Early diagenesis in Baltic Sea sediments along a redox gradient: A combined field data and reactive transport modeling study

Abstract

The restricted bathymetry of the Baltic Sea allows a strong halocline to form, limiting downward oxygen diffusion and leading to hypoxia of the deeper waters. Sediment biogeochemical cycles respond strongly to contrasting redox conditions, complicating efforts to mitigate hypoxia via reductions in anthropogenic nutrient inputs. Here, we combine field results along a water-depth and redox gradient into the Fårö Basin with reactive transport modeling of the sediment geochemistry, to better understand the factors controlling carbon and phosphorus burial in the central Baltic. Depth profiles of all major sediment and pore water constituents of the most oxic site (LF1) on the redox gradient were reproduced by the model. This modeled site was then used to reproduce the depth profiles of seasonally hypoxic (LF1.5), hypoxic (LF3) and sulfidic (F80) sites by setting the bottom water oxygen to in-situ values. The sulfidic site (F80) could not be reproduced by the model, suggesting an incomplete understanding of the processes occurring in the deep basins. In the three remaining sites, beside bottom water O2, several other parameters needed to be varied to fit the model to the sediment and pore water profiles of LF1.5 and LF3, implying a high degree of spatial heterogeneity in the sedimentary environment. Specifically, porosity, organic matter-deposition rate, and the biodiffusion coefficient are important forcings in the remineralization of organic matter, and may deviate independently of bottom water O2. At all three modeled sites, an increasing flux of organic matter through the last century was implemented to reproduce the organic carbon depth profiles. This corresponds to a 2-fold increase of primary productivity, similar to the magnitude calculated by other studies. Aerobic respiration is the dominant pathway of organic matter breakdown at all three modeled sites. However, the relative importance of this pathway decreases down-transect, compensated by elevated rates of sulfate reduction and methanogenesis. This shift is partly mitigated by a lower biodiffusion coefficient down-transect, which reduces the efficiency of downwards mixing of material through the sediment column. The majority of remineralization at the modeled sites takes place in a thin layer of the surface sediments. Preferential release of phosphorus from organic matter breakdown in this surface layer increases down-transect, indicated by increasing Corg:Porg ratios in the surface sediments. Field data potentially indicates vivianite formation in the deep basins (F80) of the Baltic Sea.