Contrasting controls of particulate organic carbon composition and age from riverine to coastal sediments of Eastern China Marginal Seas.

Terrestrial organic carbon (OC) burial in marginal seas comprises an important marine carbon sink. However, controls on the evolution of OC during sediment transport within and beyond river basins remain poorly constrained. OC contents and isotopic compositions (δ13C and Δ14C), and specific surface areas (SSA) of river particles of the Changjiang (Yangtze River) and Huanghe (Yellow River) and surface sediments in the Eastern China Marginal Seas (ECMS) were determined to better understand controls of OC composition and age along these two major river sedimentary systems of China. Although petrogenic OC (OC petro) content (0.02%) of particulate organic carbon (POC) in the Huanghe is lower than the Changjiang (0.09%), the radiocarbon age of Huanghe POC (4500 ± 420 yr) is significantly older than Changjiang POC (3230 ± 315 yr). Biospheric OC (OC bio) of POC in the Huanghe (0.32 ± 0.08%) is primarily derived from pre-aged soils in contrast to OC bio of Changjiang POC (0.87 ± 0.23%) - mainly from modern litter biomass. However, the age of OC in the Bohai Sea and Yellow Sea surface sediments is younger than those in the East China Sea (2335 ± 590 yr vs. 4135 ± 725 yr). Removal of most terrestrial OC bio during transport results in diagenetic aging in the Changjiang estuary (via frequent physical reworking in highly oxidized mobile-muds) and the remaining terrestrial OC bio experiences further aging during along-shore transport (i.e., transport time-associated aging). However, the continuous addition of young marine OC with little loss of the terrestrial OC bio occurs along the Huanghe system, which is mainly due to the relatively weak sediment dynamics and the high proportion of pre-aged OC in Huanghe POC. Compared with other globally significant river sedimentary systems, the evolution pattern of OC in the Huanghe system is similar to those of small mountain river-active margin systems, although it belongs to the passive margin system. By contrast, the Changjiang system acts like other large river-passive margins. Our findings further demonstrate that the alteration of OC composition and age in river sedimentary systems can be controlled by similar sediment dynamics and OC reactivities – across divergent continental margins. • Ages of OC in coastal sediments are decoupled with OC in riverine particles. • Sedimentary OC becomes younger along the Huanghe system from additions of marine biospheric OC. • Sedimentary OC becomes older along the Changjiang system from the removal of terrestrial biospheric OC. • OC reactivities and sediment dynamics determine the evolution of OC composition and age. [ABSTRACT FROM AUTHOR]