Steady-state interstitial water sulfate O-18/O-16 ratios as a function of salinity
Abstract
At ambient temperature and pH dissolved sulfate does not exchange isotopes with water to significant extent over millions of years. However, due to the formation of intermediate sulphur species during dissimilatory sulfate reduction, fast isotope exchange is observed under microbial mediation. The catalyzed oxygen isotope exchange between pore water sulfate and intracellular water upon sulfate reduction leads to the development of steady-state values for dissolve sulfate. At the same degrees of microbial conversion, sulfur isotope ratios may still proceed to increase due to continuous net sulfate reduction. Whereas the initial ratio of d34S-d18O variations in dissolved sulfate is controlled by metabolic rates (e.g., cellular sulfate reduction rates), the balance between sulfate reduction and sulphide oxidation, and the isotopic composition of cellular water, the steady state value is controlled only by oxygen isotope exchange with water. Although the exact relationship between measured steady-state values and equilibrium is still under debate, an ultimate control by both temperature and the oxygen isotope composition of water is expected. The latter is related to salinity and climate zone. In this study we show the results from interstitial waters obtained at different natural localities covering a wide range of salinity regimes and benthic metabolic rates. Sites include brackish coastal and euxinic systems in the Baltic Sea and the Black Sea, coastal sediments of the North Sea, and deep marine pore waters from the Mediterranean and the Pacific (ODP Legs 160, 161, 182, 201). Results are compared to previous findings by Fritz et al. (1989) on groundwaters and experimental studies by Mizutani & Rafter (1973). The field examples show that the pore waters reach steady-state values depending on the isotopic values of pore waters, and the rates can vary strongly with microbial activity in the specific setting. If conditions allow diagenetic barites to be formed in these environments at the diffusional sulfate-barium interface, these authigenic phases have the potential to serve as proxies for pore water salinity.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.B13D0552W
- Keywords:
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- 0454 BIOGEOSCIENCES / Isotopic composition and chemistry;
- 0489 BIOGEOSCIENCES / Trace element cycling