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 con-strained. OC contents and isotopic compositions (delta 13C and Delta 14C), 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 (OCpetro) 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 (OCbio) of POC in the Huanghe (0.32 +/- 0.08%) is primarily derived from pre-aged soils in contrast to OCbio 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 OCbio during transport results in diagenetic aging in the Changjiang estuary (via frequent physical reworking in highly oxidized mobile-muds) and the remaining terrestrial OCbio 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 OCbio 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.

Automated summary

The organic carbon (OC) composition and age in river sedimentary systems are controlled by sediment dynamics and OC reactivities. The Huanghe (Yellow River) system has a higher proportion of pre-aged OC and continuous addition of young marine OC, while the Changjiang (Yangtze River) system experiences diagenetic aging and transport time-associated aging. The evolution pattern of OC in the Huanghe system is similar to small mountain river-active margins, while the Changjiang system is similar to other large river-passive margins.