Sedimentary carbon stocks and microbial turnover in shallow coastal areas of the Baltic Sea

Abstract. Coastal sediments play a significant role in active global carbon cycling, and their potential is well recognised for carbon sequestration. The role of shallow nearshore sediments as carbon reservoirs remains poorly constrained, particularly in heterogeneous depositional environments with varying hydrodynamic forcing, sediment texture, and organic-matter composition. Such spatial variability may decouple sediment carbon stocks from remineralisation rates, requiring carbon storage to be evaluated together with active remineralisation. To address spatial patterns in sedimentary organic carbon (C_org) stock and turnover potential, we sampled 20 sites at 3–4 m water depth along a strong gradient of salinity and wave exposure in the coastal zone of Hanko peninsula, Finland. At each site, two cores were collected for sediment and porewater to quantify particulate and dissolved carbon and nitrogen. Sedimentary C_org stocks in the uppermost 25 cm ranged from 77 to 3903 g C m^-2. The maximum value for fractional turnover of the stock in the uppermost 10 cm, estimated from diffusive fluxes of dissolved inorganic carbon and ammonium, was ~14 % yr^-1. C_org stocks decreased with increasing depth-attenuated wave exposure, consistent with hydrodynamic sorting and reduced fine-sediment retention in exposed areas. We found that the highest proportional turnover occurred in carbon-rich sediments, implying that large C_org stocks are also zones of more active remineralisation. Furthermore, we explored whether bulk organic-matter source indicators, including C/N and δ¹³C, are related to fractional turnover estimates. This exercise indicated that bulk organic-matter source indicators covaried with depositional setting rather than directly controlling carbon reactivity. Our findings highlight the importance of sheltered nearshore sediments for carbon storage while showing that depositional carbon-rich areas can remain active zones of microbial remineralisation. Spatial assessments of shallow coastal carbon storage should therefore consider C_org stocks together with dynamic remineralisation to better constrain their carbon retention and sequestration potential.