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Volcanic ash reveals time-transgressive abrupt climate change
during the Younger Dryas
Christine S. Lane1*, Achim Brauer2, Simon P.E. Blockley3, and Peter Dulski2
1
Research Laboratory for Archaeology and the History of Art, University of Oxford, Dyson Perrins Building, South Parks Road,
Oxford OX1 3QY, UK
2
GFZ German Research Centre for Geosciences, Telegrafenberg, D-14473 Potsdam, Germany
3
Centre for Quaternary Research, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK
ABSTRACT
Knowledge of regional variations in response to abrupt climatic transitions is essential to understanding the climate system
and anticipating future changes. Global climate models typically
assume that major climatic changes occur synchronously over continental to hemispheric distances. The last major reorganization of
the ocean-atmosphere system in the North Atlantic realm took place
during the Younger Dryas (YD), an ~1100 yr cold period at the end
of the last glaciation. Within this region, several terrestrial records
of the YD show at least two phases, an initial cold phase followed by
a second phase of climatic amelioration related to a resumption of
North Atlantic overturning. We show that the onset of climatic amelioration during the YD cold period was locally abrupt, but timetransgressive across Europe. Atmospheric proxy signals record
the resumption of thermohaline circulation midway through the
Younger Dryas, occurring 100 yr before deposition of ash from the
Icelandic Vedde eruption in a German varve lake record, and 20 yr
after the same isochron in western Norway, 1350 km farther north.
Synchronization of two high-resolution continental records, using
the Vedde Ash layer (12,140 ± 40 varve yr B.P.), allows us to trace
the shifting of the polar front as a major control of regional climate
amelioration during the YD in the North Atlantic realm. It is critical
that future climate models are able to resolve such small spatial and
chronological differences in order to properly encapsulate complex
regional responses to global climate change.
of the YD also has been reported in terrestrial records and linked to associated changes in atmospheric circulation regimes (Walker, 1995; Isarin
et al., 1998; Bakke et al., 2009). Such a mid-YD transition has been suggested to have occurred simultaneously across the North Atlantic realm
(Bakke et al., 2009) although proof has not been possible due to the lack of
direct and precise synchronization of independently dated records.
Annually resolved paleoclimate records suggest that abrupt climate
changes may happen within a few decades, or even within a year (Brauer
et al., 2008; Steffensen et al., 2008), but if we want to compare the timings of rapid shifts between different archives, we require an independent
tool to line up records with annual precision. Visible volcanic ash layers
(tephra) have long been used as such time-synchronous marker horizons
close to volcanic sources (Lowe, 2011). Recent advances in the detection
of non-visible tephra layers that are present in multiple important climate
archives allows for the integration of many more records in more distant
locations (Turney et al., 2004; Blockley et al., 2012).
Here, we report a YD record from an annually laminated German
maar lake in the Eifel region (Fig. 1), spanning the time interval from the
tephra layer of the late Allerød Laacher See eruption until the early Holocene (12.9–11.5 ka). The full sediment record from Lake Meerfelder
Maar (MFM) contains a succession of 12,000 varves yielding a temporal resolution that allows insight into even seasonal changes. MFM
is the only lake record in Western Europe that contains a continuous
sequence of annual layers throughout the YD. The present-day climate
is dominated by the prevailing westerly winds (Fig. 1), and changes in
INTRODUCTION
On long time scales, of tens of thousands of years, abrupt shifts are a
common feature of the climate system (Brauer et al., 2008; Steffensen et
al., 2008; Blockley et al., 2012). However, dating uncertainties for sediment archives of past environmental change are typically multi-centennial
in scale. Therefore, it is still largely unknown if such shifts occurred synchronously or if there were regional differences in timing and amplitude.
To anticipate the regional expressions of possible future climate change,
we need to precisely understand the timing and consequences of major
changes not only between far distant regions, but also between areas located within the same climatic region. An ideal test area is the North Atlantic realm due to the large amount of available background information.
