Concomitant changes of lipid biomarker and water column mixing since mid-Holocene

Abstract

An increasing observation of in-situ production of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in marine environments has aroused concerns about the utility of BIT (the ratio of brGDGTs over isoGDGTs) index to indicate soil contribution in marine environment. In this study, GDGT biomarkers and foraminiferal carbon isotope (δ13C) are analyzed in a mid-to-late Holocene sediment core from the northern South China Sea (SCS). The BIT values exhibited an overall similar pattern as compared to Δδ13C(P-G) (the δ13C difference between subsurface and surface planktonic foraminifera) index that indicates water column productivity, implying that the variations in BIT are likely related to marine productivity status. A positive relation between BIT and Δδ13C(P-G) was observed and explained by the downward transport of organic matter due to change of upper water mixing, with higher BIT Δδ13C(P-G) values during strengthened upper water mixing. Despite these findings, further investigations on the mechanistic processes that regulate the productions and dynamics of brGDGTs and isoGDGTs in marine sediments are warranted.