Molecular Insights Into the Fate of Sedimentary Organic Carbon Fractions Across Estuarine—Offshore Continuums

Abstract Estuarine‐offshore continuums are critical components of the global ocean carbon cycle. Previous studies have primarily focused on the source‐sink processes of bulk organic carbon (OC) in these systems. However, the molecular‐scale fate of distinct sedimentary OC fractions during lateral transport remains poorly understood. Here, we quantified the proportional distributions, stable isotopic signatures, biomarkers, and functional groups of particulate OC (POC), mineral‐associated OC (MAOC), and iron‐associated OC (Fe‐OC) in the Changjiang River Estuary (CRE) and the East China Sea (ECS) inner shelf, and integrated these observations with global data sets to evaluate their broader implications. Our study demonstrates that POC, MAOC, and Fe‐OC exhibit distinct spatial distribution and preservation potential. During lateral transport, both POC and MAOC undergo a shift from terrestrial‐dominated to increasingly marine‐influenced sources. Enhanced marine OC inputs contribute substantially to POC accumulation in non‐muddy regions, whereas MAOC preferentially accumulates in muddy regions, where terrestrial and persistent molecules are progressively degraded and replaced by marine OC. Reactive iron serves as an important carrier of terrestrial OC in muddy regions. However, increasing marine OC inputs and a shift in iron–carbon binding mechanisms further reduce the long‐term preservation potential of Fe‐OC during offshore transport. These findings highlight that the contribution of mineral protection on terrestrial OC may have been overestimated, and molecular composition represents an additional key control on OC sequestration potential. This framework provides new insights into how human activities, OC fraction dynamics, and environmental perturbations interact to regulate carbon preservation across dynamically active and human‐impacted estuarine‐offshore continuums worldwide.