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

Abstract

Mineralization was studied within 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 (SO42-, HCO3-, PO43-, NH4+, H2S, and CH4), to solid-phase chemistry (C, S, N, and Fe), and to bacterial distributions. Sulfate penetrated 0.9 > 3.5 m into the sediment. Sulfate reduction rates decreased > 100-fold from maxima of 6 to 74 nmol cm-3 d-1 at the surface to between 0.1 and 1 nmol cm-3 d-1 at 1 to 2 m depth. Between 8 and 88% of the total, depth-integrated sulfate reduction took place within the uppermost 0 to 15 cm of the sediment. Maxima of sulfate reduction or bacterial densities 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 210Pb age determinations were 0.3 to 6.2 mm yr-1. The total deposition of organic 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 mmol m-2 d-1. This was equivalent to about 50% to the primary productivity in the water column. The net burial rates of organic C were 1.5 to 26 mmol m-2 d-1 corresponding to 9 to 50% of the deposited organic C. The burial of pyritic sulfur corresponded to 9 to 37% of the reducing equivalents buried as organic C.