Anaerobic mineralization in marine sediments from the Baltic Sea-North Sea transition

Abstract

Mineralization was studied withln the upper 2 m of sediments from the Belt Sea, Kattegat, and Skagerrak at 15 to 200 m water depth.Radiotracer measurements of sulfate reduction rates were related to porewater chemistry (SO,'-, HC03-, P O ~~-, NH4+, H2S, and CH,), to solid-phase chemistry (C, S , N, and Fe), and to bacterial distributions.Sulfate penetrated 0.9 to > 3 .5 m into the sediment.Sulfate reduction rates decreased > 100-fold from m a m a of 6 to 74 nmol cm-3 d-' at the surface to between 0.1 and 1 nmol cm-3 d-' at 1 to 2 m depth.Between 8 and 88 O/O of the total, depth-integrated sulfate reduction took place within the uppermost 0 to 15 cm of the sediment.Maxlma of sulfate reduction or bacterial densibes at the sulfate-methane transition indicated a zone of anaerobic methane oxidation 0.8 to > 2.5 m below the surface.The fraction of the iron pool, which was bound in pyrite, was 17 to 42 %, even in the presence of > 1 mM H2S.Only 4 to 32 % of the H2S produced from sulfate reduction was permanently buried as FeS2 while the rest was reoxidized.Sediment accumulation rates determined from 'I0pb age determinations were 0.3 to 6.2 mm yr-'.The total deposition of organlc carbon, determined from the sum of organic C mineralized by aerobic and anaerobic respiration plus net burial of organic C, was 16.7 to 52.3 mm01 m-' d-'.This was equivalent to about 50 % of the primary productivity in the water column.The net burial rates of organic C were 1.5 to 26 mm01 m-' d-' corresponding to 9 to 50 ' % of the deposited organic C. The bunal of pyritic sulfur corresponded to 9 to 37 O/u of the reducing equivalentes buried as organic C.