Fichtner Vanessa, Kirchner Ferdinand, Kutzschbach Martin, Strauss Harald, Tillberg Mikael, Whitehouse Martin, Drake Henrik
Department of Biology and Environmental Science, Linnæus University, Kalmar, Sweden.
Department of Applied Geochemistry, Technische Universität Berlin, Berlin, Germany.
Isotopes Environ Health Stud. 2024 Dec;60(6):557-584. doi: 10.1080/10256016.2024.2410293. Epub 2024 Oct 4.
The stable isotope compositions of sulphur (δS) and oxygen (δO) in barite are frequently used as proxies for microbial sulphate reduction (MSR) in diverse environments, such as in relation to anaerobic oxidation of methane in marine cold seeps. There, isotopically heavy barite is used as a marker for MSR from a sulphate pool that has undergone semi-closed system conditions. Closed-system MSR is also a commonly observed feature in igneous rock hosted fracture aquifers, as shown by extremely S-enriched pyrite. What is less well-constrained is whether δS in barite can be used as a proxy for MSR in such systems. Here we explore the microscale heterogeneity of δS and δO via secondary ion mass spectrometry and the trace element Sr via LA-ICP-MS maps in barite precipitated in granite-hosted boreholes during a 17-year experiment, at Äspö, Sweden. We compare it with δO, δS, and δS of the fracture fluids and with paragenetic pyrite with δS values reflecting closed system MSR. The δO values in barite (+9.4 to +16.9 ‰) represent two generations of barite, one with low values and one with high values. The latter are likely impacted by sulphur disproportionating or -oxidizing bacteria. The barite reflects a much smaller span in δS (+14.5 to +28.6 ‰) than the pyrite (-47.2 to +53.3 ‰). This lack of extremely high δS values is proposed to be due to that barite saturation only occurred in the early parts of the Rayleigh cycle. Additionally, fluid migration has affected the δS values to lower values, accompanied by higher Sr concentrations. Taken together, barite δS values cannot be regarded as a reliable independent proxy for MSR in deep sulphate-poor igneous rock hosted aquifers. However, the relation between the δS values of coeval barite and pyrite is regarded as a useful proxy for MSR-related fractionation during early stages of MSR.
重晶石中硫（δS）和氧（δO）的稳定同位素组成常被用作不同环境中微生物硫酸盐还原（MSR）的代理指标，比如与海洋冷泉中甲烷的厌氧氧化有关。在那里，同位素较重的重晶石被用作来自经历了半封闭系统条件的硫酸盐库的MSR的标志物。封闭系统的MSR也是火成岩裂隙含水层中常见的特征，如极度富硫的黄铁矿所示。重晶石中的δS是否能用作此类系统中MSR的代理指标，目前还不太明确。在此，我们通过二次离子质谱法研究了δS和δO的微观尺度非均质性，并通过激光烧蚀电感耦合等离子体质谱图研究了瑞典阿斯波一个17年实验中花岗岩钻孔中沉淀的重晶石中的微量元素锶。我们将其与裂隙流体的δO、δS和δS以及反映封闭系统MSR的共生黄铁矿的δS值进行了比较。重晶石中的δO值（+9.4至+16.9‰）代表两代重晶石，一代值低，一代值高。后者可能受到硫歧化或氧化细菌的影响。重晶石反映出的δS跨度（+14.5至+28.6‰）比重晶石（-47.2至+53.3‰）小得多。这种缺乏极高δS值的情况被认为是由于重晶石饱和仅发生在瑞利循环的早期阶段。此外，流体迁移已将δS值影响至较低值，同时伴有较高的锶浓度。综上所述，重晶石的δS值不能被视为深部贫硫酸盐火成岩含水层中MSR的可靠独立代理指标。然而，同期重晶石和黄铁矿的δS值之间的关系被认为是MSR早期阶段与MSR相关分馏的有用代理指标。
