Sulphur isotope ratios in sulphate and oxygen isotopes in water from a small watershed in central Sweden

• During 1988-89 water samples for sulphur and oxygen isotope measurements were collected in the Lake Mjösjön watershed (7.3 km2), central Sweden. Samples included: precipitation, throughfall, lakewater, shallow groundwater and inlet and outlet streams. The δ34S of sulphate in precipitation ranged from + 6.41‰ in winter to + 3.88‰ in summer, the higher winter values attributed to seasonal differences in the kinetic and equilibrium isotope fractionation during oxidation of atmospheric sulphur dioxide to sulphate. The δ34S in rain samples and in pine and spruce throughfall were similar, indicating no gain of sulphur from the trees. In the inflowing stream, the δ34S value increased as discharge decreased, from + 5.57‰ in spring to + 26.21‰ in summer, indicating bacterial sulphate reduction. The fluctuations in the inlet water were damped by the lake and in the outlet water, only a small decrease in the δ34S value during spring discharge was observed. During winter 1988-89, the near surface waters in the lake showed the same δ34S as snow indicating that meltwater governs the isotopic composition. During the winter, the δ34S in the near bottom waters increased while oxygen decreased due to bacterial sulphate reduction in the sediments. This also caused an increase in the alkalinity in the near bottom waters. Based on the δ18O data the water within the watershed is derived largely from meteoric water. During spring discharge, meltwater governs the inflow and outflow stream while additional groundwater influences occurred during the drier period. Most sulphur is derived from atmospheric deposition and the δ34S in sulphate increased during passage through the watershed due to bacterial sulphate reduction.

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NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geokemi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geochemistry (hsv//eng)

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