Anthropogenic perturbations to the fate of terrestrial organic matter in a river-dominated marginal sea

Abstract

River-dominated ocean margins (RiOMar) are major terrestrial organic carbon (OCterr) repositories that play an important role in the global carbon cycle. However, riverine inputs of sediment and associated OCterr have decreased substantially due to intensifying human activity. Thus, studying OCterr processing during transport to and within RiOMar is crucial. The East China Sea (ECS) receives reduced sediment loads from the Changjiang River (CJR), which is compounded further by the impoundment of the Three Gorges Reservoir. Here, we examined the CJR-ECS source-to-sink system to understand the fate of OCterr in a typical RiOMar. We analyzed the sedimentary properties (grain size and the specific surface area), bulk organic carbon properties (C/N ratio and δ13C), and molecular biomarkers (n-alkanes and GDGTs) of riverine suspended particulate matter (SPM) and surficial marine sediment collected in 2006 and 2018. The C/N ratio, δ13C value, and short-chain n-alkanes abundances in the riverine SPM indicate that phytoplankton productivity could be an important source of OCterr in the basin due to the reduced sediment loads. Seabed erosion occurred in the estuarine–inner shelf areas of the ECS as a result of reduced riverine inputs, which resulted in a change in the distribution of sedimentary OCterr. The reduced total organic carbon content and n-alkanes abundance are indicative of the low sedimentary OCterr content. Variations in GDGT-based indices are further manifestation of reduced sediment load, as they show increased in situ production of GDGTs. Spearman correlations indicate that the biomarker abundances were positively correlated with silt, which is in accordance with silt being easily shaped by hydrodynamic sorting. Biomarker abundances along four estuarine–inner shelf transects also exhibited pronounced alongshore or cross-shelf OCterr transport. Furthermore, the relationship between the biomarkers and sedimentary properties indicates that mineral protection plays an important role in regulating the fate of OCterr on continental margins. Therefore, variations in riverine inputs and the sedimentary environment caused by anthropogenic perturbations have affected the abundance, distribution, composition, and transport of sedimentary OCterr in the study area. These findings can be applicable to other RiOMar, as humans continue to shape riverine inputs globally.