Dispersal and aging of terrigenous organic matter in the Pearl River Estuary and the northern South China Sea Shelf

Large rivers discharge great amounts of terrigenous organic carbon (OC terr ) to the ocean, 90% of which are trapped in the coastal areas. The OC terr processing during transport to and within the coastal oceans are not well known. The Pearl River is the second largest river in China in terms of annual runoff. Here, we studied OC terr delivered from the river in the Pearl River Estuary (PRE) and the northern South China Sea (SCS) shelf using stable and radioactive carbon isotope compositions (6 13 C and F 14 C) of OC in surface sediments to (1) constrain sources of OC, (2) explore the role of hydrodynamic processes for OC terr transport, and (3) better understand the burial and degradation processes of OC terr . A three end -member mixing model based on 6 13 C and 1/(C/N) values of bulk OC was used to calculate relative contribu- tions of OC terr , OC derived from marine (OC mar ) and riverine primary production (OC Rpp ). Results showed that the PRE and its western coastal inshore mud deposit are dominated by OC terr (70 ? 3% and 54 ? 10%, respectively), but other areas receive increasing contributions from OC mar : the slope (49 ? 10%), the eastern coast (58 ? 2%) and the outer shelf (68 ? 3%). OC Rpp accounted for substantial proportions in the PRE (14 ? 6%) and rapidly decreased in the offshore area (3-5%), likely due to extensive aerobic respiration of organic matter (OM). Subsequently, average F 14 C values of OC terr were calculated ranging from 0.271 to 0.639 using a Monte -Carlo simulation strategy and based on the assumption that F 14 C values of OC Rpp and OC mar varied within narrow ranges. Together with OC terr contents and grain sizes of sediments, F 14 C values of OC terr were used to distinguish two regions of distinctive sedimentological characteristics in the study area. Region I, including the PRE and inner shelf, showed a decrease of OC terr content along the trajectory of westward along -shelf transport, reflecting resuspension-dominated conditions for OC terr transport. Region II, including the outer shelf and one site on the inner shelf (E701), exhibited relatively old OC terr (9180 ? 730 yr BP), which is attributed to preferential accumulation of coarser sedi- ments by bedload movement. Finally, a first -order degradation rate constant was calculated from contents and 14 C ages of OC terr , yielding a slow OC terr degradation rate of (2.88 ? 0.61) x 10 - 4 yr -1 , indicating a more refractory nature and likely effective mineral protection of OC terr . The RCO 2 efflux due to OC terr degradation was estimated to be 3.68 ? 1.39 Gg C yr -1 for the entire surface sediments in the Pearl River derived mud belt, equivalent to only 0.7 ? 0.3% of the total particulate OC flux of the Pearl River. This, hence, suggests that the OC terr is more persistent and the preservation of OC terr in the marine system is better than previously thought, but experiences substantial degradation on centennial to millennial timescales. (C) 2020 Elsevier Ltd. All rights reserved.