Spatial Upscaling of Top-Meter Sedimentary Organic Carbon Stock and Regional Storage in the Unvegetated Hwangdo Tidal Flat, Taean, Cheonsu Bay, West Coast of Korea

Unvegetated tidal flats store sedimentary organic carbon (OC), but their contribution to blue-carbon inventories remains poorly constrained because sparse cores must represent heterogeneous intertidal environments. This study quantified OC stock within the upper 1 m of Hwangdo tidal flat, South Korea, by integrating ten 100-cm cores, 107 surface-sediment samples, and an unmanned-aerial-vehicle-derived digital elevation model covering 4.841 km2. The surface survey characterized variations in elevation, sediment texture, and OC, while the cores provided profiles of grain size, dry bulk density, and OC content. Core stocks were integrated over 0–100 cm and evaluated using a spatially grouped, cross-validated adjustment based on surface elevation and uppermost-sediment OC to reduce bias from sparse core distribution. Measured core stocks ranged from 13.61 to 30.20 Mg C ha−1, with an unadjusted mean of 23.04 Mg C ha−1. Mean OC contents were low in surface sediments (0.18%) and core sediments (0.17%). Across the pooled surface and core dataset, TOC correlated positively with mud content (Spearman’s ρ = 0.47) and negatively with sand content (ρ = −0.51), indicating sediment-textural control over OC distribution. The adjustment increased the mean stock by 5.9% to 24.39 Mg C ha−1. Applying this value to 484.108 ha yielded a regional OC storage of 11.81 Gg C, with a conditional bootstrap interval of 9.12–12.90 Gg C. However, the low cross-validated Q2 (0.070) shows that the model adjusts the core mean rather than providing a spatially explicit prediction. Overall, Hwangdo’s comparatively low OC pool likely reflects the combined effects of limited inputs of terrestrial organic matter; the absence of vascular vegetation and the associated belowground biomass that would otherwise promote carbon retention; and the frequent reworking, oxygenation, and export of organic matter from its sand-dominated, hydrodynamically energetic sediments. By integrating depth-resolved cores with dense surface observations and UAV-derived topography while explicitly reporting conditional uncertainty, this study establishes a transferable framework for more defensible blue-carbon inventories and strategically targeted sampling in data-limited unvegetated tidal flats.