Contrasting sources and fates of sedimentary organic carbon in subtropical estuary-marginal sea systems

Abstract

Sediments deposited in subtropical deltas and continental shelves are the “recorders” of sources, transport processes, and accumulation of organic carbon (OC) at the land-ocean interface, but their role in the global carbon cycle remains poorly constrained. In this study, we determined the abundance and carbon isotopic composition (δ13C and Δ14C) of bulk OC, as well as characteristics of specific biomarker compounds (plant wax n-alkanes, n-fatty acids, and phytoplankton sterols) in surface sediments from the Pearl River estuary (PRE) and adjacent continental shelf of the northern South China Sea (SCS) in order to examine the sources and fates of OC delivered to a large subtropical river-marginal sea system. We observed marked spatial variability in the sources, hydrodynamic processes and accumulation of sedimentary OC in PRE and northern SCS shelf, with the emergence of two distinct regimes reflected in the bulk OC isotopic characteristics of surface sediments. In Pearl River impacted regions, relatively fresh terrestrial OC (OCterr) was subject to substantial degradation and aging during transport within the estuary and from the estuary to the shelf. In contrast, sedimentary OC in the Taiwanese river impacted region comprised a mixture of fluvially-derived refractory OCterr from the Choshui River and modern marine OC. Burial efficiencies of OCterr are also spatially heterogeneous as a result of these different OC sources and hydrodynamic processes. The overall OCterr burial efficiencies are relatively low in the Pearl River influenced region, with an average value of 27 ± 10% in the PRE and an average value of 17 ± 6% in the western shelf, in contrast to the higher values in the region impacted by Taiwanese rivers (55% ± 39%).These findings suggest both sources and hydrodynamic processes during transport exert strong influence on the fate of OC, with important implications for the role of river-dominated marginal seas in the global carbon cycle.