Low-permeability Ordovician shale and limestone formations of the Michigan Basin are a target of interest for a deep geological repository for the long-term isolation of low and intermediate-level radioactive waste. Porewater studies contribute to an understanding of the age and evolution of paleofluids as part of ongoing research. Sulfur and oxygen isotope data presented here add to the characterization of these formations. For this work, a method was developed and used to extract porewater sulfate from low permeability rocks for the analysis of sulfate concentration and its δ34S and δ18O values, using an elemental analyzer interfaced with an isotope ratio mass spectrometer (EA-IRMS). Porewater sulfate concentrations were very low, varying from 0.02 to 228.88 mmol/ kgw. Values for δ34S were all near or below those for Ordovician seawater. The earlier work examined the δ34S and the morphology of framboidal pyrite in the Ordovician formations provides strong evidence for diagenetic sulfate reduction (Jautzy et al. in prep.). The lack of enrichment for δ34S and δ18O in porewater sulfate suggests that diagenetic sulfate reduction was quantitative and that the porewater sulfate measured in these samples is not residual sulfate remaining from that time. The distribution of sulfur isotope values between that of Silurian seawater and those of the framboidal pyrite suggests an origin from the infiltration of Silurian brines with contributions from pyrite oxidation. The δ18O values are consistent with this, showing mixtures between Silurian seawater sulfate and sulfide oxidation without the involvement of atmospheric oxygen. This evidence for anoxic sulfide oxidation indicates that inadvertent exposure to atmospheric O2 during porewater leaching in the laboratory is unlikely and that in-situ oxidation involving electron acceptors with lower electromotive potential took place, possibly over geological time. The combination of sulfate mass and the stable isotopes of porewater sulfate, along with the presence of Fe (III) and Mn (IV) bearing minerals indicate potential evidence of in-situ sulfide oxidation in one third of twenty-six core samples. The relative timing of events is not clear as to whether sulfide oxidation could have occurred during the Ordovician and/or after the infiltration of evaporated Silurian seawater into the Ordovician strata. This study adds a new understanding of the redox evolution within this sedimentary system with implications for contaminant transport.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/38742 |
| Date | 24 January 2019 |
| Creators | Zhang, Jing |
| Contributors | Clark, Ian |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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