Lipid biomarkers for the reconstruction of deep-time environmental conditions

Lipids can survive long geological intervals within sediments and provide a unique tool that allows the reconstruction of past organismic diversity and environmental conditions. The lipids discussed in this review include all substances produced by organisms that are insoluble in water but extractable by organic solvents. Lipid biomarkers refer to both functionalized biolipids as well as their hydrocarbon derivatives in geological materials that contain diverse information about biotic sources and environmental conditions. In spite of diagenetic and catagenetic alteration, lipid biomarkers commonly preserve the hydrocarbon structure of their biotic counterparts and have been found in rocks up to 1.6 billion years in age. These features have promoted the use of lipid biomarkers in many fields, including petroleum geology, paleoclimatology, oceanography, meteorology, geobiology and environmental science. Here, we (i) review the use of lipid biomarker records for the reconstruction of environmental conditions in deep time, including climatic conditions (temperature), sedimentary environments (redox, salinity and chemical composition) as well as catastrophic terrestrial events (soil erosion and wildfire), and (ii) generate new insights into environmental perturbations during the Permian-Triassic transition based on investigation of lipid biomarkers. We further propose that the ratio of dibenzothiophene to phenanthrene (DBT/P) in marine carbonates may be a robust proxy for seawater sulfate concentrations in deep time. Our compiled DBT/P records show substantial variations in seawater sulfate levels through Earth history that are consistent with the results of other proxies. We discuss the future outlook for application of lipid biomarker records to deep-time environmental research