Sources and implications of hydroxylated isoprenoid GDGTs on the northwest shelf of Australia through the Pliocene-Pleistocene era

Hydroxylated glycerol dialkyl glycerol tetraethers (OH-GDGTs) have been recently recognized as alternative biomarkers for sea surface temperature (SST) estimations in oceanic environments. Yet, it is still unclear whether OH-GDGTs are applicable on the northwest shelf of Australia. Here we present results of OH-GDGTs from sedimentary cores at IODP Site U1461 covering the Pliocene-Pleistocene period. We discussed potential sources and applications for OH-GDGTs on the northwest shelf of Australia by comparison with previously reconstructed records of surface and subsurface temperatures, terrigenous inputs, water depth changes, and phytoplankton composition. The historical variations in concentrations and distribution patterns of OH-GDGTs suggest that besides oceanic Thaumarchaea production, OH-GDGTs could be also contributed from terrigenous input to some extent. The relative abundance of OH-GDGTs over isoprenoid GDGTs (%OH-GDGTs) might track the variation in water temperature of subsurface layers or the whole water column as the isoGDGT-based TEX86 proxy does, while the ring indices of OH-GDGTs (RI-OH and RI-OH') have some difficulties in tracking temperature variation on the northwest shelf of Australia through the whole Pliocene-Pleistocene period. However, when the samples were separated into two sections bounded at -1.3 Ma, the RI-OH and RI-OH' show strong positive correlations with TEX86 values. Therefore, at -1.3 Ma, -1000 m lowering of water depth as well as the exposure of continental paleo-soils/sediments that previously formed in warmer periods potentially induced additional OHGDGT-2 contributions to oceanic OH-GDGTs, eventually resulting in relatively higher RI-OH, RI-OH' and % OH-GDGTs values on the northwest shelf of Australia in the mid- and late Pleistocene. If source of terrigenous OH-GDGTs and water depth are relatively stable, these OH-GDGT-based proxies are capable of tracking water temperature of subsurface layers or whole water column.