IS THERE A RELATIONSHIP BETWEEN METHANE AND SALINITY IN SHIELD WATERS? AN ISOTOPIC SURVEY OF δ81BR AND δ37CL OF DISSOLVED HALIDES, δ13C AND δ2H OF METHANE, AND δ15N OF N2

Methane occurrences of bacteriogenic and abiogenic origins are found with high salinity groundwaters in deep crystalline rocks of both the Canadian and Fennoscandian Shields. The high salinities in the waters come from either various seawater concentration processes or different water-rock interactions. However, there has been no attempt to correlate the methane with the salinity in the waters. New isotopic techniques are now available to investigate a possible relationship between dissolved halides and methane gases in these environments. We present δ⁸¹Br and δ³⁷Cl results from eighty water samples from twenty four different sites in the Fennoscandian and Canadian Shield, and compare them with both previously published and new methane δ¹³C, δ ²H and δ¹⁵N -N₂ results.

Chlorine and bromine stable isotopes were analyzed by isotope ratio mass spectrometry, and range from -0.78‰ to 1.03‰ relative to SMOC and +0.01‰ to +2.04‰ relative to SMOB, respectively. Comparison of δ ⁸¹Br and δ³⁷Cl in groundwaters with the δ²H of associated methane generally show an increasing trend. One the other hand, there is a decreasing trend of δ²H-CH₄ with δ¹⁵N-N₂. Deviations from the trend at three sites indicate there are locations where there are either additional unknown origins and/or processes or no link between the dissolved halide and evolved gas components.

While the evolution of bromide, methane, and nitrogen in the form of NH₄⁺ have been linked during diagenesis of organic carbon in marine systems, chloride in these sedimentary systems comes from seawater or hydrothermal fluids. Thus an organic origin does not explain the link between all of the isotopic systems studied in the crystalline shield environment. It is possible that paleofluids in shield environments contained quantities of CH₃Br and CH₃Cl. Sources of methyl halides in these deep groundwaters are likely to include pyrolysis of methane and subsequent reactions of HCl or HBr in hydrothermal fluids or low-mid temperature metamorphism.