Oxidation of petrogenic organic carbon in a large river-dominated estuary

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Riverine export of petrogenic organic carbon (OCpetro) from continents to coastal oceans is a dynamic component of the global carbon budget and affects the long-term atmospheric carbon reservoir. In large fluvial systems, oxidation of OCpetro during transit releases a large flux of carbon dioxide to the atmosphere, influencing climate changes; however, the transport and fate of OCpetro and their controls along the fluvial–marine transition remain poorly constrained. Here, we combined Raman spectral, radiocarbon activity (F14C), mineralogical, and sedimentological techniques with multiple geochemical analyses to characterize the dynamics of OCpetro in the water column and sediment particles from the Yangtze River channel–estuary–shelf continuum systems.Our data show that much of the OCpetro present in suspended sediment (POCpetro) exported by the Yangtze River is “labile” fractions (mostly disordered materials) that can be degraded or lost during transport across the estuarine continuum, whereas the OCpetro deposited in seabed sediment are characterized by highly recalcitrant, unreactive, and graphitic carbon phases. As discrete “free” particles, coarse plant debris (>63 μm) with ages of several thousand years observed in the proximal delta of the Yangtze River exhibit nearly identical characteristics of disordered materials, and the presence of aged vascular plant detritus (carbon-rich and 14C-depleted materials) may lead to an overestimate of OCpetro in marine sediments. Using a Bayesian endmember mixing approach, a binary mixing model, and the ratio of fraction modern carbon (F14C) to Al/OC, we found a large decrease in POCpetro concentration and loading from the suspended sediments to seabed sediments, suggesting a loss of mineral-bound OCpetro fraction during sediment transport through the estuary and deposition on the shelf. We estimated that, on average, 46 ± 35 % of the POCpetro initially present in suspended sediments delivered to the Yangtze River Estuary during a flood event was primarily oxidized at the sediment–water interface, leaving the most graphitic carbon components transported laterally and efficiently reburied in shelf sediments. We found that during estuarine mixing, flocculation process-induced microaggregates may provide transient physical protection for POCpetro in the form of inclusions/aggregates with carbonate minerals; however, when POCpetro is physically and chemically separated from its mineral matrix via disaggregation and dissolution, it may be easily oxidized by microbial activity. In contrast, OC-phyllosilicates interactions exert a first-order control on the preservation of OCpetro in marine sediments. Our findings suggest that the importance of POCpetro oxidation and loss in carbon cycling and budget assessments of estuaries may be underestimated.