The effect of particle density on the sources, distribution, and degradation of sedimentary organic carbon in the Changjiang Estuary and adjacent shelf

• Significant differences in SOC among density fractions from the Changjiang Estuary
• SOC became more degraded with increasing sediment density.
• Hydrodynamic sorting controls the selective transport and degradation of SOC.
• Different controls on the sorting of density fractions and size fractions

Hydrodynamic sorting processes have a significant influence on the dispersal of sedimentary organic carbon (OC) in large-river delta-front estuaries (LDEs). Density fractionation was conducted on 6 sediments from the Changjiang LDE to investigate the sources, distribution, and degradation of OC among density fractions. Mass, elemental composition (C and N) and stable carbon isotope ratios, lignin, and specific surface area (SSA) of density fractions and bulk sediments were determined. Mass was concentrated in high density fractions (2.0–2.5 g cm− 3 and > 2.5 g cm− 3) (up to 67.8%) while OC was dominated in 2.0–2.5 g cm− 3 fraction (up to 77.3%) due to extremely low OC in the highest density fraction (%OC < 0.10%). The less dense fractions (< 1.6 g cm− 3 and 1.6–2.0 g cm− 3) contained the highest OC (up to 21.4%) and lignin concentrations (up to 3.17 mg g− 1 or 2.85 mg/100 mg OC), and thus were most depleted δ13C (down to − 25.2‰). A three end-member mixing model using δ13C and lignin content (Λ8) as source markers and Monte-Carlo simulation, to distinguish the relative importance of terrestrial vascular plant, soil, and marine plankton OC sources, showed that less dense fractions were dominated by terrestrial OC (the sum of soil- and vascular plant-derived OC), whereas marine OC was the predominant OC source in high density fractions. Distributions of OC from different sources in density fractions and bulk sediments both indicated a differential transport and deposition of OC along the coast, showing a gradual decreasing of terrestrial OC and addition of marine OC. Ratios of vanillic acid to vanillin (Ad/Al)v, 3,5-Bd to total V phenols (3,5-Bd/V) and total p-hydroxy phenols to sum of vanillyl and syringyl (P/(S + V)) among lignin oxidation products increased with increasing particle density suggesting selective degradation of lignin among the different density fractions. Distinctively low OC/SSA loadings (< 0.40 mg m− 2) were observed in the highest density fraction and the bulk sediments, indicating low preservation of OC in these sediments, consistent with the results of lignin decay parameters.