Source and distribution of glycerol dialkyl glycerol tetraethers along lower Yellow River-estuary–coast transect

Abstract

To assess the source of glycerol dialkyl glycerol tetraethers (GDGTs) and their usefulness as proxies for terrestrial organic matter inputs and temperature in the Yellow River-dominated margin, we measured isoprenoid and branched GDGT concentrations in surface sediments along a lower Yellow River-estuary–coast transect. Branched GDGTs dominated over isoprenoid GDGTs in the riverbed sediments and had similar compositions from river to coast. In contrast, isoprenoid GDGTs displayed an increasing abundance and a decreasing GDGT-0 to crenarchaeol ratio (1.6 to 0.6) toward the sea. Such distribution patterns of GDGTs, combined with the result from a principal component analysis (PCA), confirmed the different origin of branched and isoprenoid GDGTs with branched GDGTs being primarily from soil erosion of the Chinese loess plateau (CLP) whereas, in addition to allochthonous terrestrial inputs, aquatic Thaumarchaeota partially contributes to the isoprenoid GDGT pool in estuarine and coastal areas. The branched GDGT-derived temperature (avg. 11°C) is consistent with the annual mean air temperature (MAT) of the CLP in the middle river basin, a major source region for the Yellow River sediments, whereas the isoprenoid-derived temperature (12.7 to 28.4°C) deviated widely from the annual mean temperature in the study region. Application of a binary mixing model based on δ13C, the branched and isoprenoid tetraether (BIT) index and branched GDGT concentrations showed consistent decreases in the relative amount of terrestrial organic carbon toward the sea, but estimates from the latter two proxies were lower than those from the δ13C.