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Science Peaks East African mountain lakes: Archives for past climate change and glacier dynamics Hilde Eggermont The sedimentary record of Africa’s alpine lakes contain valuable and largely untapped records of past climate change and glacier dynamics. Our findings suggest that the influence of late 19th century reductions in precipitation in triggering Rwenzori glacier recession is weaker than previously thought Tropical glaciers are enormously important to our understanding of past, present, and future climate change, yet lack of continuous quantitative records of alpine glacial extent in tropical East Africa prior to the 20th century has left the precise timing and drivers of recent glacial advances and recession in the region equivocal. Sediments accumulating on the bottom of alpine glacial lakes chronicle the history of central African climate and environmental dynamics, and can thus provide an historical perspective to understand the climatic controls on glacier mass balance, the relative impacts of human-induced global warming versus natural climate variability, and the long-term tropical mountain ecosystem and glacier stability in East Africa. The main goal of our research program (2005 - present) is to investigate century-scale variability in rainfall and temperature and its impact on alpine glaciation and ecosystems in central equatorial Africa during the mid- to late Holocene through multi-proxy analysis of sediment records from East African mountain lakes. Our current efforts are to: • Actively monitor the physical and chemical limnology of a large set of East African mountain lakes, and collect associated climate data • Calibrate sedimentological, geoche14 Lake Batoda located at 4017 m in the Rwenzori Mountains (Uganda), surrounded by tree groundsel. © Hilde Eggermont mical and biological climate-proxy indicators through analysis of their variation in the surface sediments of lakes and pools located along a large altitudinal (temperature) gradient • Analyse long sediment cores from the most promising sites to construct a regional picture of African paleoclimate history over the past 500010,000 years. • Document the poorly known biodiversity of aquatic algae, insects and micro-crustacea in the unique setting of tropical high-elevation lakes; and analyse their long-term response to climate change and glacier dynamics. To this end, we maintain an active field and laboratory research programme in three East African mountain regions: the Rwenzori Mountains (Uganda-DR Congo; Eggermont et al. 2009b), Mount Kenya (Kenya) and the Bale Mountains (Ethiopia); the former two still have snow-capped summits. In January 2008, we recovered long cores from seven lakes in the Rwenzori that extend to ~10,000 yr BP. The lakes include three lakes that currently receive glacial meltwater (Upper and Lower Kitandara Lake, Lac du Speke) and four lakes that do not (Mahoma, Upper Kachope, Batoda, Kopello). Comparisons between these classes of lake allow us to disentangle the effects of climate-induced vs. glacier-induced environmental perturbations. Previous paleoclimatic research in tropical mountains has been severely constrained by uncertainty about the relative influence of temperature and precipitation on climate proxies. We are Mountain Research Initiative Newsletter no. 4, May 2010 East African mountain lakes New Afrotropical paleothermometers: (a) Chironomid-reconstructed air temperature from 65 sites in East Africa against observed mean annual air temperature (Eggermont et al., 2009a); (b) MBT reconstructed air temperature from 65 sites in East Africa plotted against observed mean annual air temperature (Tierney et al. submitted; Loomis et al. unpublished data) developing novel, state-of-the-art paleoecological and organic geochemical proxies for temperature using lake surface sediment measurements calibrated against an extensive dataset of physical, chemical, sedimentological and biological data on ~50 lakes and pools located along an elevational gradient between 2500 and 5000 m asl (Eggermont et al., 2007, 2009a-b; Russell et al., 2009). Chironomidae (Insecta: Diptera; nonbiting midges) are powerful biological indicators of past environmental change, owing to their great taxonomic and ecological diversity, short generation times, efficient long-distance dispersal, and good preservation of their larval remains in lake sediments (Walker 2001). As a first step to develop chironomids as paleothermometers in an African context, we analysed their distribution in the surface sediments of 65 lakes and permanent pools in southwestern Uganda (including virtually all lakes on the Ugandan side of the Rwenzori Mountains) and central and southern Kenya (including a handful of lakes on Mt Kenya) spanning a large mean annual temperature gradient (1.1-24.9°C) (Eggermont and Verschuren, 2007; Eggermont et al., 2009a). The results (Figure 3a) highlight the great importance of chironomids as paleothermometers, but they likewise indicate that refine- ment/elaboration with more Mt Kenya lakes is desirable to increase model performance. Furthermore, expansion of the calibration dataset with other regions (such as the Bale Mountains in Ethiopia, and high elevation sites in Tanzania) is also necessary to enhance applicability of the