Cyclisation degree of tetramethylated brGDGTs in marine environments and its implication for source identification

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are ubiquitous in marine environments. Nevertheless, it is unclear whether these brGDGTs are derived from terrestrial soils or produced in situ. Recently, it was proposed that the weighted average number of cyclopentane moieties of tetramethylated brGDGTs, #Rings(tetra), could be used to identify sources of sedimentary brGDGTs in marine environments. However, little is known about #Rings(tetra) in seawater. In this study, we report #Rings(tetra) data in suspended particulate materials (SPM) in the surface shelf waters of the East China Sea (ECS) and coastal South China Sea (SCS), as well as in core-top sediments of the deep SCS. According to a two-end member model, #Rings(tetra) values in SPM increase offshore from 0.48 to 0.86 in the ECS and from 0.39 to 0.83 in the SCS, suggesting increasing marine-produced brGDGTs. An offshore increasing trend of #Rings(tetra) is also observed in the shelf surface sediments of the ECS and SCS, suggesting increased contributions of marine-produced brGDGTs. However, the offshore increasing trend in core-top sediments of the SCS ceases at a water depth ca. 100 m, with constantly low #Rings(tetra) values (0.3 +/- 0.1) at sites deeper than 780 m and a transition value of 0.58 at 329 m. Instead of interpreting the low #Rings(tetra) values in deep-sea sediments as the result of increased contributions of terrestrial-derived brGDGTs, we propose that brGDGTs in deep-sea sediments are produced in situ. The lower pH in the deep-sea bottom water and sediment porewater than in the seawater column could be the reason for the low #Rings(tetra) values. We further propose that pH reconstructions of deep-sea sediment cores (e.g., >1000 m water depth) with the least terrestrial influence should reflect pH changes in marine bottom environments.