Assessment of Carbon Storage in Marine Sediments of the South Korean EEZ: An Insight into Surface Sediment Grain Size in the Retention of Organic Carbon

As part of the South Korean government's long-term national objectives for achieving greenhouse gas neutrality, we are developing a project to establish a systematic framework for assessing organic carbon storage in seafloor sediments across the coastal and offshore regions of the South Korean Peninsula's Exclusive Economic Zone (EEZ), utilising sediment core and surface sediment analyses. In Korean waters, existing sedimentary carbon data are mostly limited to the surface layers, which are highly vulnerable to physical and chemical disturbances, resulting in considerable uncertainty in carbon storage evaluations. Through this project, we aim to address these critical limitations.

For this study, we collected sediment core samples from 48 stations in Korean EEZ waters, and sediments were analysed for surface and core sediment properties, water content, total carbon (TC), total inorganic carbon (TIC), and total organic carbon (TOC). Water content was calculated from the weights of wet and dry sediment and used as a proxy for sediment porosity and structural characteristics. Contrary to general expectations, our results show that samples characterised by lower mean surface grain size values (φ) exhibit relatively high-water content (R² = 0.4658) and a weak negative relationship to TOC (R² = 0.3061), resulting in elevated TOC values in sediments that are texturally classified as sandy. Furthermore, TOC concentrations show a weak positive relationship with increasing water content (R² = 0.2326), suggesting possible mechanisms such as poorly sorted sands containing interstitial fine particles, enhanced intragranular porosity associated with biogenic or carbonate-rich sands, and rapid burial of organic matter under energetic depositional conditions that limit early diagenetic degradation.

Our findings highlight the limitations of using mean surface grain size as a key predictor of surface sedimentary organic carbon and emphasise the importance of incorporating water content and sediment physical properties when assessing carbon storage potential. Understanding these physical and biogeochemical controls is crucial for enhancing estimates of marine carbon burial and for the development of robust frameworks to assess the role of seafloor sediments in climate mitigation strategies.