Pore water alkalinity below the permanent halocline in the Gdańsk Deep (Baltic Sea) - Concentration variability and benthic fluxes

In the present work, spatial and temporal variability of benthic fluxes of total alkalinity (AT) and the effect of pyrite burial on sedimentary AT release was studied in the Gdańsk Deep (max. depth of 118 m) located in the southern Baltic Sea. Cores of surface sediment were collected in summer and winter at three locations. Concentrations of carbonate and non-carbonate (dissolved sulfide, ammonia, phosphate, silicate, borate and sulfate) constituents of AT were analyzed in pore water and their sediment-water diffusive fluxes were estimated using Fick's I Law. Benthic flux of AT was calculated as the sum of fluxes of particular components. Pyrite burial rate in sediment was estimated based on analysis of pyrite sulfur. The average diffusive flux of AT was 1397 ± 511 μmol m−2 day−1 and was dominated by bicarbonate and carbonate. The main non-carbonate component of AT flux was hydrogen sulfide with contribution of 1-30%. Assuming complete oxidation of sulfide released from the sediment to the bottom water, the net flux of AT in the study area was 1263 ± 518 μmol m−2 day−1. The average pyrite burial rate estimated for the last several hundred years was 242 ± 28 μmol m−2 day−1. This indicates that pyrite formation on average generates 38% of the net AT flux from sediment into the water column of the Gdańsk Deep. These findings suggest that anaerobic respiration of organic matter and subsequent pyrite formation may have considerable effect on the benthic fluxes of alkalinity in the Baltic Sea.