however, there are comparably few other records from the North Atlantic region that indicate a major climatic change at this time. The implication of this could be that this climate change consisted of a change primary in humidity and that any change in temperature was of minor importance. A wet-shift in the composite record at around 3000 cal yr BP (Figure 6 h) could be traced in all the investigated bogs from Värmland (Paper IV; Figure 6). This wet-shift begins slightly before a drastic increase in Δ14C at c. 2800 cal yr BP (Figure 6 g) that has been linked to a global climate change (e.g., van Geel et al., 1998, 2000; Viau et al., 2002; Blaauw et al., 2004). There are several indications of a climate change in Scandinavia at this time (e.g., Denton and Karlén, 1973; Karlén and Kuylenstierna, 1996; Lauritzen and Lundberg, 1999a; Rosqvist et al., 2004) and a number of palaeorecords indicate a lowering in temperatures also on a larger scale at this time (e.g., O´Brien et al., 1995; Bond et al., 1997, 20 2001; Mayewski et al., 1997, 2004). Throughout north-western Europe many wet-shifts (Figure 6 f) are reported in bogs at this time (Nilssen and Vorren, 1991; Kilian et al., 1995; Hughes et al., 2000; Langdon et al., 2003; Barber et al., 2004; Blaauw et al., 2004) and together with increased lake-levels in the Jura mountains (Magny, 2004) this implies that this was a period of increased humidity in Europe. The lowering of temperatures together with the decrease in solar activity could have led to a more humid climate, perhaps due to decreased evaporation which increased surface wetness conditions on north-western European bogs. The period between c. 1700 and 900 cal yr BP is characterised by mainly wet conditions in Värmland according to the composite record. Many other European bogs seem to have reacted to an increase in surface wetness during this time as well (Figure 6 f)(Nilssen and Vorren, 1991; Hughes et al., 2000; Chiverrell, 2001; Langdon et al., 2003; Barber et al., 2004). An increase in lake-levels in southern Sweden (Digerfeldt, 1988), decreasing temperatures in Norway (Lauritzen and Lundberg, 1999a) and glacier advances in Scandinavia (Denton and Karlén, 1973; Karlén and Kuylenstierna, 1996; Matthews et al., 2000; Rosqvist et al., 2004) indicates that there was a regional shift in climate at this time. Several palaeorecords indicate a climatic transition around 1500 cal yr BP in Europe and the North Atlantic region (Figure 6 a, b, d, e)(Mayewski et al., 1997, 2004; Bond et al., 2001; Magny, 2004). The three periods of increased bog surface wetness deduced from the composite record discussed here are all in concordance with climate changes found in other records and in other regions, especially within the North Atlantic region. The climate shift c. 30002800 cal yr BP is registered in several records around north-west Europe, this climate change has also been found in palaeorecords elsewhere in the world and a decrease in solar irradiation seem to have affected the climate at this time. Many palaeoclimate records also display a climate change starting c. 1700 cal yr BP. In this case, however, there is no direct evidence of a change in solar activity and indications of a global change are few (e.g., Mayewski et al., 2004). Finally, few archives other than bogs and lakes indicate a climate change around 3700 cal yr BP around the North Atlantic region and few records indicates cooler conditions during this time. Despite the Norwegian speleothem record and indications of glacier advances in Scandinavia (Lauritzen and Lundberg, 1999a; Snowball et al., 1999) this climate change seems to have been more a change of humidity than in temperature in Scandinavia. From this it can be The colour of climate: changes in peat decomposition as a proxy for climate change argued that bogs react to climate changes at different spatial scales and to different types of changes (i.e., temperature and humidity). It is clear that the bogs investigated in this study have reacted to the larger Holocene climatic shifts throughout the North Atlantic region. However, the peat decomposition processes react also to changes in local conditions that to some extent can bias the interpretation. In this respect a composite record could be useful in order to filter some of the locally induced changes in bog surface wetness. Causes of hydrological shifts and periodicities in bogs Since both external and internal forcing cause changes in climate (Emeis and Dawson, 2003), variation in solar activity is a potential forcing factor for climatic variability on decadal to millennial timescales (Chambers et al., 1999; Bond et al., 2001). Increases in Δ14C (Figure 6 g) at c. 5500 and c. 2800 cal yr BP indicate a decrease in solar irradiation at these times, which could have caused climate changes (van Geel et al., 1998, 2000; Mayewski et al., 2004). The decrease in solar activity at 5500 cal yr BP could have caused the wet-shift present in Kortlandamossen (Paper IV; Figure 6) and the distinct shift to low solar activity at 2800 cal yr BP is recorded in all investigated sites in Värmland as a period of wet conditions (Paper IV; Figure 6). Three different cycles were found in the humification data from Stömyren and Kortlandamossen: c. 250 yr (both bogs), c. 200 yr (Stömyren) and c. 400 yr (Kortlandamossen). Periodicities of 250±25 yr were identified in distal glaciolacustrine sediments in southern Norway (Matthews et al., 2000), in sediments in the Gotland Basin (Kunzendorf and Larsen, 2002) and at Draved Mose in Denmark (Aaby, 1976). Both the c. 200 yr and c. 400 yr cycle have been found in other studies, and could be of solar origin (Wijmstra et al., 1984; Stuiver and Braziunas, 1989; Chambers et al., 1997; Dean and Schwalb, 2000; Chambers and Blackford, 2001; Dean et al., 2002; Clilverd et al., 2003; Yu, 2003). The changes and periodicities discussed might