Cross-shore heterogeneity in organic carbon storage and source composition in bare tidal flats of South Korea

Bare tidal flats are increasingly recognized as blue carbon reservoirs, but system-scale estimates are prone to bias when cross-shore heterogeneity is not adequately captured. We quantified sedimentary total organic carbon (TOC) content, organic carbon (OC) stocks, sequestration rates, and source composition along shore-normal transects in five Korean bare tidal flat systems during 2022-2024. Surface-layer TOC showed no significant temporal variation, but pronounced spatial gradients in OC stocks occurred among and within systems. Mud-rich Suncheon Bay recorded the highest mean OC stock (56.3 Mg C ha^-1) and apparent OC sequestration rate (0.52 Mg C ha^-1 yr^-1), whereas the sand-dominated Nakdong River estuary exhibited the lowest (7.5 Mg C ha^-1). Within-system OC stocks varied up to 6-fold; extrapolating solely from carbon-rich upper flats would overestimate system means by nearly 89%. Comparisons with sediment-type-based estimates confirmed that such extrapolations fail to capture substantial internal heterogeneity. Stable isotope profiles revealed stronger marine-derived influence in most systems, but terrestrial influence remained highest in the Nakdong River estuary despite low OC, demonstrating that source composition alone does not determine carbon storage. Downcore isotope shifts at GH3 and ND1-ND2 were broadly consistent with CRS-derived event-depth markers for Han River channel-engineering (1982) and Nakdong barrage construction (1987), suggesting that anthropogenic modifications may have influenced OC source composition. These findings provide field-based evidence that reliable system-scale OC estimation in bare tidal flats requires shore-normal transects capturing within-system heterogeneity and offer a basis for reducing bias in regional blue carbon inventories.