Understanding the sources and fate of organic matter (OM) sequestered in continental margin sediments is of importance because the mode and efficiency of OM burial impact the carbon cycle and the regulation of atmospheric CO₂ over long time scales. We carried out molecular (lignin‐derived phenols from CuO oxidation), elemental, isotopic (δ¹³C, Δ¹⁴C), and sedimentological (grain size and mineral surface area) analyses in order to examine spatial variability in the abundance, source, age of surface sediments of the East China Sea. Higher terrigenous organic matter values were found in the main accumulating areas of fluvial sediments, including the Changjiang (Yangtze) Estuary and Zhejiang‐Fujian coastal zone. Isotopic and biomarker data suggest that the sedimentary OM in the inner shelf region was dominated by aged (Δ¹⁴C = −423 ± 42‰) but relatively lignin‐rich OM (Λ = 0.94 ± 0.57 mg/100 mg OC) associated with fine‐grained sediments, suggesting important contributions from soils. In contrast, samples from the outer shelf, while of similar age (Δ¹⁴ C = −450 ± 99‰), are lignin poor (Λ = 0.25 ± 0.14 mg/100 mg OC) and associated with coarse‐grained material. Regional variation of lignin phenols and OM ages indicates that OM content is fundamentally controlled by hydrodynamic sorting (especially, sediment redistribution and winnowing) and in situ primary production. Selective sorption of acid to aldehyde in clay fraction also modified the ratios of lignin phenols. The burial of lignin in East China Sea is estimated to be relatively efficient, possibly as a consequence of terrigenous OM recalcitrance and/or relatively high sedimentation rates in the Changjiang Estuary and the adjacent Zhejing‐Fujian mud belt.

Information on the age dynamics of particulate organic matter (POM) in large river systems is currently sparse and represents an important knowledge gap in our understanding of the global carbon cycle. Here we examine variations in organic geochemical characteristics of suspended sediments from the Changjiang (Yangtze River) system collected between 1997 and 2010. Higher particulate organic carbon content (POC%) values were observed in the middle reach, especially after 2003, and are attributed to the increase of in situ (aquatic) primary production associated with decreased total suspended matter concentrations. Corresponding Δ¹⁴C values from depth profiles taken in 2009 and 2010 indicate spatial and temporal variations in POC sources within the basin. Two isotopic mass balance approaches were explored to quantitatively apportion different sources of Changjiang POM. Results indicate that contributions of biomass and pre‐aged soil organic matter are dominant, regardless of hydrological conditions, with soil‐derived organic carbon comprising 17–56% of POC based on a Monte Carlo three‐end‐member mixing model. In contrast, binary mixing model calculations suggest that up to 80% of POC (2009 samples only) derived from biospheric sources. The emplacement of the Three Gorges Dam and resulting trapping of sediment from the upper reach of the watershed resulted in a modification of POM ¹⁴C ages in the reservoir. With the resulting decline in sediment load and increase in the proportion of modern POC in the lower reach, these changes in POM flux and composition of the Changjiang have significant implications for downstream carbon cycle processes.

The Changjiang is the most important source of freshwater and dissolved organic matter (DOM) for the East China Sea. However, knowledge regarding the sources, seasonal fluxes, and fluvial transport of terrigenous DOM (tDOM) in the Changjiang is lacking. To fill this knowledge gap, we measured dissolved organic carbon (DOC) and dissolved lignin in water samples collected in the middle and lower Changjiang under different hydrological conditions. Additional samples were collected biweekly in the lower Changjiang. Through comparisons with other rivers, we found that the DOC in the Changjiang is mainly from soil organic matter and has a higher fraction of tDOM during flood. Mass balance model results indicate that approximately 33% of the dissolved lignin discharged into the middle and lower Changjiang is removed during its transport to the lower reach during both low‐discharge and flood periods. Based on a comparison of the removal rates under these two contrasting hydrological conditions and considering the lower organic carbon content and fine grain size of the Changjiang's suspended particles, we speculate that the major process for the removal of dissolved lignin is sorption, and potentially flocculation by suspended particles. Changjiang discharges 1.4 ± 0.10 Tg yr⁻¹ and 8.6 ± 0.30 Gg yr⁻¹ DOC and dissolved lignin to the estuary during the period of July 2010 to June 2011, respectively. Seasonal distributions of DOC and dissolved lignin fluxes are controlled by water discharge, which will be affected by future climate change and the Three Gorges Dam.