Global scale production of brGDGTs by benthic marine bacteria: Implication for developing ocean bottom environmental proxies

Abstract

The in-situ production of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in marine environments has been generally accepted. However, whether it takes place in the water column or the sediments remains elusive. Here, we compiled a dataset composed of globally distributed marine sediments from continental shelves to hadal trenches to address this important knowledge gap. Firstly, a cutoff of ΣIIIa/ΣIIa ≥0.92 was applied to distinguish the marine vs. terrestrial signals of brGDGT production. For marine derived brGDGTs, the cyclization of branched tetraethers (CBT) index, related to ambient pH, displays a large variability, suggesting that marine derived brGDGTs in sediments are mainly produced within sediments where the pH of sediment porewater is highly variable, rather than in the water column where pH is relatively constant. This is also supported by good correlations between the bottom water temperature and the methylation of branched tetraethers (MBT’) index, between the water depth and the ΣIIIa/ΣIIa index, as well as between the brGDGT-based proxies and sediment redox conditions during glacial/interglacial cycles. We thus propose that the benthic production of brGDGTs in marine sediments is ubiquitous on a global scale, and is largely affected by the ambient temperature and pH. Our findings support that brGDGT-based proxies have potential of recording environmental conditions of the ocean bottom.