IMPACTS OF PLEISTOCENE GLACIATION ON HYDROBIOGEOCHEMICAL PROCESSES IN SEDIMENTARY BASINS

Past and present continental glaciation is an important driver of basinal-scale fluid and solute transport, as well as biogeochemical processes at depth in sedimentary basins. Recent empirical and numerical modeling studies of formation waters from multiple aquifers within the Illinois, Michigan and Appalachian basins illustrate the timing, mechanisms, and magnitude of Pleistocene freshwater circulation. In addition, isotopic and microbial studies of natural gas reservoirs associated with Pleistocene age groundwater demonstrate the importance of freshwater recharge on enhancing biogenic gas generation. Influx of freshwater may have added terminal electron acceptors that actually served to oxidize hydrocarbons and were exhausted prior to methanogenesis. Groundwater flow systems within sedimentary basins are often highly impacted for relatively short periods of time (tens of thousands of years) as the ice sheets advance and retreat across sedimentary basins. Volumetrically significant quantities of relatively fresh groundwater are emplaced episodically during periods of glaciation. However, in some situations, permafrost cover blocks recharge. Because periods of glaciation in the midcontinent region are infrequent, a clear glacial end-member isotopic signature is not always apparent. The current distribution of geochemical tracers is likely the result of many cycles of glaciation. In some settings, overturns in geochemical profiles (e.g. saline water overlaying freshwater) suggest that geochemical conditions are far from equilibrium. Synthesis of ice sheet-permafrost-aquifer interactions in other sedimentary basins worldwide provides further insight into particularly useful natural tracers of the timing and source of freshwater recharge, penetration depths of glacial meltwater, extent of freshwater dilution of basinal brines, and distribution of biogenic gas accumulations. Glaciated sedimentary basins may be future storage sites of anthropogenic waste (e.g. radionuclides and carbon dioxide); thus, it is important to understand how future continental glaciation may alter subsurface hydrologic and biogeochemical conditions. In addition, Pleistocene age groundwater is an important, high quality, fossil water resource in northern latitude regions worldwide.