In particular, the Younger Dryas (YD), the last major climatic fluctuation
of the last glaciation (Broecker et al., 1988; Alley et al., 1993; Bakke et al.,
2009; Brauer et al., 2008), represents an ideal natural laboratory to study
the complexity of abrupt shifts of the climate system. Past investigations
concentrated on the large shifts and their potential trigger mechanisms
at the onset and end of the YD (Broecker et al., 1988; Isarin et al., 1998;
Brauer et al., 2008; Lohne et al., 2013), but changes within the YD have
received less attention. There is common agreement that the YD is related
to a weakening of the North Atlantic Meridional Overturning Circulation
(AMOC) during the early YD (McManus et al., 2004; Jennings et al.,
2006; Bakke et al., 2009) followed by a mid-YD recovery (Jennings et al.,
2006; Elmore and Wright, 2011; Pearce et al., 2013). A bipartite structure
*E-mail: [email protected].
GEOLOGY, December 2013; v. 41; no. 12; p. 1251–1254; Data Repository item 2013355
Figure 1. Map showing average Holocene position of polar front
and present-day westerlies at ~62°N, alongside postulated polar
front position and prevailing westerlies at ~50°N during Younger
Dryas (Broecker et al., 1988) and at time of Vedde Ash (12,140
± 40 varve yr B.P.). Also shown are locations of Meerfelder Maar
(MFM, Germany) and Kråkenes (Norway).
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the depositional environment detected by detailed micro-facies analyses
have been demonstrated to be sensitive proxies for atmospheric changes
(Brauer et al., 2008).
METHODS
We applied a combination of varve counting and cryptotephra investigations (Blockley et al., 2005) to the Lateglacial sequence in MFM in
order to refine the YD chronology and to facilitate robust correlations with
other regional climate archives.
MFM varve counting was carried out on large-scale thin sections using a petrographic microscope (Brauer et al., 1999). The floating MFM
chronology has been linked to an absolute time scale using the Preboreal
Ulmener Maar Tephra (Brauer et al., 1999) and additionally confirmed by
69 radiocarbon dates obtained from terrestrial macrofossils (Brauer et al.,
2000). The given error of 12,140 ± 40 yr for the Vedde Ash reflects the
counting uncertainty for the counted interval between two tephra layers,
the aforementioned Ulmener Maar Tephra and the late Allerød Laacher
See Tephra. The counting error has been revealed from multiple counting
of six sediment cores by three different investigators.
Volcanic glass shards were located to within 1 cm in the MFM sediment cores by sieving (>25 μm) and density extraction (2.0–2.5 g/cm3)
techniques (Blockley et al., 2005), then the exact varve position of the
ash was determined via microscopic inspection of a sediment thin section.
Glass-shard major- and minor-element oxide compositions were measured on the JEOL JXA 8600 wavelength dispersive X-ray spectrometer–
electron microprobe (WDS-EMP) at the Research Laboratory for Archaeology and the History of Art, University of Oxford (UK). An accelerating
voltage of 15 kV, a 6 nA beam current, and a 10 μm beam were used.
The WDS-EMP instrument was calibrated using a suite of characterized
mineral standards. Ten elements were measured in each sample with varying count times (Na—10 s; Si, Al, K, Ca, Fe, Mg—30 s; Ti, Mn—40 s;
P—60 s). Secondary glass standards from the MPI-DING suite (Jochum
et al., 2006) were analyzed between and within analytical runs to check instrumental precision and accuracy. Glass compositional data are reported
in Table DR1 in the GSA Data Repository1.
RESULTS AND DISCUSSION
We located volcanic glass shards in a concentration of ~7000
shards/g dry sediment between 800 and 801 cm depth, midway through
the YD sequence, in the MFM sediment core. The glass shards are chemically correlated to the Icelandic eruption of the Vedde Ash (Mangerud et
al., 1984; Lane et al., 2012) (Fig. 2). Subsequent scrutiny of thin sections
showed that the ash layer is concentrated within the spring snow melt
of an individual varve dated to 12,140 ± 40 varve yr B.P. in the MFM
varve chronology, in agreement with the reported age of the Vedde Ash of
12,121 ± 114 yr B.P. (after converting the b2k [before the year 2000] age
to B.P.; the stated uncertainty is the “maximum counting error,” equivalent to 2σ) from the Greenland ice cores (Greenland Ice Core Chronology
2005 [GICC05]; Rasmussen et al., 2006). We use this isochron as a datum
around which we align the MFM record to that of Kråkenes, a glacier-fed
lake at 62°N (Norway) (Bakke et al., 2009), where the Vedde Ash is also
observed and dated to 12,066 ± 42 calibrated yr B.P. based on Bayesian
age-depth modeling of high-resolution AMS 14C dates (Mangerud et al.,
1984; Lohne et al., 2013), and consider the relative timing of the mid-YD
climate transition recorded in these two sites (Fig. 3).
