Origin of hydroxyl GDGTs and regular isoprenoid GDGTs in suspended particulate matter of Yangtze River Estuary

Abstract

Hydroxylated isoprenoid glycerol dialkyl glycerol tetraethers (OH-GDGTs) were found having the potential to track past sea surface temperatures (SST), in analogy to the TEX86 – a paleothermometer based on isoprenoid GDGTs (iGDGTs). Especially in estuarine regions with high terrestrial input, the ring index of OH-GDGTs (RI-OH) was more robust to trace the summer SST than TEX86. In order to clarify the potential sources of sedimentary GDGTs and to further elucidate the potential of OH-GDGTs as SST proxy, we investigated the seasonal distribution of core OH-GDGTs and iGDGTs in the suspended particulate matter in a transition section in the Yangtze River Estuary. The concentrations of OH-GDGTs and iGDGTs were higher in the estuary than those in the lower Yangtze River, which suggested the OH-GDGTs and iGDGTs in estuarine sediment mainly came from marine autochthonous organisms, whereas the terrestrial contribution was relatively low. The higher content of OH-GDGTs and iGDGTs in summer than in winter indicated that sedimentary OH-GDGTs and iGDGTs mainly originated from the summer SPM deposition. In addition, the ratio of OH-GDGTs versus iGDGTs suggested that OH-GDGTs were relatively abundant in the upper water layer compared to iGDGTs, regardless of season. Furthermore, the distributions of iGDGTs and OH-GDGTs were found to be influenced by hydrodynamics. The increasing warm bias of iGDGT and OH-GDGT reconstructed temperatures with water depth suggested that either GDGTs in deeper waters derive mainly from surface water or GDGTs cyclization in deeper waters was influenced by reduced ammonium oxidation rates in response to the limited supply of ammonium and oxygen, especially in summer.