In paleohydrogeological studies, the geochemical and isotope geochemical composition of fracture calcites can be utilised to gain information about the evolution of the composition of deep groundwaters in crystalline bedrock. The aim of our study was to investigate the latest hydrogeochemical evolution of groundwaters in the crystalline bedrock at Olkiluoto, which is the planned site for deep geological disposal of spent nuclear fuel. Samples were collected from drill cores intercepting water-conducting fractures at the upper ~500 m of the bedrock. The latest fracture calcite generations were identified using optical microscopy and electron microprobe. They occur as thin ~10-200 μm crusts or small euhedral crystals on open fracture surfaces. These latest calcite fillings were carefully sampled and analysed for the isotopic composition on carbon and oxygen. In addition, fluid inclusion homogenisation temperatures were determined on selected calcite samples. Fluid inclusion data indicated a low temperature of formation for the latest fracture calcite fillings. The δ(18)O values of calcite in these fracture fillings vary only slightly, from-7.3 to-11.5 ‰ (Vienna Pee Dee Belemnite, VPDB), whereas the δ(13)C values fluctuate widely, from-30 to+31 ‰ (VPDB). The δ(13)C values of latest calcite fillings show a systematic pattern with depth, with high and variable δ(13)C values below 50 m. The high δ(13)C values indicate active methanogenesis during the formation of the latest calcite fillings. In contrast, the present-day methanic redox environment is restricted to depths below 200-300 m. It is possible that the shift in the redox environment at Olkiluoto has occurred during infiltration of SO2-(4)-rich marine waters, the latest of such events being the infiltration of brackish waters of the Littorina Sea stage of the Baltic Sea at ~8000-3000 BP.
We explore needle sugar isotopic compositions (δ 18 O and δ 13 C) in boreal Scots pine (Pinus sylvestris) over two growing seasons.A leaf-level dynamic model driven by environmental conditions and based on current understanding of isotope fractionation processes was built to predict δ 18 O and δ 13 C of two hierarchical needle carbohydrate pools, accounting for the needle sugar pool size and the presence of an invariant pinitol pool.Model results agreed well with observed needle water δ 18 O, δ 18 O and δ 13 C of needle water-soluble carbohydrates (sugars + pinitol), and needle sugar δ 13 C (R 2 = 0.95, 0.84, 0.60, 0.73, respectively). Relative humidity (RH) and intercellular to ambient CO 2 concentration ratio (C i /C a ) were the dominant drivers of δ 18 O and δ 13 C variability, respectively. However, the variability of needle sugar δ 18 O and δ 13 C was reduced on diel and intra-seasonal timescales, compared to predictions based on instantaneous RH and C i /C a , due to the large needle sugar pool, which caused the signal formation period to vary seasonally from 2 d to more than 5 d. Furthermore, accounting for a temperature-sensitive biochemical 18 Ofractionation factor and mesophyll resistance in 13 C-discrimination were critical.Interpreting leaf-level isotopic signals requires understanding on time integration caused by mixing in the needle sugar pool.
Variation in 13 C/ 12 C-isotope ratios of fracture filling calcite was analyzed in situ to investigate carbon sources and cycling in fractured bedrock. The study was conducted by separating sections of fracture fillings, and analyzing the 13 C/ 12 C-ratios with secondary ion mass spectrometry (SIMS). Specifically, the study was aimed at fillings where previously published sulfur isotope data indicated the occurrence of bacterial sulfate reduction. The results showed that the δ 13 C values of calcite were highly variable, ranging from-53.8‰ to +31.8‰ (VPDB). The analysis also showed high variations within single fillings of up to 39‰. The analyzed calcite fillings were mostly associated with two calcite groups, of which Group 3 represents possible Paleozoic fluid circulation, based on comparison with similar dated coatings within the Baltic Shield and the succeeding Group 1-2 fillings represent late-stage, low temperature mineralization and are possibly late Paleozoic to Quaternary in age. Both generations were associated with pyrite with δ 34 S values indicative of bacterial sulfate reduction. The δ 13 C values of calcite, however, were indicative of geochemical environments which were distinct for these generations. The δ 13 C values of Group 3 calcite varied from-22.1‰ to +11‰, with a distinct peak at-16‰ to-12‰. Furthermore, there were no observable depth dependent trends in the δ 13 C values of Group 3 calcite. The δ 13 C values of Group 3 calcite were indicative of organic matter degradation and methanogenesis. In contrast to the Group 3 fillings, the δ 13 C values of Group 1-2 calcite were highly variable, ranging from-53.8‰ to +31.8‰ and they showed systematic variation with depth. The near surface environment of <30 m (bsl) was characterized by δ C values indicative of degradation of surface derived organic matter, with δ 13 C values ranging from-30.3‰ to-5.5‰. The intermediate depth of 34-54 m showed evidence of localized methanotrophic activity seen as anomalously 13 C depleted calcite, having δ 13 C values as low as-53.8‰. At depths of ~60-400 m, positive δ 13 C values of up to +31.8‰ in late-stage calcite of Group 1-2 indicated methanogenesis. In comparison, high CH4 concentrations in present day groundwaters are found at depths of >300 m. One sample at a depth of 111 m showed a transition from methanogenetic conditions (calcite bearing methanogenetic signature) to sulfate reducing (precipitation of pyrite on calcite surface), however, the timing of this transition is so far unclear. The results from this study gives indications of the complex nature of sulfur and carbon cycling in fractured crystalline environments and highlights the usefulness of in situ stable isotope analysis.
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