Origin and preservation of archaeal intact polar tetraether lipids in deeply buried sediments from the South China Sea

Intact polar lipids-glycerol dibiphytanyl glycerol tetraethers (IPL-GDGTs) are assumed to be degraded to core lipids (CL) upon cell death, which thus can serve as markers for live archaea in marine deep biosphere. However, the degradation models of sedimentary IPL-GDGTs suggested that they are mainly fossil compounds and can be preserved over geological timescales. Here we investigated the CL- and IPL-GDGTs from deeply buried sediments (0.1-485 mbsf, similar to 7.3 Ma) in the South China Sea (SCS). The depth profiles of IPL-GDGT concentrations paralleled those of CL-GDGTs. The sea surface temperatures (SST) derived from CL- and IPL-TEX 86 ranged 23.1-28.8 degrees C and 22-33.3 degrees C, respectively. They are close to the SST in the SCS, suggesting that CL- and IPL-GDGTs mostly originate from pelagic archaea. The composition and distribution of the IPL-GDGTs differed among the polar headgroups. Hexose-phosphohexose (HPH)-GDGTs were dominated by GDGT-0, monohexose (1G)-GDGTs were mainly composed of GDGT-0 and crenarchaeol, and dihexose (2G)-GDGTs consist primarily of GDGT-2 and crenarchaeol isomer. The compositions of HPH-, 1G- and 2G-GDGTs are similar to those in previously studied water column samples, supporting that sedimentary IPL-GDGTs predominantly derive from the water column. HPH-GDGT abundances fast declined to be undetectable at similar to 31 mbsf (similar to 0.6 Ma) while 1G- and 2G-GDGTs dominated the deeply buried sediments, indicating that GDGTs with glycosidic headgroups are better preserved versus GDGTs with phosphorous headgroups over geological timescales. Our results demonstrate that IPL-GDGTs, especially glycosidic GDGTs, are not suitable as biomarkers for live archaea in the deep biosphere