result from changes in solar activity in a complex cryosphere-ocean-atmosphere system and implies that solar activity plays an important role in climate oscillations over the North Atlantic region (e.g., Karlén and Kuylenstierna, 1996; Chambers et al., 1999; van Geel et al., 1999; Bond et al., 2001; Chambers and Blackford, 2001; Carslaw et al., 2002; Fröhlich and Lean, 2002; Clilverd et al., 2003; Blaauw et al., 2004; Magny, 2004). In the two records (Ältabergsmossen and Gullbergbymossen; Paper III) that have been compared to meteorological records decreased precipitation could be determined as a cause for lowering of the water tables, however, higher summer temperatures at the initiation of the dry period could also have influenced the bog surface wetness. The results from the five analysed peat sequences from Värmland show that shifts in bog surface wetness are present at times of decreased solar activity as well as when there is no indication of such a change. Additionally do the results from all analysed bogs in this study show that changes to wetter conditions on bogs occur when either temperature decreases or precipitation increases, or if they both change at the same time. This implies that no explicit forcing mechanism, other than climate, can be determined in this study. Problems and potentials with peat humification The normal distribution of the frequency of the standard sample (Figure 8) implies that the method of chemical extraction of humic substances and the analytical protocol used are acceptable. However, this does not take into account the problems of NaOH alteration of the humic acids from high to low molecular weight (Caseldine et al., 2000). The problems of poorly understood processes in the standard methodology need attention, but nevertheless a thoroughly applied procedure can provide a palaeohydrological proxy (Caseldine et al., 2000). Transformation of the measured absorbance to percentage humification should be avoided, because of the variety of humic acids produced by different peats under different conditions (Blackford and Chambers, 1993). In the present study most humification records have been normalised, both to facilitate comparison between different sequences with sometimes large differences in “mean” humification levels, and to reduce the risk of comparing levels of humification measurements as degree of decomposition which may lead to misinterpretations. The process of secondary decomposition (Tipping, 1995) described in Paper III has great implications for palaeoenvironmental interpretations of humification records. The onset of the effects of drier conditions could appear to start earlier (deeper) than the actual hydrological shift. However, this process can be discriminated by the compaction that usually takes place in longer sequences. When analysing long series, even in contiguously sampled peat sequences, the temporal resolution could be too low to detect the process of secondary decomposition. However, 21 Anders Borgmark when performing high-resolution analyses of peatsequences (i.e., sub-recent peat), the process of secondary decomposition should be taken into account when interpreting the data. Perhaps the most complicating factor with peat humification as a proxy for past climate change is that the method only produces a relative measurement of change, it is not possible to estimate the magnitude of change or to say that a certain degree of humification equals a certain width of the acrotelm. One way to overcome this problem might be to use e.g., testate amoebae assemblages for which transfer functions have been developed, which makes it possible to obtain quantitative estimates of peat moisture and water table depth (Warner and Charman, 1994; Charman and Warner, 1997; Woodland et al., 1998; Charman et al., 2000; Charman, 2001; Mitchell et al., 2001; Booth, 2002). When comparing peat humification records and water tables inferred from testate amoebae assemblages from a sequence, it might be possible to transform the humification data to bog surface wetness using a two-step transfer function. A function of this kind could be set up by using a relatively small number of samples from which testate amoebae assemblages are determined. The testate amoebae transfer function is then used to infer a water table for these assemblages, the results are transferred to the matching peat humification values and from that surface wetness could be inferred on the entire sequence. Due to the nature of the decomposition process, this two-step transfer function is valid only on the actual sequence analysed. However, usage of the relatively fast and unproblematic determination of peat humification, together with a small number of analyses of testate amoebae assemblages, would make it possible to retrieve a better result than if only one method is used and less time consuming than if both methods were used separately (i.e., more samples of testate amoebae analysed). Conclusions • Comparison of replicate sequences from one bog gives an idea of the type and scale of active hydrological processes i.e., if they are autogenic (microand mesoscale) or allogenic (meso- and macroscale). Comparison and correlation of sequences from several bogs within a specific area provides indication of the macroscale processes, of which climate is the most important factor. • Changes in peat bog surface wetness can be correlated with other Fennoscandian palaeoclimate records as well as climate shifts registered across the 22 North Atlantic region. This implies that the peat humification displays changes in bog hydrological conditions due to variations in the climate in a scale of at least north-western Europe and probably the North Atlantic region. Increased surface wetness has been inferred from a composite record, c. 3700-3200, 3000-2800 and 1700-900 cal yr BP. • Cyclic patterns of inferred bog surface wetness