chironomid-based temperature models. “Previous paleoclimatic research in tropical mountains has been severely constrained by uncertainty about the relative influence of temperature and precipitation on climate proxies.” Organic geochemical methods have provided fundamentally important new insights into the precipitation and temperature history of the continents. For instance, application of the TEX86 temperature proxy, based upon distributional variations in aquatic Crenarcheotal membrane lipids, to sediment cores from Africa’s great lakes has documented temperature variations on orbital and millennial time-scales over the past 60,000 years (Powers et al., 2005; Tierney et al., 2008). However, Mountain Research Initiative Newsletter no. 4, May 2010 the application of TEX86 is limited to large, low elevation lakes by ‘contamination’ from membrane lipids derived from soil Crenarchaea. More recently, variations in the relative abundance of a new class of nine microbial membrane lipids, quantified by the so-called MBT/CBT indices, have been found to correlate to environmental temperature and pH (Weijers et al., 2007). We have investigated the distribution of these compounds in our 65-lake dataset, and found that these membrane lipids indeed respond to and can be used to predict temperature with considerable accuracy (Tierney et al., submitted; Loomis et al. unpublished data). Questions still remain regarding lipid source – an issue that we are investigating through paired analyses of soil, river, and lake sediment. Yet, this method clearly holds promise in unravelling the temperature history of East Africa’s Afroalpine environments. Using short sediment cores taken at the deepest part of the Rwenzori lakes, we investigated changes in lacustrine sedimentation, glacial extent, and biogeochemical processes in the Rwenzori Mountains during the last 700 years by comparing sedimentological (organic and siliciclastic component) and or15 Long coring at Lac du Speke located at 4235 m in the Rwenzori Mountains (DR Congo) beneath the glaciers on western Mount Speke. © Hilde Eggermont 16 ganic geochemical profiles (carbon and nitrogen abundance, ratio, and isotopic composition of sedimentary organic matter) from lakes occupying presently glaciated catchments against profiles from lakes located in catchments lacking glaciers (Russell et al., 2009). The siliciclastic content of sediments in ‘glacial lakes’ significantly decreased towards the present, whereas ‘nonglacial lakes’ generally showed weak trends in their siliciclastic content over time, demonstrating that changes in the siliciclastic content of glacial lake sediments records fluctuations in glacier extent. ever, recession was underway by 1870 AD, during a regionally wet episode. These data suggest that precipitation variability does not appear to have been the primary control on Rwenzori glaciers over long time-scales. Lastly, our organic geochemical data indicate that glacial retreat has significantly affected carbon cycling in Afroalpine lakes, but trends in aquatic ecosystem functioning are variable among lakes and require more detailed analysis. Radiometric dating of our sediment cores indicated that prior to their late 19th-century recession Rwenzori glaciers stood at expanded ‘Little Ice Age’ positions for several centuries under a regionally dry climate regime. Evidence for this drought includes lithostratigraphic data from crater lakes situated immediately south east of the Rwenzori (Russell et al., 2007; Bessems et al. 2008) and rising and high %Mg in calcite values from Lake Edward. How- been the primary control on „These data suggest that precipitation variability does not appear to have Rwenzori glaciers over long time-scales.“ We also evaluated the limnological and ecological sensitivity of Rwenzori mountain lakes to climate change by comparing the species assemblage of larval chironomid remains recovered from sediments deposited recently with fossil assemblages recovered from the base of the short sediment cores which are dated to within or briefly after the Little Ice Age (Eggermont et al., 2010). Application of the chironomid-based temperature models indicated significantly warmer mean annual air temperatures (on average +0.38 ± 0.11 °C) at present compared to between ~85 and ~645 years ago. Inferred temperature changes are independent of whether lakes are located in glaciated or nonglaciated catchments, and of the age of the core base, suggesting that at least part of the signal is due to relatively recent, anthropogenic warming. The fairly uniform and marked historical warming trend in Rwenzori lakes documented by this study highlights their ecological vulnerability and their value as early-warning systems for detecting the limnological and ecological effects of global warming. Future fieldwork will focus on the expansion of the calibration data sets with more lakes from Mt Kenya, and recovery of long sediment cores from Mt Kenya lakes. Together with ongoing multiproxy work on the long cores Mountain Research Initiative Newsletter no. 4, May 2010 East African mountain lakes from Rwenzori, these data will allow us to construct a regional picture of African paleoclimate history and glacier dynamics over the past 5000-10,000 years. In addition, we aim to disentangle the relative importance of local, regional