Impact of water depth on the distributions and proxies of isoprenoidal hydroxylated GDGTs in the Mediterranean Sea and the Red Sea

Hydroxylated Isoprenoidal Glycerol Dialkyl Glycerol Tetraethers (OH-isoGDGTs) have recently been utilized as paleothermometers in the marine environment. However, their ability to reconstruct temperature in the Mediterranean and Red Sea has not been adequately investigated. Previous research has shown that archaeal communities inhabiting different water depths in these basins exert a substantial influence on the regular isoGDGT distributions and associated proxies such as the ${TEX}_{86}$. However, the impact of these archaea on OH-isoGDGTs and their corresponding proxies remains unclear. In this study, we examined the distribution of OH-isoGDGTs and their associated proxies ($\%OH$, RI-OH, RI-OH′ and ${TEX}_{86}^{OH}$) in surface sediments of the Mediterranean and the Red Sea. We observe strong correlations between the fractional abundances of OH-isoGDGTs, relative to all isoGDGTs and OH-isoGDGTs, and water depth which suggests that deep-water archaeal communities have a lower OH-isoGDGT abundance compared to the shallow communities. As a result, $\%OH$ and ${TEX}_{86}^{OH}$ are strongly correlated with water depth, particularly at depths <500 m in the Mediterranean Sea. Interestingly, RI-OH and RI-OH′ show no correlation with water depth in the Mediterranean Sea. Instead, they correlate more strongly with satellite-derived sea surface temperature compared to other isoGDGT-based proxies, indicating their potential as paleothermometers. Finally, unlike ${TEX}_{86}$ and ${TEX}_{86}^{OH}$, the $\%OH$, RI-OH and RI-OH′ do not exhibit distinct 'Red Sea cluster' and display comparable values to sediments from other tropical oceans. Further research on sedimentary OH-isoGDGT distributions with broader geographical coverage within these basins and enrichment cultures of deep-water archaea are needed to confirm these observations.