Archaeal lipid diversity, alteration, and preservation at the Cathedral Hill deep sea hydrothermal vent, Guaymas Basin, Gulf of California, and its implications regarding the deep time preservation paradox

Abstract

Archaea are among the earliest evolved organisms. Today, they exist in nearly every habitable environment and their highly recalcitrant membrane lipids are both abundant and easily detected in modern sediments. Yet, unlike bacteria and eukaryotes, the lipid biomarker signatures of archaea are almost entirely absent in the ancient rock record (>145 Ma). We present a comprehensive study of archaeal lipids from the Cathedral Hill hydrothermal vent complex in Guaymas Basin, Gulf of California. Here, porewaters reach 155 °C by 21 cm below the sea floor (cmbsf), which enables the near-complete tracking of sedimentary organic matter (SOM) accumulation, diagenetic alteration and thermochemical transformation into petroleum-like hydrocarbons within and beyond the habitable zone of life. Identified intact polar lipids (IPLs) of living archaea included mono- and di-glycosidic archaeols (1G-AR, 2G-AR) and glycerol dialkyl glycerol tetraethers (1G- and 2G-GDGTs). The 1G- and 2G-ARs, potentially derive from methane-oxidizing archaeal groups ANME-1 and ANME-2 and anaerobic thermophilic methanogens, reach sediment depths of ∼ 135 cm and 50 °C temperatures. The 1G-and 2G-GDGT lipids reach a depth of ~180 cm where vent fluid temperatures are ~145 ± 15 °C. Core lipids (CLs) include archaeols (AR) and isoprenoidal and branched GDGTs (iGDGTs, brGDGTs). The core GDGTs (cGDGTs) closest to the vent center have high rates of lipid loss. Up to 95% of all identified archaeal biomarkers, including lipids supplied from the upper water column, are degraded or recycled in the surface sediments and do not reach conditions of late diagenesis and catagenesis. Only ∼ 0.11%, proportional to the lipid bound component in the protokerogen, marks thermochemically cracked biphytane – a prominent archaeal hydrocarbon biomarker. This extreme reduction indicates archaeal lipids do not necessarily become incorporated into kerogen thereby helping to explain the deep time archaeal lipid preservation paradox.