Sedimentary Branched Tetraethers in an African Lake Record 170 KYR of Tropical Temperature Change: Assessment of Calibrations

Abstract

Summary In order to improve our understanding of Earth’s climate history, proxies that accurately reconstruct past temperatures are needed. The distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs), the membrane lipids of certain bacteria, is correlated to temperature in modern settings, and therefore is at the basis of several paleothermometers. Here we apply several brGDGT based temperature calibrations developed specifically for lakes to the sediment sequence from Lake Chala, in equatorial East African, to generate temperature records which cover the last 170 kyr in unprecedented high temporal resolution (210 years on average). Surprisingly, application of the recently developed East African lake calibration leads to ambiguous results. By contrast applying a calibration created before the discovery of 5- and 6-Me brGDGT isomers produces a temperature record strongly reminiscent of established climate records such as the isotope records from Antarctica and contains periodicities relating to orbital precession and obliquity. Therefore it appears that 6-Me brGDGTs are key to the temperature signal archived in Lake Chala. In Lake Chala today, 5- and 6-Me brGDGTs generally occupy different parts of the water column, and thus their relative abundance may be indirectly related to temperature through changes in lake depth or seasonal mixing.