Biogeochemical controls on glycerol dialkyl glycerol tetraether lipid distributions in sediments characterized by diffusive methane flux

Abstract

The TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms) is a proxy for sea surface temperature (SST) based on the distribution of isoprenoidal glycerol dialkyl glycerol tetraether (GDGT) membrane lipids synthesized by marine pelagic Thaumarchaeota. One of the caveats of this proxy is the production of additional GDGTs by sedimentary Euryarchaeota involved in anaerobic oxidation of methane (AOM) that occurs at deep-sea methane seeps but is also widespread in many continental shelf settings. Here, GDGT distributions are investigated through the sulfate-methane transition zone (SMTZ) in Aarhus Bay, Denmark, to examine the extent the TEX86 proxy is compromised in such a continental shelf setting where AOM is characterized by a diffusive rather than rapid advective methane flux. Both free extractable and non-extractable lipid fractions were analyzed as it was expected that pelagic-derived and sediment-derived GDGTs could become incorporated into the molecular and sedimentary matrix to a different extent. The results show a large change of TEX86 values mainly due to the relative high amounts of GDGT-2 produced by AOM-related Archaea, both in the free and non-extractable lipid fractions. This additional GDGT input renders calculation of SSTs based on TEX86 inappropriate at the SMTZ. The AOM-related GDGT signature, however, did not persist into deeper sediments, perhaps reflecting rapid remineralization of the GDGTs at the SMTZ. Although the process of AOM at Aarhus Bay might not be representative for all continental margin settings, it illustrates that AOM in a variety of settings, not just cold seeps, can influence sedimentary GDGT distributions.