In MFM, a change in depositional regime from one dominated by
wind-driven diatom blooms in spring to one featuring distinct spring snow
1
GSA Data Repository item 2013355, Table DR1 (representative major- and
minor-element oxide data [WDS-EMP] from the Vedde Ash cryptotephra layer
in MFM, alongside associated secondary standard analyses), is available online
at www.geosociety.org/pubs/ft2013.htm, or on request from editing@geosociety
.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
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Figure 2. Varves from Meerfelder Maar containing Vedde Ash. Tephraderived glass shards (bottom right) were found within a varve directly
below a notable spring snow melt layer, as indicated, at depth of
801 cm below lake floor (12,140 ± 40 varve yr B.P.). Wavelength dispersive electron probe micro-analysis results confirm correlation to
rhyolitic and intermediate phases of Vedde Ash (Lane et al., 2012). Inset cross symbols on bi-plots indicate representative ±2σ uncertainty,
based upon replicate secondary standard analyses (see Table DR1
[see footnote 1]).
melt is observed within 1 yr (Fig. 3). This occurs at 12,240 ± 40 varve yr
B.P., which is precisely 100 yr before the deposition of the Vedde Ash
(Fig. 3). We interpret this as a climatic amelioration associated with a
northward shift of westerly wind systems. In contrast, in Kråkenes, the
abrupt shift from a cold and stable early YD to a more unstable phase, due
to increased westerly winds, does not occur until ~20 yr after the Vedde
Ash layer (Bakke et al., 2009). The location of the Vedde Ash layer in
the two sediment sequences, combined with the sequences’ multi-decadal
resolution age models (Brauer et al., 2008; Lohne et al., 2013), confirms
without doubt that this transition is time-transgressive. It is important to
highlight that within both lake records we observe a mid-YD shift within
multiple proxy data sets; however, for simplicity we show the same proxy
signals in Figure 3, e.g., micro–X-ray fluorescence (XRF) Ti counts, as a
proxy for detrital flux for spring snow (MFM) and glacial melt (Kråkenes;
Bakke et al., 2009).
We interpret the 120 yr offset between the mid-YD climate shift in
MFM and that in Kråkenes as resulting from a gradual northward retreat of
the average polar front position from an early-YD position south of 50°N
(Broecker et al., 1988; Isarin et al., 1998) to ~62°N, close to its present-day
position (Fig. 1). We interpret this as a dynamic atmospheric response to
increasing AMOC strength during the YD. Our data confirm that the YD
climate transitions observed in MFM and Kråkenes were indeed locally
abrupt; however, they form part of a time-transgressive phenomenon occurring over at least 120 yr, rather than a single synchronous event. While
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Years B.P.
Figure 3. Comparison of the timing of the mid–Younger Dryas (YD) transition observed in Kråkenes (Norway) and Meerfelder Maar (MFM,
Germany), alongside North Greenland Ice Core Project (NGRIP) ice core record. Sites are plotted on their independent age models (NGRIP
converted from b2k [before the year 2000] to years B.P.), which agree within 20 yr at position of Vedde Ash (dashed line). Mid-YD transition
in each record is marked by color change from yellow to blue. In Kråkenes, Ti X-ray fluorescence (XRF) count rate records abrupt change
in glacier melt (Bakke et al., 2009) occurring 20 yr after Vedde Ash. In Meerfelder Maar, change from sediments dominated by wind-driven
diatom blooms (higher varve thickness) to one dominated by spring snow melt layers (higher Ti XRF count rate) occurs 100 yr before Vedde
Ash. Note that elevated Ti and varve thickness values at base of MFM time series is related to two varves of increased thickness immediately
following deposition of Laacher See Tephra layer (12,880 ± 40 varve yr B.P.; Brauer et al., 1999).
the recession of polar air masses may not have been linear, we calculate an
approximate average retreat rate of 10 km/yr between the two study sites.
