BRANCHED GDGT BASED TEMPERATURE RECONSTRUCTION IN PALEOLIMNOLOGY: INSIGHTS FROM REGIONAL CALIBRATIONS IN MEXICAN AND GUATEMALAN LAKES

The potential to use bacterial derived branched glycerol dialkyl glycerol tetraethers (brGDGTs) to reconstruct mean annual air temperatures from soils sparked significant interest in the terrestrial paleoclimate community, where a high-fidelity paleotemperature proxy is desperately needed. Initial applications of the original soil-based MBT/CBT proxy to lacustrine sediments from Valles Caldera, New Mexico were promising, producing a viable temperature record spanning two glacial/interglacial cycles, with glacial temperatures near present day average winter and interglacial temperatures near present day average summer temperatures. In continuing research, however, the original soil-based MBT/CBT proxy calibration was shown generally to predict low temperatures when applied to reconstructions in lacustrine sediments. While the bacteria synthesizing brGDGTs remain unknown (but potentially attributed to the highly diverse phylum Acidobacteria), much evidence points to the potential for these bacteria to live not only in soils but also in sediments and possibly lake waters as well. As a result, several additional lacustrine calibrations have been published in an attempt to produce more realistic temperature reconstructions. Here, we present data from a suite of lakes in Mexico and Guatemala, including a core-top calibration with present day annual mean air temperatures as well as down-core reconstructions of temperature from multiple lakes using both our new calibration and those published to date by other groups. Intriguingly, the various calibrations produce quite different results, even sometimes producing reasonable values in one lake and ridiculous temperatures in another, clearly indicating a significant role for regional differences in controlling the brGDGT distribution. This is likely due to different brGDGT-producing microbial communities thriving under varying environmental conditions. We do observe significant differences in the distribution of brGDGTs in these Mexican lakes compared with other systems, and explore their relationships between brGDGT distributions and environmental conditions in our Mexican and Guatemalan lakes compared with other systems.