Characteristics and Implications of Isoprenoid and Hydroxy Tetraether Lipids in Hadal Sediments of Mariana and Yap Trenches

As one of the least-explored regions on Earth, the hadal zone remains poorly understood in terms of sedimentary organic geochemistry. This study examined the composition of isoprenoid glycerol dialkyl glycerol tetraethers (Iso-GDGTs) and hydroxy GDGTs (OH-GDGTs) in sediment cores with water depths ranging from 5058 to 10,911 m from the Mariana and Yap Trenches. The results showed a higher proportion of GDGT-0 (25–73%, mean = 49%) in the trench Iso-GDGTs pool than those from shallower ocean counterparts (<5000 m). Properties, including higher values of the [GDGT-2/GDGT-3] ratio (7.17–20.03, mean = 13.02) and the [GDGT-0/Crenarchaeol] ratio (0.49–3.94, mean = 1.48), as well as a dominance of OH-GDGT-0 in the OH-GDGTs, are similar to characteristics observed in other cold deep marine environments rather than tropical shallow ocean. Ring Index decreased with increasing water depth of the sampling sites, corresponding with increased fractional abundance of acyclic Iso-GDGTs and OH-GDGTs. Paleothermometer TEX86 in the surface sediments (>5000 m) revealed a significant negative linear correlation with water depth (TEX86H: R2 = 0.89, p < 0.01; TEX86L: R2 = 0.89, p < 0.01), leading to a bias toward lower proxy-derived temperatures. This is likely due to the increased incorporation of cold- and pressure- adapted species of archaea in dark ocean. Overall, this study provided implications of the extreme water depth effect on the distribution of isoprenoid and hydroxy tetraether lipids, as well as the proxy application in hadal environments.