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

Abstract

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 CO2–CH4 systematics. Groundwaters collected from boreholes in kimberlites and limestone, and from groundwaters in overlying Tyrell Sea sediment (TSS) were analyzed for δ13CDIC, δ2HH2O, δ18OH2O, dissolved inorganic carbon (DIC), and metal concentrations. Methane gas samples from borehole and TSS groundwaters were analyzed for concentration, δ13CCH4, and δ2HCH4. The CH4 concentrations and Δ13CDIC–CH4 (isotope separation) values indicate biological carbonate reduction in TSS groundwaters overlying kimberlites. Whereas, Δ13CDIC–CH4 values from TSS groundwaters over limestone and from boreholes within limestone and kimberlite indicate the biological consumption of methane (oxidation). The δ2HH2O values from TSS over kimberlites are consistent with the variation in Δ13CDIC–CH4, as they are less negative compared to where they should fall on the local meteoric water line, suggesting that methanogens are using lighter δ2HH2O values to produce CH4. Biological DIC reduction requires H+ ions from H2O to form CH4. There is evidence in the water geochemistry to support the isotopic results, as the ratio of methane to calculated Fe3+ (as amorphous Fe hydroxide), SO42−, and O2(aq) is largest in the majority of TSS groundwaters over kimberlites (where Δ13CDIC–CH4 values indicate CH4 production). Low temperature serpentinization of olivine in kimberlite is not considered for CH4 production, as redox conditions in kimberlite groundwaters do not support abiogenic methane production. The findings here suggest that kimberlites are indirectly influencing the CO2–CH4 system by consuming oxidized ions in the overlying TSS, thereby creating a favorable environment for methane 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.