Source, composition, and distributional pattern of branched tetraethers in sediments of northern Chinese marginal seas

Abstract

Branched glycerol dialkyl glycerol tetraethers (brGDGTs), biomarkers for organic carbon source and paleoenvironment, can be produced in both land and sea. The source assignment is prerequisite for the accurate application of brGDGTs-based indicators. Here, we reported brGDGTs in surface sediments of northern Chinese Marginal Seas (CMS), including the Bohai Sea, Yellow Sea and East China Sea. Two source indicators, ∑IIIa/∑IIa and #ringstetra, have an average of 0.86 ± 0.18 and 0.74 ± 0.08 in the Bohai Sea, 0.61 ± 0.34 and 0.73 ± 0.16 in the East China Sea, and 1.32 ± 0.45 and 0.87 ± 0.12 in the Yellow Sea, suggesting variable contributions of terrestrial and marine-derived brGDGTs in the Bohai Sea and East China Sea, but the predominance of marine in-situ brGDGTs in the Yellow Sea. The distribution of brGDGTs presents a zonal pattern from nearshore to offshore in the Bohai Sea/East China Sea, but a complex and patched pattern in the Yellow Sea. This difference is attributed to different influences of terrestrial brGDGTs from the Yangtze River and Yellow River. However, the branched and isoprenoid tetraether (BIT) index, an indicator for terrestrial organic carbon in marine environments, has low values (an average of <0.1) in all three marginal seas, demonstrating the limitation of the BIT index in high productive marginal seas. Of those samples containing pure marine derived brGDGTs (∑IIIa/∑IIa ≥ 0.92), significant correlations between ∑IIIa/∑IIa and CBT5ME and between water temperature and MBT′5ME suggest that the production of brGDGTs by marine bacteria is also affected by ambient pH and temperature, although a global calibration is still lacking.