Organic Carbon Cycling in East China Sea Shelf Sediments: Linkages with Hypoxia

The Changjiang River provides the main source of sediment and terrestrial derived organic carbon (OC) to the Changjiang large delta-front estuary (LDE) in the East China Sea (ECS). This study analyzed bulk OC, biomarkers including lignin and plant pigment, black carbon (BC) on ECS sediments sampled in winter 2009 and 2010 in order to study the OC cycling under the influence of natural and anthropogenic disturbance. Low-oxygen tolerant foraminiferal microfossils were analyzed in another two sediment cores to study the historical hypoxia events in the Changjiang LDE.

Bulk carbon to nitrogen (C/N) ratio and stable isotope ?13C in the surface sediment samples indicated a mixture source of terrestrial, deltaic and marine derived OC. Refractory BC and reworked marine OC seemed to comprise most of the OC pool with older, less reactive signatures as deduced from ?14C, and BC analyses. Winter wind/wave energy and hydrodynamic sorting had a substantial winnowing effect on surface sediment OC redistribution. As a result, the highest lignin concentration shifted to the south during the 2010 cruise after the summer flood event. In addition, algal inputs from local deltaic lakes due to eutrophication and/or lateral transport likely caused the observed lack of benthic-pelagic coupling of pigment concentrations between the surface sediments and the water column after the summer flood in 2010.

For the down-core sediment, the mass accumulation rate distribution followed the dispersal pathway of the ECS sediment. Terrestrial and marine derived OC showed significant spatial and temporal distribution. Lignin rich materials were better preserved in sediments closer to the coast while offshore sediments tended to be composed of lignin-poor, degraded OC, that were likely hydrodynamically sorted to a long distance during transport. Besides eutrophication, plant pigments indicated that marine-derived OC was mostly deposited in the sediment mixed layer with decay in the underlying sediment accumulation layer. The total OC standing stock since 1900 is approximately 1.62?1.15 kgC m^-2, about 1/10 of the total OC stock in all the middle and lower lakes in the Changjiang catchment.

There has been an increase in the number of hypoxic bottom water events on the Changjiang LDE over the past 60 yrs indicated from the increases in low-oxygen tolerant foraminiferal microfossils due to excess deposition of OC and summer stratification.