Disentangling Effects of Sea Surface Temperature and Water Depth on Hydroxylated Isoprenoid GDGTs: Insights From the Hadal Zone and Global Sediments

Hydroxylated isoprenoid glycerol dialkyl glycerol tetraethers (OH‐GDGTs) preserved in marine sediments are thought to be controlled by sea surface temperature (SST). However, water depth may also exert a significant influence on OH‐GDGTs. Here, we investigated sedimentary OH‐GDGTs in the Kermadec and Atacama trench regions (2,560–9,560 m water depth). Sedimentary OH‐GDGTs in hadal trenches were dominated by OH‐GDGT‐0 (72 ± 8%), potentially reflecting an adaption of source organisms to ambient cold deep water. This result, combined with global data set, revealed that the predominance of OH‐GDGT‐0 is a ubiquitous phenomenon in deep‐sea sediments, leading to a considerable underestimation of RI‐OH′‐derived SSTs. By considering both SST and water depth effects, we developed more accurate OH‐GDGT‐based paleothermometers for both shallow regions and the global ocean, encompassing the full‐ocean‐depth range. Our findings highlight the importance of accounting for the effect of water depth on OH‐GDGTs and provide improved tools for reconstructing paleo‐SSTs. Plain Language Summary Building quantitative proxies that can accurately estimate SSTs is one of the most common themes in paleoceanography. Archeal‐derived hydroxylated isoprenoid glycerol dialkyl glycerol tetraethers (OH‐GDGTs) preserved in marine sediments have a potential to reflect past sea surface temperatures (SSTs). However, the source organisms of OH‐GDGTs can live throughout the water column, implying that sedimentary OH‐GDGTs record an integrated water column signal rather than only SST. Currently, the effect of water depth on sedimentary OH‐GDGTs remains vague. We investigated the distribution of OH‐GDGTs in 13 sediment cores in the hadal zone, which represents the deepest and least explored habitats on Earth's surface. The data from this study and literature revealed that the deep‐sea (including hadal trench) sediments are characterized by a predominance of OH‐GDGT‐0, which causes a considerable underestimation of OH‐GDGT‐derived SSTs. We evaluated the effects of both SST and water depth on OH‐GDGTs and established new calibrations that more accurately reconstruct paleotemperatures at a global scale. Key Points Sedimentary hydroxylated isoprenoid glycerol dialkyl glycerol tetraethers (OH‐GDGTs) were first investigated in the Kermadec and Atacama trenches with water depth ranging from 2,560 to 9,560 m Water depth has large impact on OH‐GDGT distributions with higher OH‐GDGT‐0% and lower RI‐OH′ valu