have been found with frequencies of 200, 250 and 400 yr. These periodicities and the wet-shifts at c. 5500 and 2800 cal yr BP correlate to variations in sun irradiance and it is plausible that this external forcing caused the shifts in climate. In this study it was evident that, given the physical properties of the data, frequencies longer than c. 500 yr could be the result of a first-order autoregressive process implying that great caution should be taken when interpreting this type of data. • Peat humification reflects shifts in the surface wetness of raised peat bogs. The processes of decomposition are, however, not fully understood. Secondary decomposition could result in a lag between peat humification and biological proxies, especially when analysing high-resolution records. Due to the complexity of peat decomposition processes it is important to interpret humification data carefully, with special emphasis on the amplitude needed in the humification record for interpretation of possible climate driven changes. • In order to better assess the relationship between precipitation and temperature and their resulting influence on bog surface wetness more studies of the processes involved in peat decomposition are necessary. Usage of multiproxy high-resolution studies of sub-recent peat that could be correlated to known climatic conditions is probably the best way to study decomposition processes. • Chemical extraction of humic substances, despite limitations in the extraction process, is a straightforward method that produces replicable results. To delimit the number of errors, well-defined mediumsized ombrotrophic bogs without signs of drainage and/or peat cutting should be used in this type of study. The colour of climate: changes in peat decomposition as a proxy for climate change Acknowledgements Under dessa år har min familj varit ett stort stöd, även om de inte alltid har förstått vad jag sysslat med har Mamma, Pappa, Ullis, Kattis och Fredde alltid funnits där. Utan Elin och Axel hade livet varit halvt och bra mycket tråkigare. Mitt allra största och djupaste tack går till Johanna, som inte bara har stått ut med mig under den här tiden, utan också stöttat mig när jag behövt det som mest. Stefan Wastegård initiated this project and together with Ulrik Kautsky (SKB) we formulated the main focus of the work. Stefan has, as project leader and main supervisor, been there with a good balance between whip and carrot. Ann-Marie Robertsson, my other supervisor, has always kindly read my texts. I could not have done this without the two of you. Jan Lundqvist has always been supportive of my work, and his stories about “the good old days” will always be remembered - and I hope that some of the mess in Värmland has been cleaned up now! Fieldwork and the majority of the radiocarbon dates were funded by the Swedish Nuclear Fuel and Waste Management Co (SKB) and by scholarships from the foundations of Helge Ax:son Johnson and Gerard De Geer. For the first year of PhD studies I received a scholarship from Gustaf and Ellen Kobb´s Foundation. The remaining years were funded by Stockholm University and a grant from The Swedish Natural Science Research Council to Stefan Wastegård. Financial support has also been provided by The Swedish Foundation for International Cooperation in Research and Higher Education (STINT), Wallenbergs jubileumsfond, C F Liljevalch J:ors resestipendium, Carl Mannerfelt´s Foundation and The Swedish Natural Science Research Council (grant to Stefan Wastegård). This thesis has benefited greatly from the comments, questions and criticism from Ninis Rosqvist, Jan Risberg and Barbara Wohlfarth and I’m grateful to you all. Terri Lacourse made a huge effort in correcting the language of the thesis. My ´brother in arms´ Kristian Schoning has been a great support while driving around in the forests in search for the optimal bog, and coring the potential ones. Since we have yet to find it, there is still work to do… Many people that have contributed with bits and pieces over the years and you are all acknowledged. However, I have to mention some of you; Greger Lindeberg introduced me to the magical world of frequency analysis and showed how dark that magic can be. Anders Moberg has been very helpful with answers to questions about time-series and statistics. Ann Karlsson helped with futher development of the humic acid extraction methodology. Ann, and later Sven Karlsson, have been invaluable in the lab. Katarina Borgmark finished the last batch of humification measurements and eased my burden. As a source of knowledge about almost everything from H. P. Lovecraft to Return to Castle Wolfenstein has Jens Heimdahl been more than just a colleague and we have had lots of fun together during the years. I have had several fruitful and enjoyable discussions about peat, climate and everything in-between with Jonas Bergman. Keith Barber kindly arranged my visit at the Department of Geography, University of Southampton (funded by STINT), he and Paul Hughes, Pete Langdon, Antony Blundell, Paul Coombes and the rest of the gang made my time there (and at the pubs!) a memorable and learning experience. Per, Greger, Anna, Kristian, Gun, Tiit, Jens, Martina and all the other past and present PhD students in Quaternary Geology have made me proud of being part of this group, as everyone at the Department of Physical Geography and Quaternary Geology has. The “innebandy”-players took my mind of science for at least one hour every week…it has been great fun! References Aaby, B. 1976. Cyclic climatic variations in climate over the past 5,500 yr. reflected in raised bogs. Nature 263, 281-284. Aaby, B. 1986. Palaeoecological studies of mires. In Handbook of Holocene Palaeoecology and Palaeohydrology. B. E. Berglund (Ed.). John Wiley & Sons Ltd. 145-164. Aaby, B. and Tauber, H. 1974. 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