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

Abstract

Large rivers discharge great amounts of terrigenous organic carbon (OCterr) to the ocean, 90% of which are trapped in the coastal areas. The OCterr 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 OCterr 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 (δ13C and F14C) of OC in surface sediments to (1) constrain sources of OC, (2) explore the role of hydrodynamic processes for OCterr transport, and (3) better understand the burial and degradation processes of OCterr.A three end-member mixing model based on δ13C and 1/(C/N) values of bulk OC was used to calculate relative contributions of OCterr, OC derived from marine (OCmar) and riverine primary production (OCRpp). Results showed that the PRE and its western coastal inshore mud deposit are dominated by OCterr (70 ± 3% and 54 ± 10%, respectively), but other areas receive increasing contributions from OCmar: the slope (49 ± 10%), the eastern coast (58 ± 2%) and the outer shelf (68 ± 3%). OCRpp 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 F14C values of OCterr were calculated ranging from 0.271 to 0.639 using a Monte-Carlo simulation strategy and based on the assumption that F14C values of OCRpp and OCmar varied within narrow ranges. Together with OCterr contents and grain sizes of sediments, F14C values of OCterr 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 OCterr content along the trajectory of westward along-shelf transport, reflecting resuspension-dominated conditions for OCterr transport. Region II, including the outer shelf and one site on the inner shelf (E701), exhibited relatively old OCterr (9180 ± 730 yr BP), which is attributed to preferential accumulation of coarser sediments by bedload movement. Finally, a first-order degradation rate constant was calculated from contents and 14C ages of OCterr, yielding a slow OCterr degradation rate of (2.88 ± 0.61) × 10–4 yr−1, indicating a more refractory nature and likely effective mineral protection of OCterr. The ΣCO2 efflux due to OCterr 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 OCterr is more persistent and the preservation of OCterr in the marine system is better than previously thought, but experiences substantial degradation on centennial to millennial timescales.