Intact polar glycosidic GDGTs in sediments settle from water column as evidenced from downcore sediment records

Abstract

The origins of intact polar lipid glycerol dialkyl glycerol tetraethers (IPL-GDGTs) with glycosidic headgroups that are present in deep-sea sediments are still unclear. Here, we tried to clarify this issue by comparing variations in TEX86 temperatures and compositions of IPL (dihexose (DH) and monohexose (MH) head groups) and core lipid (CL) GDGTs in a sediment core covering a period since the last glacial time in the northern South China Sea (SCS). Haptophyte-derived alkenones were also analyzed to provide an independent paleo-sea surface temperature (SST) record. The sedimentary DH-GDGTs were composed mainly of DH-GDGT-1, -2, -3, and the crenarchaeol isomer. In contrast, both the sedimentary MH- and CL-GDGTs were composed mainly of crenarchaeol and GDGT-0. However, in each pool of GDGTs, crenarchaeol correlated significantly with other GDGT compounds, suggesting a shared origin. The DH-TEX86-derived temperature values were systematically higher than, but well correlated with CL-TEX86 and UK'37-derived SST values, showing an expected glacial-interglacial increase. This finding suggests that the DH-GDGTs could have originated mainly from the upper water column and may have been well preserved in sediments for a long time. However, the MH-TEX86-derived temperature did not display such a climatic pattern but exhibited unexpected higher values during the last glacial period. We propose that the elevated glacial MH-TEX86-derived temperature was likely associated with the ammonia limitation and subsequently reduced ammonia oxidation rate in seawater during the last glacial period. This study indicates that the sedimentary glycosidic GDGTs could settle mainly from the water column rather than being produced in situ.