Publication Date:
2021-02-11
Description:
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 (d13C
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 d13C 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) x 10–4 yr-1, indicating a more refractory nature and likely
effective mineral protection of OCterr. The RCO2 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.
Repository Name:
EPIC Alfred Wegener Institut
Type:
Article
,
isiRev
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