Lateral Transport Controls Particulate Organic Carbon Stocks and Fluxes in the Baltic Sea

Abstract. The sediments of the Baltic Sea represent a substantial regional carbon sink. Yet, detailed mapping of organic carbon (OC) content and stock in the Baltic Sea is lacking, and our understanding of the transport pathways of particulate OC (POC) in the water column is limited. Here, we generate high-resolution (500×500 m) maps of surface OC content from available data using a deep neural network. The results are combined with porosity and Holocene sediment thickness maps to derive OC stocks per maritime zone of each Baltic Sea country. The total surface (top 10 cm) OC stock is estimated to 1.29 ± 0.36 GtC and the spatially averaged surface stock to 3.14 ± 0.86 kgC m^-2. A process-based, 3D numerical model is then used to simulate fluxes of resuspended POC. The results imply that horizontal transport of resuspended POC, rather than in-situ biological production, is the key factor determining the flux and distribution of sediment OC. Net horizontal transport of resuspended POC across maritime boundaries reaches the order of 1 MtC yr^-1, with substantial interannual variability. These fluxes are in the same magnitude as the recent net OC accumulation rate, underscoring the importance of laterally derived, allochthonous carbon in sedimentary Blue Carbon habitats. Regional numerical modelling may be useful in addressing the issues of double counting and additionality in Blue Carbon accounting and management.