Land-sea continuum: long-term fate of terrestrial organic carbon in coastal environments of the Baltic Sea

Natural climate solutions to climate change are recognised as a mechanism for drawing down carbon from the atmosphere. Together with geoengineering, this represents a potential solution for reducing anthropogenic carbon dioxide (CO₂) concentrations in the atmosphere, which are contributing to global warming. The storage of carbon in marine sediments at the land-sea interface of the continental shelves is a globally significant process. These sediments play a crucial role by providing both sequestration and efficient carbon burial, enabling marine systems to serve as effective natural climate solutions. For the subarctic, there is emerging evidence that at millennial time scales, approximately 50% of organic carbon (OC) buried in shallow-water marine sediments may be organic material derived from terrestrial and inland water systems via riverine discharge. In the Baltic Sea, these processes have mostly been investigated in the deeper basins, and with limited data available on the origins of this stored carbon through time. Furthermore, evidence suggests that multiple anthropogenic influences affecting e.g., the riverine delivery of OC and nutrients and the physical integrity or redox status of sediments could lead to altered storage capacity of the system, including rapid remineralisation of old organic carbon into greenhouse gas. To determine the location of Baltic organic carbon storage hotspots and their sensitivity to anthropogenic stressors, we developed a spatially-explicit predictive model using gridded environmental variables and organic carbon data from our synthesis Baltic Sediment Blue Carbon Database (>12,500 entries). We further use stable isotopes of H, C and N to quantify and determine the source (terrestrial vs. marine) of OC transported to and buried in Swedish coastal sediments of the Gulf of Bothnia along gradients of riverine discharge with contrasting land-use. This approach helps constrain the impact of climate change on the delivery and fate of terrestrially-derived and aquatic OC in coastal sediments and identifies marine burial hotspots to better understand OC processing along the land-sea continuum.