The impact of declining oxygen conditions on pyrite accumulation in shelf sediments (Baltic Sea)

Deterioration of oxygen conditions in water below the halocline has been observed in the Baltic Sea. Deoxygenation is linked to the reduced frequency and volume of inflows of highly saline surface water from the North Sea (major Baltic inflows—MBIs) in the second half of the twentieth century and the increased organic matter respiration due to eutrophication. In the present study, the impact of worsening oxygen conditions on pyrite content in the Gdańsk Deep (max. depth of 118 m, southern Baltic Sea) sediments was determined. Geochemical parameters (acid volatile sulfides, pyrite sulfur, reactive iron, organic carbon, sedimentation rate and sediment age) were analyzed in relation to the variation in bottom water oxygen concentration and the occurrence of MBI. The obtained results demonstrate that pyrite content in the study area decreased after 1960. The declining pyrite content coincided with the deterioration of oxygen conditions (concentration < 2 ml l −1 ) in bottom water. In the same period, reactive iron concentration decreased and organic carbon increased in sediment. In the period 1616–1960, average pyrite accumulation rate was 322 µmol m −2 day −1 . In the subsequent years, its average accumulation rate decreased to 210 µmol m −2 day −1 . Fluctuations of oxygenation of bottom water in the study area were manifested by highly variable degree of pyritization (36 ± 11%) and particulate organic carbon to pyrite sulfur ratio (2.8–37).