Anomalous YDB Aquifer Oxygen Isotopes
Analysis of modern and Pleistocene hydrologic exchange between Saginaw Bay (Lake Huron) and the Saginaw Lowlands area
B25290.1
Geological Society of America Bulletin 2004;116;3-15
John R. Hoaglund III, Jonathan J. Kolak, David T. Long and Grahame J. Larson
The exchange of water and solutes between the Michigan Basin and the large, freshwater lakes of the Great Lakes region warrants investigation because the juxtaposition of these entities gives rise to one of the highest known salinity gradients (Fig. 4.7 in Hanor, 1979). We present a numerical analysis of groundwater flow under both modern and Pleistocene conditions to determine temporal variations in the exchange of fluids between the Michigan Basin and Saginaw Bay (Lake Huron). Saline water has been documented near the sedimentwater interface in Lake Michigan (Callender, 1969), Lake Ontario (Drimmie et al., 1992), and in Saginaw Bay (Lake Huron) (Kolak et al., 1999), although the rate of exchange between these saline waters and the overlying water column remains unknown.
In addition to the occurrence of saline water and brine at relatively shallow depths, portions of the Michigan Basin also contain groundwater with a stable isotopic composition that is significantly lighter than modern meteoric recharge (Clayton et al., 1966; Long et al., 1988; Meissner et al., 1996; Wahrer et al., 1996; Ging et al., 1996), indicating the preservation of a paleorecharge signal emplaced when the climate was significantly cooler than today. Attempts to address fluid transport within the Michigan Basin must account for the distribution of the salinity and stable isotope signatures; each signature has a different origin. An extensive study of the hydrogeology, geochemistry, and paleohydrology of the Michigan Basin has been conducted as part of the U.S. Geological Survey Regional Aquifer System Analysis (RASA) initiative (e.g., Mandle and Westjohn, 1989; Westjohn et al., 1994; Westjohn and Weaver, 1996a, b, c; Holtschlag, 1996, 1997; Ging et al., 1996; Meissner et al., 1996; Wahrer et al., 1996; Hoaglund et al., 2002b).
Modeling of ice-induced hydraulic loading from the Port Huron ice advance shows that hydraulic loading produced a groundwaterflow reversal localized to the region of the ice sheet and its proglacial margin. In response to hydraulic loading, vertical gradients between the heads simulated in the Glaciofluvial aquifer and both the Saginaw and Marshall bedrock aquifers show a general pattern of groundwater flow downward under the icel sheet and upward into proglacial Lake Saginaw. There is a strong relation between these areas of simulated downward vertical gradients and the isotopically light groundwater presently observed in the Marshall aquifer. Chloride concentrations and d18O values serve to constrain possible mixing scenarios between the water masses; a multi-event mixing scenario provides a plausible explanation of the geochemical signature observed in porewater samples from the Saginaw Lowlands area.
The geochemical evolution of Saginaw Bay porewater samples is less well constrained, but it appears to have evolved along a different mixing trajectory. The Michigan confining unit plays a prominent role in both the geochemical evolution and present spatial variations of Saginaw Bay porewater chemistry. The Michigan confining unit appears to have limited the extent of subglacial recharge during the Pleistocene, particularly in the Saginaw Bay area, thus preserving a high-chloride source, derived from brine in the Marshall aquifer, below the bay. Fractures in the Michigan confining unit associated with an anticlinal structure presently regulate the degree of communication between Saginaw Bay sediments and the high-chloride source at depth.