Apparent Aging and Rejuvenation of Terrestrial Organic Carbon Along the River‐Estuary‐Coastal Ocean Continuum.

The fates of terrestrial organic carbon (OCterr) during fluvial transport from land to ocean are still not well constrained. This study systematically examines the evolution and dynamics of OCterr along the river‐estuary‐coastal ocean continuum in three fluvial systems discharging to the Chinese marginal seas. The 14C‐depleted characteristics of bulk OC and molecular components of riverine suspended sediments and marine sediments suggest that the Chinese marginal seas are a significant sink of pre‐aged OCterr. Our study reveals significant apparent aging of OCterr within estuaries, likely due to degradation of (younger) labile components, and apparent rejuvenation of OCterr in shelf systems, likely reflecting inputs of younger OCterr from proximal sources along the sediment dispersal pathway. The aging and rejuvenation of OCterr along the river‐ocean continuum confounds the use of plant wax lipid 14C to constrain lateral transport times, and sheds light on more complex OCterr dynamics in marginal seas. Plain Language Summary: Determining the factors driving degradation of terrestrial organic carbon (OCterr) is important for understanding the fate of OCterr in marginal seas and its impact on the global carbon cycle. In this study, we use bulk and molecular‐level carbon isotopic measurements as well as sedimentological data to investigate how OCterr characteristics evolve along the river‐estuary‐coastal ocean continuum for three different river systems discharging into the Chinese marginal seas. Marked decreases in the OCterr abundance and 14C contents reveal marked aging associated with degradation processes within estuaries over relatively short transport distances, likely due to degradation of younger and labile OCterr. Both aging and apparent "rejuvenation" of OCterr after estuarine transport was observed in different shelf systems, the latter likely resulting from inputs of fresh OCterr from proximal sources. Our findings suggest both sources and transport processes influence terrestrial OC and biomarker signatures in marginal sea sediments, highlighting the complexity of OCterr dynamics in coastal systems. Key Points: Bulk and molecular 14C values suggest millennial‐aged terrestrial OC transport along the Chinese river‐estuary‐coastal ocean continuumRapid degradation and significant aging of terrestrial OC co‐occur during estuarine processingApparent aging and rejuvenation of terrestrial OC along‐shelf transport confounded the transport time estimated by plant wax lipid 14C [ABSTRACT FROM AUTHOR]