The influence of near-surface sediment hydrothermalism on the TEX₈₆ tetraether-lipid-based proxy and a new correction for ocean bottom lipid overprinting

Abstract

The diversity and relative abundances of tetraether lipids produced by archaea and bacteria in soils and sediments are increasingly used to assess environmental change. For instance, the TetraEther indeX of 86 carbon atoms (TEX86), based on archaeal isoprenoidal glycerol dialkyl glycerol tetraether (iGDGT) lipids, is frequently applied to reconstruct past sea-surface temperatures (SSTs). Yet, it is unknown how the ratio fully responds to environmental and/or geochemical variations and if the produced signals are largely the adaptive response by Thaumarchaeota to oceanographic effects associated with climate or seasonal temperature changes in the upper water column. We present the results of a four push-core transect study of surface sediments collected along an environmental gradient at the Cathedral Hill hydrothermal-vent system in Guaymas Basin, Gulf of California. The transect crosses a region where advecting hydrothermal fluids reach 155 ∘C within the upper 21 cm below the seafloor (cm b.s.f.) close to the vent center to near-ambient conditions at the vent periphery. The recovered iGDGTs closest to the vent center experienced high rates of turnover with up to 94 % of the lipid pool being lost within the upper 21 cm b.s.f. Here, we show that the turnover is non-selective across TEX86 GDGT lipids and does not affect the ratio independently. However, as evident by TEX86 ratios being highly correlated to the Cathedral Hill vent sediment porewater temperatures (R2=0.84), the ratio can be strongly impacted by the combination of severe lipid loss coupled with the addition of in situ iGDGT production from archaeal communities living in the vent sediments. The resulting overprint produces absolute temperature offsets of up to 4 ∘C based on the TEX86H calibration relative to modern climate records of the region. The overprint is also striking given the flux of iGDGTs from the upper water column is estimated to be ∼ 93 % of the combined intact polar lipid (IPL) and core GDGT lipid pool initially deposited on the seafloor. A model to correct the overprint signal using IPLs is therefore presented that can similarly be applied to all near-surface marine sediment systems where calibration models or climate reconstructions are made based on the TEX86 measure.