Thermochemical decomposition reveals distinct variability of sedimentary organic carbon reactivity along the Yangtze River estuary-shelf continuum

Continental shelves host 90% of annual organic carbon (OC) deposition in the global ocean and are regarded as “hot spots” of carbon burial and decomposition. Numerous studies have thus investigated OC sources, recent accumulation, long term preservation and key processes involved. Nonetheless, OC reactivity or lability, as a key property governing the fate of OC in the long term, received less attention, primarily due to a lack of proper technique of investigation. In this study, we conducted thermochemical decomposition analysis of OC using ramped-temperature pyrolysis/oxidation technique to investigate the reactivity of sedimentary OC along the Yangtze River estuary-shelf continuum. Our results reveal that sedimentary OC in the Yangtze River estuary-shelf region is relatively more stable than global average level, which is attributed to the winnowing of sediments due to frequent sedimentation-resuspension cycles. In general, OC reactivity increases gradually from the estuary to the inner shelf, which is governed by organo-mineral interactions and the progressive absorption of marine OC. Based on our results, we propose that OC reactivity is a key OC property to be considered in future organic carbon cycle frameworks. •Organic carbon in the Yangtze River estuary-shelf is thermochemically recalcitrant.•Organic carbon reactivity increases offshore along the estuary-shelf continuum.•Carbon sources and organo-mineral interaction control organic carbon reactivity.•Carbon reactivity should be considered a key factor in carbon cycle frameworks