Influence of hydrothermal vent activity on GDGT pool in marine sediments might be less than previously thought

Abstract

Hydrothermal systems have a pronounced effect on deep-sea chemistry and some plumes are reported to occupy large parts of ocean volume. However, the impact of these systems on the marine organic carbon cycle is not well understood. Here, we compare the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) in several short (0–23 cm) and one long (304 cm) sediment core of hydrothermal vent sites at the Central (CIR) and Southeast Indian Ridges (SEIR) and to those of nearby normal marine sediment as reference, to characterize the impact of hydrothermal activity on deep sea sediment organic matter. We detected a range of GDGTs in some of the sediments. BrGDGT and GMGTs were absent at the normal marine reference location, but present in some of the hydrothermal sediments with relatively high concentrations of brGMGT-IIIa and brGDGT-Ia, hinting at in situ hydrothermal production. The isoGDGT distributions were similar between the hydrothermal and normal marine sites, with predominance of GDGT-0 and crenarchaeol. isoGDGT-0/crenarchaeol ratios (G-0/Cren) < 2 suggested Thaumarchaeota as the major source of GDGTs in these sediments. Consistent with a dominant thaumarchaeotal source, TEX86 in the surface samples reflected modern sea surface temperature (SST) at the core locations. Increasing trends of δ13Corg values from surface to bottom sediments were observed only in hydrothermal influenced sediments and not at the reference station. Altogether these results indicate a minor impact of hydrothermal activity on the GDGT distribution in this region, urging for caution in up-scaling previous results which proposed a significant impact of hydrothermal vents system on nearby open ocean marine sediments.