Premium
Sulphur cycling in organic‐rich marine sediments from a Scottish fjord
Author(s) -
BOTTRELL SIMON H.,
MORTIMER ROBERT J.G.,
DAVIES IAN M.,
MARTYN HARVEY S.,
KROM MICHAEL D.
Publication year - 2009
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1111/j.1365-3091.2008.01024.x
Subject(s) - anoxic waters , sulfur , biogeochemical cycle , geology , sediment , pore water pressure , environmental chemistry , fjord , pyrite , cycling , mineralogy , chemistry , oceanography , geomorphology , geotechnical engineering , organic chemistry , archaeology , history
In this study, the biogeochemical transformations of sulphur in organic‐rich marine sediments in a Scottish fjord are investigated by a combination of pore water and sediment geochemistry with sulphide diffusive gradient thin‐film probes and sulphate isotopic data (δ 34 S and δ 18 O). Particular attention is paid to sulphur cycling in the upper sediment profile where sulphate reduction occurs but free sulphide is below the detection limits of conventional pore water geochemical analysis but quantifiable by sulphide diffusive gradient thin film. In the uppermost part of the sediment core, δ 18 O sulphate decreased from near‐sea water values to +7‰, indicating that anoxic sulphide oxidation dominated during this interval. Sulphate δ 34 S remained unchanged as there was no net sulphate reduction (i.e. reduction was balanced by re‐oxidation). Below 4 cm depth, there was a slight increase in sulphate δ 34 S from 20‰ to 23‰ associated with minor accumulation of iron sulphide. The δ 18 O of the sulphate also increased, to around +10‰ at 10 cm depth, as a result of the isotopic exchange of sulphate–oxygen with pore water and/or sulphur disproportionation reactions mediated during sulphur cycling. These processes continued to increase the δ 18 O of the sulphate to 14‰ at 20 cm depth with no further change in the δ 34 S of the sulphate. Below 20 cm depth, free sulphide is detectable in pore waters and both the δ 34 S of the sulphate and sulphide increase with depth with an offset controlled by kinetic fractionation during bacterial sulphate reduction. The δ 34 S of the sedimentary organic fraction shifted towards lower, more bacteriogenic, values with depth in the profile, without any increase in the size of this sulphur pool. Thus, the organic sulphur fraction was open to interaction with bacteriogenic sulphide without the occurrence of net addition. Therefore, caution should be exercised when using sulphur isotopic compositions to infer simple net addition of bacteriogenic sulphide to the organic sulphur fraction.