Source, transport and fate of terrestrial organic carbon from Yangtze River during a large flood event: Insights from multiple-isotopes (δ13C, δ15N, Δ14C) and geochemical tracers

Abstract

Climate change-triggered episodic and intensive flood events are increasing in frequency globally; these events play a disproportionate role in the carbon cycle at land-ocean interfaces by influencing the delivery of terrestrial organic matter to the sea. However, the transport and burial of flood-driven terrestrial organic carbon (Corg) in river-dominated marginal seas remains poorly understood. Here, water column particles (surface, intermediate, and bottom layers) and surface sediments from the Yangtze River estuary system were collected during a large flooding event (>60,000 m3 s−1) for a multiproxy geochemical study. We analyzed N/C, δ13Corg, δ15N, F14C, grain size, and elemental (Al and Si) to characterize the sources, ages, and alterations of particulate organic carbon (POC) and sedimentary Corg from the river channel to the estuarine mixing zone to the continental shelf and fingerprint the fate of Corg burial in marine sediments.We found that terrestrial POC delivered by the Yangtze flood is characterized by low F14C (0.625 ± 0.002; Δ14C = −379.8 ± 2.3‰, 14C age = 4,135 yr before present (BP)) and δ13Corg (−27.1 ± 0.5‰) values and high δ15N (3.9 ± 0.6‰), and N/C (0.15 ± 0.01) values, and was adsorbed on fine-grained particles. Using a Bayesian-Monte Carlo simulation approach, we estimated that terrestrial POC is dominated by C3 plant detritus (42 ± 16% on average), followed by petrogenic organic carbon (33 ± 7%), and soil (26 ± 22%). Moving offshore, hydrodynamic sorting of lithogenic particles exerts first-order control on POC by selecting particles with unique signatures. Petrogenic organic carbon and aged plant debris associated with coarse-sized particles preferentially accumulated within the proximal delta, whereas POC derived from catchment soil and adsorbed on fine-grained particles was transported offshore along the sediment dispersion system. We estimated that, on average, 76 ± 18% of POC initially present in the water column is oxidized at the water column-seabed interface during seaward transport and constitutes a source of atmospheric carbon dioxide. In contrast, the stable form of the organic matter-aluminosilicate mineral association controls the preservation of the sedimentary Corg. Our results suggest that frequent flooding events in the near future will likely increase lithospheric Corg delivery to marine sediments and have the potential to promote petrogenic Corg burial efficiency in the river-dominated marginal seas and accelerate biospheric POC oxidation and CO2 release, thus generating a positive feedback to global warming.