Evaluation of the Potential for Dissolved Oxygen Ingress into Deep Sedimentary Basins during a Glaciation Event

Sergio A Bea,Kerry T B Macquarrie,Danyang Su,K Ulrich Mayer

doi:10.1155/2018/9475741

Abstract

Geochemical conditions in intracratonic sedimentary basins are currently reducing, even at relatively shallow depths. However, during glaciation-deglaciation events, glacial meltwater production may result in enhanced recharge (Bea et al., 2011; and Bea et al., 2016) potentially having high concentrations of dissolved oxygen (O2). In this study, the reactive transport code Par-MIN3P-THCm was used to perform an informed, illustrative set of simulations assessing the depth of penetration of low salinity, O2-rich, subglacial recharge. Simulation results indicate that the large-scale basin hydrostratigraphy, in combination with the presence of dense brines at depth, results in low groundwater velocities during glacial meltwater infiltration, restricting the vertical ingress of dilute recharge waters. Furthermore, several geochemical attenuation mechanisms exist for O2, which is consumed by reactions with reduced mineral phases and solid organic matter (SOM). The modeling showed that effective oxidative mineral dissolution rates and SOM oxidation rates between 5 × 10−15 and 6 × 10−13 mol dm−3 bulk s−1 were sufficient to restrict the depth of O2 ingress to less than 200 m. These effective rates are low and thus conservative, in comparison to rates reported in the literature. Additional simulations with more realistic, yet still conservative, parameters reaffirm the limited ability for O2 to penetrate into sedimentary basin rocks during a glaciation-deglaciation event.

Highlights

In intracratonic sedimentary basins, reducing geochemical conditions are widespread and commonly dominate even at shallow depths close to the ground surface
One of the explanations for the rapid development of reducing conditions with increasing depth is that O2 in the recharge water is consumed due to interactions with redox-buffering minerals present as coatings on the aquifer matrix and solid organic matter (SOM) [2, 3]
The objective of this paper is to evaluate dissolved O2 ingress and attenuation in a hypothetical sedimentary basin during a glaciation/deglaciation event by exploring key physical and geochemical parameters that affect the persistence of O2, thereby contributing to an improved understanding of the phenomena governing redox stability of deep geological repositories in sedimentary environments

Summary

Introduction

In intracratonic sedimentary basins, reducing geochemical conditions are widespread and commonly dominate even at shallow depths close to the ground surface. McIntosh et al [7] summarized geochemical and isotopic evidence for freshwater ingress in North American Paleozoic basins as a consequence of glaciation cycles. The source of the freshwater recharge was inferred to be subglacial meltwater, because the 14C ages correspond to periods of ice cover, and the low δ18O values provide evidence for mixing of Geofluids ice sheet derived waters with saline brines and modern precipitation [10, 11]. Several other associated studies reported evidence for recharge and storage of glacial meltwaters in the Michigan Basin, documented by low salinity, 14C ages ranging from modern to >50 ka, and oxygen isotope values ranging from −18 to −13‰ [4, 12,13,14]. Noble gas signatures have been used to infer the ingress of cold glacial meltwaters [15, 16]

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Results

Conclusion