oa Calibration of Bacterial 3-Hydroxy Fatty Acid-Based Paleoclimate Proxies in Global Soils, Marine Sediments and Lakes

3-Hydroxy fatty acids (3-OH-FAs), derived from Gram-negative bacterial outer membranes, have received recent attention for their potential as both: a) terrestrial temperature and pH palaeoclimate proxies; and b) marine sea-surface temperature proxies. However, the distribution and performance of 3-OH-FA based proxies at a global scale and in lake sediments remains largely unknown. This is a critical gap in knowledge as lake sediments are the most common terrestrial archive for paleoclimate. Here we explore (including the application of machine learning tools) 3-OH-FA lipid distributions in 186 globally distributed soils. We also investigate co-located lake sediments and catchment soils in China and the USA. We find that the ratio of anteiso to normal 3-OH-FAs of the C15 or C17 homologues (RAN15 or RAN17) shows a significant linear relationship with mean annual air temperature (MAAT) in global soils. However, in lakes these do not appear to be suitable for temperature calibration. Analysis of gene community composition reveals that Gram-negative bacterial populations from lake sediments are distinct from both soils and marine sediments (but are closer to the former). We find strong relationships between alternative 3-OH-FA based proxies and MAAT in lakes. Thus 3-OH-FA geochemical tools continue to show promise for palaeoclimate reconstruction.