The Baltic Sea is an ideal natural laboratory to study the methane cycle in the framework of diagenetic processes. With its brackish character and a gradient from nearly marine to almost limnic conditions, a strong permanent haline stratification leading to large vertical redox gradients in the water column, and a sedimentation history which resulted in the deposition of organic-rich young post-glacial sediments over older glacial and post-glacial strata with very low organic content, the Baltic allows to study the role of a variety of key parameters for early diagenetic processes including the methane cycle. Within the BONUS + Project “Baltic Gas”, a 3.5 week scientific expedition of RV Maria S. Merian in August 2010 was dedicated to study the methane cycle in the various basins of the Baltic Sea, with strong emphasis on the metabolic reactions of early diagenesis and the occurrence of shallow gas deposits. Various subbottom profiling systems were used to map the thickness and structure of organic-rich deposits and build the base for a detailed coring program for biogeochemical analysis, including methane, sulfur compounds, iron, and other compounds. Methane gradients in connection with the information of the areal extend of organic-rich deposits are used to estimate the diffusive flux from the sediments into the water column and the rate of methane oxidation, with changing importance of sulfate as oxidant along the salinity gradient. On selected key stations, rate measurements of methanogenic and methanotrophic reactions were executed. The methane distribution in the water column was comprehensively assessed, revealing amongst other findings a drastic increase in bottom water methane concentration between the post bloom summer situation and the situation in the winter of 2009, in connection to the occurrence of a benthic nepheloid layer. Air-sea flux measurements were executed along the ship’s track comprising all major basins of the Baltic. The talk gives an interdisciplinary overview of the first results of this research campaign.
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