Sulphate reduction and anaerobic methane oxidation in sediments of Lake Grevelingen
Mineralogy, Petrology and Geochemistry
Final Report Abstract
Human induced eutrophication in the Baltic Sea area due to enhanced nutrient loads from land build a great challenge for the filter function of Baltic Sea coastal areas. A consequence is the spreading of hypoxia in the in the deeper parts of the Baltic coastal zone. Under oxic conditions phosphorus burial in surface sediments is known to be highly effective but phosphorus burial under hypoxic/anoxic conditions is still insufficiently understood. Therefore, a 4 m piston core was taken from the seasonally hypoxic Erstaviken (Stockholm Archipelago) on board of RV “Electra” in March 2017 and sliced under anoxic conditions to study how different stages of hypoxia in the past influenced phosphorus burial in the basin sediments. Sequential phosphorus extraction by the SEDEX method revealed shifting importance of different phosphorus burial species allowing a division into three stages: 1. Hypoxic conditions in the Medieval Climate Anomaly (MCA) was marked by high phosphorus burial being mainly attributed to phosphorus binding to Fe(II)-phases but also to phosphorus incorporation in Ca-rhodochrosite and authigenic apatite. 2. The following period of cooling (Little Ice Age) resulted in a lower biological productivity and reduced P burial. This is at least visible in a long core taken from the same position in 2015. The core taken for this study, showed still higher organic carbon and phosphorus enrichments well into the period of LIA. Nevertheless, Mn-rich vivianite appeared to be a key sink for phosphorus especially at the end of this period when productivity increased again due to enhanced anthropogenic activity. 3. Modern sediments reflected new establishment of hypoxic conditions with high contents of organic carbon and organic phosphorus. However, phosphorus burial was not as efficient as in medieval hypoxic periods with phosphorus burial rates not increasing in proportion with the increased sediment accumulation rates. Additional microstructural analyses on resin-embedded sediments by µXRF- and SEM-EDS will provide further certainty about the binding forms of P and occurrence of vivianite in the LIA sediments.
