The Baltic Sea is the largest brackish water body in the world with just 53m average water column depth. It has been eutrophicatic since the earlier 20th century. Baltic Sea sediment is a source of methane - releasing methane into water column and further global emission. I seek to better understand the biogeochemical controls on methane and sulfur cycling by pursuing: 1) High resolution and accurate deep profiles of methane and potential oxidation including iron and sulfur species, 2) The formation and consumption rate of methane, 3) The role of sulfate penetration depth on carbon preservation and the response to increases and decreases in salinity gradients. Samples for my thesis work come from Himmerfjarden Estuary (Sweden), Gdansk Bay (Poland), Aarhus Bay (Denmark) and through out the Baltic Sea. Wet chemistry, molecular marker and isotope analysis are fundamental tools used in my thesis work.