Marginal seas are important burial centers of both marine and terrestrial organic carbon (OC), accounting for the burial of more than 90% of sedimentary OC in marine environments Burdige, 2005;Hedges & Keil, 1995). These marginal seas receive vast amounts of sediments, freshwater, and nutrients from their adjacent rivers and coastal erosion, especially in those large delta-front estuaries (Bianchi & Allison, 2009;Kuehl et al., 2020). Large seasonal shifts of terrigenous inputs impact sedimentation rates, sources/age of OC, and primary productivity -all critical in controlling burial and decomposition of OC in marginal seas (Bauer et al., 2013;Bianchi et al., 2014;Liu et al., 2010). Recent work on differentiation of rock-derived or petrogenic OC (OC petro ) from physical erosion of bedrock in river drainage basins Abstract A regional synthesis of organic carbon (OC) burial was conducted using a comprehensive data set to reveal some of the key drivers and human multi-stressors controlling OC burial and transport in the Eastern China Marginal Seas (ECMS). Both OC and Δ 14 C values of suspended particulate matter (SPM) in the Changjiang River, were significantly higher than estuarine mobile-muds, suggesting selective decay of more labile younger OC from both marine and terrestrial sources and the accumulation of more recalcitrant older OC. Some of this decay is likely to be associated with iron-redox cycling in mobile-muds. In contrast, OC, δ 13 C, and Δ 14 C values increased along the Yellow River sediment dispersal pathway, indicating adding of young marine OC and less decay of terrestrial OC. OC burial efficiency in mud areas in the Bohai Sea (∼43%) was significantly higher than those in the Yellow (∼11%) and East China Seas (∼16%), owing to rapid deposition. Burial flux of biospheric OC in mud areas of the ECMS is 7.00 ± 0.79 Mt yr −1 , corresponding to atmospheric CO 2 drawdown by silicate weathering in major river drainage basins of mainland China. The burial flux of petrogenic OC was estimated to be 0.81 ± 0.25 Mt yr −1 , accounting for >1.9% of total burial in the global ocean. While the ECMS is an important OC sink, river damming has greatly reduced OC burial. Thus, the overall impact on anthropogenically altered riverdominated marginal seas remains an important and rapidly changing component of the coastal ocean carbon budget.Plain Language Summary A comprehensive regional synthesis of organic carbon (OC) burial and its drivers, were investigated across the Eastern China Marginal Seas (ECMS). Variation of OC content and carbon isotopic composition from suspended particulate matter to mobile muds, in Changjiang sediment dispersal pathways, indicated selective decomposition of younger more labile marine and terrestrial OC, which resulted in the accumulation of older more recalcitrant OC. However, continuous adding of young marine OC, with little loss of terrestrial OC, in Yellow River sediment dispersal pathway, resulted in more recalcitrant terrestrial OC buried in this relatively more quiescent sedimentary ...

To better understand the role of reactive Fe (Fe R ) in the preservation of sedimentary organic carbon (SOC) in estuarine sediments, we examined specific surface area, grain size composition, total OC (TOC), lignin phenols, Fe R , Fe R -associated OC (Fe-OC) and lignin phenols (Fe-lignin), and δ 13 C of Fe R -associated OC (δ 13 C Fe-OC ) in surface sediments of the Changjiang Estuary and adjacent shelf. An estimated 7.4 ± 3.5% of the OC was directly bound with Fe R in the Changjiang Estuary and adjacent shelf. Unusually low TOC/specific surface area loadings and Fe-OC/Fe ratios in mobile muds suggest that frequent physical reworking may reduce Fe R binding with OC, with selective loss of marine OC. More depleted 13 C Fe-OC relative to 13 C of TOC ( 13 C bulk ) in deltaic regions and mobile muds showed that Fe R was largely associated with terrestrial OC, derived from extensive riverine OC and Fe inputs. A higher proportion of hematite in the mobile muds compared to the offshore samples indicated that Fe oxides are likely subjected to selective sorting and/or become mature during long-term sediment transport. When considering the percentage of Fe-OC to SOC and SOC burial rates in different marine environments (e.g., nondeltaic shelf, anoxic basins, slope, and deep sea), our findings suggest that about 15.6 ± 6.5% of SOC is directly bound to Fe R on a global scale, which is lower than the previous estimation (~21.5%). This work further supports the notion of a Rusty Sink where, in this case, Fe R plays an important role in the preservation and potential transport of terrestrial OC in the marine environment.

No abstract