ABSTRACT
Subglacial landscapes, revealed in regions of recent ice-sheet retreat, provide a window into ice-sheet dynamics and interactions with evolving subglacial topography. Here, we document landscape evolution beneath the southern Laurentide Ice Sheet of North America since the end of the Pliocene, 2.6 million years (Ma) ago, by reconstructing the isostatically adjusted preglacial surface and modern bedrock topography at 250 m horizontal resolution. We use flow routing to reconstruct drainage networks and river longitudinal profiles, revealing the pattern and extent of their glacially forced reorganization. The overall mean Quaternary (2.6 Ma ago to present) erosion rate is 27 m/Ma, rising within ice-streaming corridors to 35 m/Ma (and locally reaching 400 m/Ma) and falling to 22 m/Ma in nonice-streaming regions. Our results suggest that subglacial erosion was sufficient to lower the southern Laurentide Ice Sheet into warmer environments, thereby enhancing ablation and reducing ice-sheet extent over time.
ABSTRACT
The imprint of glacial isostatic adjustment has long been recognized in shoreline elevations of oceans and proglacial lakes, but to date, its signature has not been identified in river long profiles. Here, we reveal that the buried bedrock valley floor of the upper Mississippi River exhibits a 110-m-deep, 300-km-long overdeepening that we interpret to be a partial cast of the Laurentide Ice Sheet forebulge, the ring of flexurally raised lithosphere surrounding the ice sheet. Incision through this forebulge occurred during a single glacial cycle at some time between 2.5 and 0.8 million years before present, when ice-sheet advance forced former St. Lawrence River tributaries in Minnesota and Wisconsin to flow southward. This integrated for the first time the modern Mississippi River, permanently changing continental-scale hydrology and carving a bedrock valley through the migrating forebulge with sediment-poor water. The shape of the inferred forebulge is consistent with an ice sheet ~1 km thick near its margins, similar to the Laurentide Ice Sheet at the Last Glacial Maximum, and provides evidence of the impact of geodynamic processes on geomorphology even in the midst of a stable craton.
ABSTRACT
Long term monitoring of the physical and chemical effects of using coal-combustion residues (CCRs), in particular fixated flue gas desulfurization (FGD) sludge, as a major component in the reclamation of a pyritic refuse deposit was undertaken to determine the beneficial and detrimental consequences of placing these controversial materials in an unrestricted environment. Monitoring wells, neutron probe access tubes, and weirs were installed before and after reclamation to observe hydrologic conditions and determine how the use of FGD sludge as a recharge barrier was affecting hydrochemical response to ambient weather conditions. Data were collected for six months prior to reclamation and then for an additional 13 years (more intensively during the first 5 years). Statistical analyses of water levels in the pyritic refuse deposit indicate a shift from precipitation- to barometric-controlled fluctuations. These findings, along with minimal variability in soil moisture within the CCR cap and transient perching of groundwater above the cap, are evidence that recharge of the refuse aquifer has been minimized. Statistically significant improvements in the quality of groundwater on-site and surface water leaving the site include long-term declines in acidity, As, and Fe concentrations within the refuse aquifer, attributed to a decrease in recharge of oxygenated water as supported by an analysis of calculated mineral saturation indices. Long-term declines in acidity and associated trace metals discharging from the site are attributed to the post-reclamation loss of sulfate salts brought to the surface by capillary forces. The results of this study indicate that strategic usage of CCRs in reclamation programs can produce beneficial effects, including acid drainage reductions, that are beyond those achieved using traditional reclamation approaches such as the utilization of mine spoil as capping and fill material.
