The influence of buried kimberlite on methane production in overlying sediment, Attawapiskat region, James Bay Lowlands, Ontario

摘要

Shallow groundwaters were collected over and near buried kimberlites in the Attawapiskat River region of the James Bay Lowlands, Ontario, Canada in order to study the impact kimberlites have on CO_2-CH_4 systematics. Groundwaters collected fromboreholes in kimberlites and limestone, and fromgroundwaters in overlying Tyrell Sea sediment (TSS) were analyzed for δ~(13)C_(DIC), δ~2H_(H2O), δ~(18)O_(H2O), dissolved inorganic carbon (DIC), and metal concentrations. Methane gas samples from borehole and TSS groundwaters were analyzed for concentration, δ~(13)C_(CH4), and δ~2H_(CH4). The CH_4 concentrations and Δ~(13)C_(DIC-CH4) (isotope separation) values indicate biological carbonate reduction in TSS groundwaters overlying kimberlites. Whereas, Δ~(13)C_(DIC-CH4) values from TSS groundwaters over limestone and from boreholes within limestone and kimberlite indicate the biological consumption of methane (oxidation). The δ~2H_(H2O) values from TSS over kimberlites are consistent with the variation in Δ~(13)C_(DIC-CH4), as they are less negative compared to where they should fall on the local meteoric water line, suggesting that methanogens are using lighter δ~2H_(H2O) values to produce CH_4. Biological DIC reduction requires H~+ ions from H_2O to form CH_4. There is evidence in the water geochemistry to support the isotopic results, as the ratio of methane to calculated Fe3+ (as amorphous Fe hydroxide), SO_4~(2?), and O_(2(aq)) is largest in the majority of TSS groundwaters over kimberlites (where Δ~(13)C_(DIC-CH4) values indicate CH_4 production). Low temperature serpentinization of olivine in kimberlite is not considered for CH_4 production, as redox conditions in kimberlite groundwaters do not support abiogenic methane production. The findings here suggest that kimberlites are indirectly influencing the CO_2-CH_4 system by consuming oxidized ions in the overlying TSS, thereby creating a favorable environment formethane producing bacteria. In contrast, isotopes and geochemistry suggest methane oxidation in areas overlying limestone. The broader implication of this study is that variable lithology underlying sediment cover may impact biological methane production or consumption.