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

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.

The Impact of Ice Sheet Geometry on Meltwater Ingress and Reactive Solute Transport in Sedimentary Basins

“…The meltwater ingress during retreat is controlled by ice sheet basal pressure heads equal to 90% of ice sheet thickness (e.g., Claesson et al, 2016). The mean total stress 𝐴𝐴 𝐴𝐴𝑧𝑧𝑧𝑧 in the mechanical loading model is one-dimensional and vertically constrained (Bea et al, 2016(Bea et al, , 2018. During ice-free periods the hypothetical basin is assumed to have the highest water table elevation at the center of the basin, in line with present day conditions in the Michigan Basin (McIntosh et al, 2011;Normani et al, 2017).…”

“…On the other hand, lateral flow can result in steeper hydraulic gradients during ice sheet retreat. Such a condition can arise if high hydraulic heads are present below an ice sheet, while hydraulic heads at the margins of the ice sheet are topography controlled (Bea et al, 2018;McIntosh et al, 2011McIntosh et al, , 2012. Under these circumstances, highly saline water at greater depth may be mobilized, leading to deeper ingress of fresh glacial meltwater (McIntosh et al, 2011).…”

MIN3P-HPC: A High-Performance Unstructured Grid Code for Subsurface Flow and Reactive Transport Simulation

“…We use a set of hypothetical parameters to define the simulation cases. The chosen parameters are within the range of values observed in the field (Bain et al 2001;Mayer et al 2001Mayer et al , 2002Bea et al 2016Bea et al , 2018 and thus are suitable for demonstrating the capabilities of our reactive transport code.…”

“…7a and 7(b). Physical and chemical input parameters are given in Table 4 and are within the range of parameters relevant for field conditions (e.g., Bain et al 2001;Mayer et al 2001Mayer et al , 2002Bea et al 2016Bea et al , 2018. Four different mesh sizes were used for both structured and unstructured grids.…”

“…As for the first demonstration example, a solution rich in carbonic acid enters the solution domain across the inflow boundary and reacts with the mineral calcite, and reaction products exit across the upper right boundary. The physical and chemical input parameters are given in Table 5 and are within the range of parameters relevant for field conditions (e.g., Bain et al 2001;Mayer et al 2001Mayer et al , 2002Bea et al 2016Bea et al , 2018. The simulation was carried out for a period of 500 days.…”