A heterocyte glycolipid-based calibration to reconstruct past continental climate change.

Understanding Earth's response to climate forcing in the geological past is essential to reliably predict future climate change. The reconstruction of continental climates, however, is hampered by the scarcity of universally applicable temperature proxies. Here, we show that heterocyte glycolipids (HGs) of diazotrophic heterocytous cyanobacteria occur ubiquitously in equatorial East African lakes as well as polar to tropical freshwater environments. The relative abundance of HG diols and keto-ols, quantified by the heterocyte diol index (HDI), is significantly correlated with surface water temperature (SWT). The first application of the HDI to a ~37,000 year-long sediment record from Lake Tanganyika provides evidence for a ~4.1 °C warming in tropical East Africa from the last glacial to the beginning of the industrial period. Given the worldwide distribution of HGs in lake sediments, the HDI may allow reconstructing SWT variations in polar to tropical freshwater environments and thereby quantifying past continental climate change.