Sedimentary core brGDGTs in the East China Sea are mainly produced in situ as evidenced by their similar distributions with brGDGTs derived from intact polar lipids

Abstract The provenance of brGDGTs in marine sediments was initially thought to be from terrestrial soils, and hence brGDGTs-based proxies have been used for reconstructing terrestrial environments. However, this notion has been challenged by the finding that the distribution of brGDGTs in marine sediments can differ from global surface soils. To constrain the sources of brGDGTs in marine sediments in the East China Sea (ECS), we analyzed brGDGTs occurring as both ‘fossil’ core and in ‘living’ intact polar lipids (core brGDGTs and IPL-derived brGDGTs, respectively). The results showed that the distribution of core brGDGTs was similar to IPL-derived brGDGTs but differed clearly from those in the global soils database. In addition, for both IPL-derived and core brGDGTs, the methylation index, MBT'5ME, decreased with increasing latitude, likely caused by northward decreases of bottom water temperature. The degree of cyclization of tetra-methylated brGDGTs, #Ringstetra, increased with water depth, with highest values > 0.7, likely reflecting an increase in seawater pH. Our results indicate that core brGDGTs may be predominantly produced in situ and that the soil contribution is minimal in this setting. Nonetheless, differences in MBT'5ME and #Ringstetra between IPL-derived and core brGDGTs exist, showing increasing offsets offshore. These offsets could be due to decreased sedimentation rates and hence increased age differences between IPL-derived and core brGDGTs offshore, which likely reflects changes in temperature and pH of seawater, most likely the bottom waters, during the past decades.