and temporal processes driving the community ecology of Afroalpine Cladocera (water fleas; Crustacea). The study comprises various spatial and temporal scales in the isolated islands of East Africa’s highest mountain ranges, and integrates traditional techniques (downcore fossil analysis, DNA barcoding) with more novel approaches such as palaeogenetics. Besides providing fundamental insight into evolution- ary and ecological drivers of multiscale community ecology, this project will provide a means to assess global change effects on the biodiversity and ecological integrity of tropical cold-water lakes. As such, theoretical ecological concepts will eventually feed back to bioconservation policy. Authors & Principal Investigators Hilde Eggermont, Limnology Unit, Ghent University, K.L. Ledeganckstraat 35, B-9000 Ghent, Belgium; Royal Belgian Institute of Natural Sciences, Freshwater Biology, Vautierstraat 29, 1000 Brussels, Belgium; [email protected] James Russell, Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA; [email protected] Dirk Verschuren, Limnology Unit, Ghent University, K.L. Ledeganckstraat 35, B-9000 Ghent, Belgium; [email protected] Project participants Shannon Loomis, Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA Leen Audenaert, Limnology Unit, Ghent University, K.L. Ledeganckstraat 35, B-9000 Ghent, Belgium Bob Nakileza, Mountain Research Center, Makerere University, Uganda Daniel Olago, Chiromo Campus, Riverside Drive, P. O. Box 30197, Nairobi, Kenya Stephen Rucina, National Museums of Kenya, Department of Palynology and Palaeobotany, Nairobi, Kenya Geoffrey Mibei, Chiromo Campus, Riverside Drive, P. O. Box 30197, Nairobi, Kenya Erustus Kanga, Kenya Wildlife Service, Langata Road, Nairobi, Kenya Anouska Kinahan, Technical Advisor Frankfurt Zoological Society –Bale Mountains Conservation Project, Robe, Bale, Ethiopia Weblinks Rwenzori Project website: http://www.geo.brown.edu/georesearch/esh/Rwenzori.htm Personal webpages: http://www.ecology.ugent.be/limno/HE.php http://research.brown.edu/myresearch/James_Russell Mountain Research Initiative Newsletter no. 4, May 2010 17 References Bessems, I., Verschuren, D., Russell, J.M., Hus, J., Cumming, B., 2008. Paleolimnological evidence for widespread late-18th century drought across equatorial East Africa. Palaeoecology, Palaeoclimatology, Palaeoecology 259: 107-120. Eggermont, H., Heiri, O., Russell, J., Vuille, M., Audenaert, L., and Verschuren, D. 2009a. Chironomidae (Insecta: Diptera) as paleothermometers in the African tropics. Journal of Paleolimnology DOI 10.1007/s10933-009-9339-2. Eggermont, H., Russell, J., Schettler, G., Van Damme, K., Bessems, I., and Verschuren, D. 2007. Physical and chemical limnology of alpine lakes and pools in the Rwenzori Mountains (Uganda-Congo). Hydrobiologia 592: 151-173 Eggermont, H., Van Damme, K., and Russell, J.M. 2009b. Rwenzori Mountains (Mountains of the Moon): headwaters of the White Nile. H.J. Dumont (ed). The Nile: Origin, Environments, Limnology and Human Use, pp. 243-261. Monographiae Biologicae, Springer Science. Eggermont, H., and Verschuren, D. 2007. Taxonomy and diversity of Afroalpine Chironomidae (Insecta: Diptera) on Mount Kenya and the Ruwenzori Mountains, East Africa. Journal of Biogeography 34: 69-89. Eggermont, H., et al (2010). Ecological and hydrological sensitivity of Rwenzori Mountain lakes to climate warming. In H. Eggermont, M. Kernan, K. Martens (Eds). Global change impacts on mountain lakes. Special issue Hydrobiologia 648: 123-142 Powers, L.A., Johnson, T.C., Werne, J.P., Castañeda, I.S., Hopmans, E.C., Sinninghe Damsté, J.S. and Schouten, S. 2005. Large temperature variability in the southern African tropics since the Last Glacial Maximum. Geophysical Research Letters 32: doi:10.1029/2004GL022014. Russell, J.M., H. Eggermont & D. Verschuren, 2007. Spatial complexity of Little Ice Age Climate in East Africa: sedimentary records from two crater lake basins in western Uganda. Holocene 17: 183-193 Russell, J.M., H. Eggermont, R. Taylor & D. Verschuren, 2009. Paleolimnological Records of Recent Glacial Recession in the Rwenzori Mountains, Uganda-D. R. Congo. Journal of Paleolimnology 41: 253-271 Russell, J.M. & T.C. Johnson, 2007. Little Ice Age drought in Equatorial Africa: ITCZ Migrations and ENSO variability. Geology 35: 21-24. Tierney, J.E., Russell, J.M., Huang, Y., Sinninghe Damsté, J.S., Hopmans, E.C. and Cohen, A.S., 2008. Northern hemisphere controls on tropical Southeast African climate during the past 60,000 years. Science 322: 252-255. Tierney, J.E., Russell, J.M., Eggermont, H., Hopmans, E.C., Sinninghe-Damsté, J.S., and Verschuren, D. submitted. Environmental controls on branched tetraether lipid distributions in tropical East African lake sediments: a new lacustrine paleothermometer. Geochimica et cosmochimica Acta. Walker, I.R., 2001. Midges: Chironomidae and related Diptera. In: Smol J.P., Birks H.J.B., Last W.M. (eds) Tracking Environmental Change Using Lake Sediments. Zoological Indicators. Kluwer Academic, Dordrecht, pp 43-66. Weijers, J.W.H., Schouten, S., van den Donker, J.C., Hopmans, E.C. and Sinninghe Damsté, J.S., 2007. Environmental controls on bacterial tetraether membrane lipid distribution in soils. Geochimica et Cosmochimica Acta 71: 703-713. 18 Mountain Research Initiative Newsletter no. 4, May 2010