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Silicate weathering in anoxic marine sediments
Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances,...
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| Published in: | Geochimica et cosmochimica acta 2008-06, Vol.72 (12), p.2895-2918 |
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| Main Authors: | , , , , , , , |
| Format: | Article |
| Language: | English |
| Subjects: | |
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| Online Access: | Get full text |
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| Summary: | Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances, and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration. Pore waters separated from the sediment cores are highly enriched in dissolved magnesium, total alkalinity, humic substances, and boron. The high contents of dissolved organic carbon in the deeper methanogenic sediment sections (50–150mgdm−3) may promote the dissolution of silicate phases through complexation of Al3+ and other structure-building cations. A non-steady state transport-reaction model was developed and applied to evaluate the down-core trends observed in the solid and dissolved phases. Dissolved Mg and total alkalinity were used to track the in-situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces. The modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments. Depth-integrated rates of marine silicate weathering in methanogenic sediments derived from the model (81.4–99.2mmol CO2m−2year−1) are lower than the marine weathering rates calculated from the solid phase data (198–245mmol CO2m−2year−1) suggesting a decrease in marine weathering over time. The production of CO2 through reverse weathering in surface sediments (4.22–15.0mmol CO2m−2year−1) is about one order of magnitude smaller than the weathering-induced CO2 consumption in the underlying sediments. The evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments. The global rate of CO2 consumption through marine silicate weathering estimated here as 5–20Tmol CO2year−1 is as high as the global rate of continental silicate w |
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| ISSN: | 0016-7037 1872-9533 0016-7037 |
| DOI: | 10.1016/j.gca.2008.03.026 |