During this short interval, different climate conditions were experienced
simultaneously on either side of the polar front, in central and northern
Europe (Fig. 3). From these findings we predict that future investigation
of suitable intermediate sites, between 50°N and 62°N, should reveal a
transition occurring between those observed at MFM and Kråkenes.
The Vedde Ash further provides a direct tie line between lake sites
in Northern Euorpe and the North Greenland Ice Core Project (NGRIP)
ice core record, where it is located midway through Greenland Stadial
1 (GS-1; Mortensen et al., 2005). As illustrated in Figure 3, the NGRIP
stable oxygen isotope signal indicates pervading cold conditions during
GS-1, while a steadily decreasing dust signal implies that some aspect of
the atmospheric system (beyond the North Atlantic) was changing (Ruth
et al., 2004). The lack of a distinct shift in Greenland δ18O data during
GS-1 provides further support for our interpretations because Greenland
was always located to the north of the shifting polar front throughout the
Lateglacial and therefore does not record the dynamics at the center of
action of the polar front.
Our data highlight some of the significant additional complexities
that need to be investigated in further studies of climate change, and in
particular studies of regional variability. We have shown that such complexities would be lost if individual records were wiggle-matched by their
paleoclimate proxies on the assumption that abrupt changes are time-synchronous. From the perspective of the paleo-data community, exploring
this aspect of abrupt climate change requires the systematic analyses and
cross correlation of annual- or near-annual–resolution records (Blaauw,
2012). Our work shows that correlations based on tephra isochrons are an
ideal way to achieve this goal. These results are further important for the
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modeling community, as state-of-the-art climate models are currently validated against high-resolution past climate data, most commonly from the
Greenland ice core records (Roche et al., 2010). These findings have two
key implications for future climate model development and for model–to–
paleo-data comparisons. Firstly, we suggest that during periods of transition the atmospheric system is dynamic, and models need to be able to encapsulate decadal- to centennial-scale time-transgressive changes within
one region, which are also very abrupt (1 yr) at individual sites. Secondly,
our data suggest that the atmospheric reorganization was expressed most
clearly at the margins of polar and Atlantic air masses. There is no similar
clear signal for this transition recorded within the Greenland record (Rasmussen et al., 2006; Steffensen et al., 2008), which remained well within
the zone of polar air masses during GS-1. To study the regional expression
of climate change, therefore, model-versus-data comparisons need to include a geographically wider range of very high-resolution records, which
requires the robust synchronization of records using independent methods.
Using this approach we have found convincing evidence that in the
past, climatic changes have been reached diachronously, even within the
same broad climate regime. This finding has implications for the anticipation of fine-scale regional responses to future global climate change.
ACKNOWLEDGMENTS
This study is a contribution to the INTIMATE (Integrating Ice Core, Marine,
and Terrestrial Records) project, which is funded as an European Cooperation in
Science and Technology Action. We thank V.C. Smith for electron probe analyses;
the 2005 MFM coring team; the Maar Museum in Manderscheid for local support;
Andreas Hendrich for graphical help; and S.O. Rasmussen and J.J. Lowe for advice on the manuscript. We also thank three reviewers, M. Blaauw, D. Lowe, and
J.P. Steffensen, for their contributions to the final article. Lane and Blockley were
funded by the UK Natural Environment Research Council consortium RESET
(grants NE/E015670/1 and NE/E015905/1) and The Leverhulme Trust (Lane). This
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study is a contribution to the climate initiative REKLIM Topic 8 “Abrupt climate
change derived from proxy data” (Brauer). This is RHOXTOR (Royal Holloway
and Oxford Tephrochronology Research) Paper number RHOX/034.
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Printed in USA
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