Composition, abundance and age of total organic carbon in surface sediments from the inner shelf of the East China Sea
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abstract
Surface sediments were collected from the inner shelf of the highly dynamic East China Sea (ECS) in December 2009 and November 2010. In between the two cruises, a big flood event occurred in July, 2010. Elemental, stable C, N isotope, radiocarbon isotope (14C), black carbon (BC) analysis, and chemical biomarkers were utilized to study the effect of the sources, short-term events and physical processes on the composition, abundance, and age of the surface sedimentary OC in this dynamic system. Bulk N/C ratio of 0.10 to 0.14 and 13C of -23.1 to -20.9 in these samples indicated a mixture source of terrestrial, deltaic and marine derived OC during both cruises. The TOC was 1,980 to 5,040yr old based on 14C, and BC extracted from two 2009 sediment samples was older: 5,480 and 5,920yr old. While the Changjiang River was a main source of the terrestrial derived OC, refractory BC and reworked marine organic matter seemed to comprise the older, less reactive carbon pool in the surface sediments of the ECS. Monte Carlo simulations indicated that more than 50% of the TOC was derived from the marine end-member, followed by the deltaic OC. The chlorophyll a of the 2010 sediment samples (11.0610.15nmolg-1 OC, n=7) was significantly lower than that of the 2009 samples (34.5820.29nmolg-1 OC, n=7). However, zeaxanthin, a biomarker for cyanobacteria, had significantly higher concentration in the 2010 sediment samples (9.263.48nmolg-1 OC, n=7) than that of the 2009 samples (5.504.62nmolg-1 OC, n=7). The lack of benthic-pelagic coupling of pigment concentrations in surface sediments after the flood of 2010 was likely due to lateral transport of surface sediments to and from this region. While there were no significant linkages between in situ shelf-derived phytoplankton sources of OC and surface sediment pigment concentrations in 2010, other algal inputs from local deltaic lakes may have contributed to the compositional changes in concentration of sediment carotenoids. The flood of summer 2010 likely played a critical role in the observed shift of the locations for the highest terrestrial biomarker concentrations in sediments. Little evidence remained from the 2010 flood event, which suggested that winter wind/wave energy and hydrodynamic sorting had a substantial effect on sediment OC redistribution. 2012.
