Archaeal Tetraether Lipids as Tracers for Past Marine Environmental Change

Archaea are single-celled microorganisms that are abundant in marine environments and play a key role in the carbon and nitrogen cycles. Archaea can biosynthesize a wide variety of isoprenoid membrane lipids, including isoprenoid glycerol dibiphytanyl glycerol tetraethers (isoGDGTs). Experimental and empirical evidence demonstrates that the number of cyclopentane moieties in isoGDGTs is related to growth temperature, leading to the formulation and development of the TEX₈₆ ocean temperature proxy. The TEX₈₆ proxy has been widely applied to marine sediments spanning the last 190 million years and has revealed unique insights into the evolution of ocean temperatures over diverse timescales. Other indices have been developed using archaeal lipids to reconstruct ecological and biogeochemical processes that operate within the marine realm. However, several knowledge gaps (e.g., depth of export production, non-temperature controls, application in non-analogue climates) limit the application of archaeal lipid-based proxies in modern and ancient marine environments. In this review, we critically assess the utility of archaeal lipids as tracers for (paleo) environmental change, with an emphasis on the TEX₈₆ paleotemperature proxy.