Assessing Water Depth Effects on Archaeal Lipids and Refining the TEX86 Proxy: Insights from Hadal Trenches to a Global Scale

The TEX₈₆ proxy, based on archaeal isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs), is among the most widely used tools for reconstructing past sea surface temperatures (SST). However, the impact of water depth on isoGDGT distributions and the reliability of TEX₈₆ remains unclear. To address this, sediments from five hadal trench regions and a comprehensive global dataset were analyzed. The findings reveal that as water depth increases, the fractional abundance of low-ring-number isoGDGTs (e.g., GDGT-1 and GDGT-2) increases, while that of high-ring-number isoGDGTs (e.g., GDGT-3 and crenarchaeol) decreases. Notably, the crenarchaeol regio-isomer, despite its high-ring structure, display an unusually higher fractional abundance in deep-sea sediments, suggesting a distinct source or history compared to other isoGDGTs. Despite the influence of water depth, isoGDGT compositions and TEX₈₆ maintain strong correlations with SSTs across varying depths, including the deepest oceanic regions, indicating the dominance of shallow-water-derived signals in sedimentary isoGDGT pools. The influence of water depth on TEX₈₆ is even more minimal, likely due to compensatory changes in GDGT-2 and GDGT-3 and the proxy’s inherent ratio structure. Remarkably, TEX₈₆ provides more reliable SST estimations in deep-sea sediments compared to coastal sediments. By excluding the crenarchaeol regio-isomer from TEX₈₆, the resulting proxy demonstrates a stronger correlation with SSTs, reduced sensitivity to water depth, and improved performance in polar regions. Consequently, newly TEX₈₆-SST calibration models were established to enhance SST estimations globally and across regions with varying water depths.