Sediment Resuspension Accelerates the Recycling of Terrestrial Organic Carbon at a Large River‐Coastal Ocean Interface

Abstract

AbstractWidespread sediment resuspension and transport processes on continental margins can modify deposits and influence the preservation of particulate organic carbon (POC) in marine sediments. However, it remains unclear how post‐depositional processes interact with physical mineral protection to affect the transport and fate of terrestrial POC along the river‐estuary‐shelf paths. Here, we synthesized literature data and newly obtained results from multiple analyses of sedimentology, mineralogy, and inorganic and organic geochemical tracers. Our goal was to quantitatively evaluate the impact of sediment reworking on the redistribution and further transformations of terrestrial POC at the Yangtze River‐ocean interface. Our results reveal that sediment resuspension resulting from physical forces along with mineral protection of phyllosilicates plays a crucial role in regulating the recycling and fate of terrestrial POC during its transport across the coastal ocean continuum. Physical processes lead to the resuspension of sequestered POC from suboxic/anoxic muddy sediments into the overlying water column. Concurrently, the interplay of energetic forcing and elevated oxygen levels has the potential to disrupt the organo‐mineral associations. The decrease in mineral‐carbon stabilization increases the likelihood that reactive POC inclusion/aggregation with minerals becomes accessible to surrounding microorganisms, making it susceptible to microbial/oxidative degradation. Consequently, mostly phyllosilicate‐protected 14C‐depleted POC (primarily soil‐derived) in <63 μm suspended sediment (>90% of the total mass) remains available for export and reburial in continental shelf sediments. The lateral transport of resuspended sediments from estuaries, previously underestimated, represents a potential contributor to the remobilized millennial‐aged POC components involved in active biogeochemical cycling on continental margins.