TY - JOUR
T1 - Paleowaters in Silurian-Devonian carbonate aquifers
T2 - Geochemical evolution of groundwater in the Great Lakes region since the Late Pleistocene
AU - McIntosh, J. C.
AU - Walter, L. M.
N1 - Funding Information:
Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society (PRF Grant 39126, L.M.W.) for partial support of this research. Additional support was provided by the National Science Foundation (NSF-EAR-0208182, L.M.W.). J. C. McIntosh acknowledges support from the Geological Society of America, the American Association of Petroleum Geologists, the Scott Turner Fund and Rackham Graduate School, University of Michigan, and the Morton K. and Jane Blaustein Foundation. We are especially grateful to Demmy Spounias and Heather Whittington for their invaluable help with planning field sampling expeditions, field sampling, lab analyses, and interpretation of the results. We also thank Patti McCall, Naila Moreira, and Larry Jordan Jr. for field sampling assistance. Corey Lambert, Andrea Klaue, Lora Wingate, and Maria Marcano helped with laboratory analyses. We appreciate Anna Martini and Joel Blum’s comments on an earlier version of this manuscript. W. Berry Lyons and Alan Stueber generously shared unpublished groundwater data with us for comparison. We thank Ian Fairchild and two other anonymous reviewers for their helpful comments.
PY - 2006/5/15
Y1 - 2006/5/15
N2 - Changes in the climatic conditions during the Late Quaternary and Holocene greatly impacted the hydrology and geochemical evolution of groundwaters in the Great Lakes region. Increased hydraulic gradients from melting of kilometer-thick Pleistocene ice sheets reorganized regional-scale groundwater flow in Paleozoic aquifers in underlying intracratonic basins. Here, we present new elemental and isotopic analyses of 134 groundwaters from Silurian-Devonian carbonate and overlying glacial drift aquifers, along the margins of the Illinois and Michigan basins, to evaluate the paleohydrology, age distribution, and geochemical evolution of confined aquifer systems. This study significantly extends the spatial coverage of previously published groundwaters in carbonate and drift aquifers across the Midcontinent region, and extends into deeper portions of the Illinois and Michigan basins, focused on the freshwater-saline water mixing zones. In addition, the hydrogeochemical data from Silurian-Devonian aquifers were integrated with deeper basinal fluids, and brines in Upper Devonian black shales and underlying Cambrian-Ordovician aquifers to reveal a regionally extensive recharge system of Pleistocene-age waters in glaciated sedimentary basins. Elemental and isotope geochemistry of confined groundwaters in Silurian-Devonian carbonate and glacial drift aquifers show that they have been extensively altered by incongruent dissolution of carbonate minerals, dissolution of halite and anhydrite, cation exchange, microbial processes, and mixing with basinal brines. Carbon isotope values of dissolved inorganic carbon (DIC) range from -10 to -2‰, 87Sr/86Sr ratios range from 0.7080 to 0.7090, and δ 34 S s(-) SO4 values range from +10 to 30‰. A few waters have elevated δ13CDIC values (>15‰) from microbial methanogenesis in adjacent organic-rich Upper Devonian shales. Radiocarbon ages and δ18O and δD values of confined groundwaters indicate they originated as subglacial recharge beneath the Laurentide Ice Sheet (14-50 ka BP, -15 to -13‰ δ18O). These paleowaters are isolated from shallow flow systems in overlying glacial drift aquifers by lake-bed clays and/or shales. The presence of isotopically depleted waters in Paleozoic aquifers at relatively shallow depths illustrates the importance of continental glaciation on regional-scale groundwater flow. Modern groundwater flow in the Great Lakes region is primarily restricted to shallow unconfined glacial drift aquifers. Recharge waters in Silurian-Devonian and unconfined drift aquifers have δ18O values within the range of Holocene precipitation: -11 to -8‰ and -7 to -4.5‰ for northern Michigan and northern Indiana/Ohio, respectively. Carbon and Sr isotope systematics indicate shallow groundwaters evolved through congruent dissolution of carbonate minerals under open and closed system conditions (δ13CDIC = -14.7 to-11.1‰ and 87Sr/86Sr = 0.7080-0.7103). The distinct elemental and isotope geochemistry of Pleistocene- versus Holocene-age waters further confirms that surficial flow systems are out of contact with the deeper basinal-scale flow systems. These results provide improved understanding of the effects of past climate change on groundwater flow and geochemical processes, which are important for determining the sustainability of present-day water resources and stability of saline fluids in sedimentary basins.
AB - Changes in the climatic conditions during the Late Quaternary and Holocene greatly impacted the hydrology and geochemical evolution of groundwaters in the Great Lakes region. Increased hydraulic gradients from melting of kilometer-thick Pleistocene ice sheets reorganized regional-scale groundwater flow in Paleozoic aquifers in underlying intracratonic basins. Here, we present new elemental and isotopic analyses of 134 groundwaters from Silurian-Devonian carbonate and overlying glacial drift aquifers, along the margins of the Illinois and Michigan basins, to evaluate the paleohydrology, age distribution, and geochemical evolution of confined aquifer systems. This study significantly extends the spatial coverage of previously published groundwaters in carbonate and drift aquifers across the Midcontinent region, and extends into deeper portions of the Illinois and Michigan basins, focused on the freshwater-saline water mixing zones. In addition, the hydrogeochemical data from Silurian-Devonian aquifers were integrated with deeper basinal fluids, and brines in Upper Devonian black shales and underlying Cambrian-Ordovician aquifers to reveal a regionally extensive recharge system of Pleistocene-age waters in glaciated sedimentary basins. Elemental and isotope geochemistry of confined groundwaters in Silurian-Devonian carbonate and glacial drift aquifers show that they have been extensively altered by incongruent dissolution of carbonate minerals, dissolution of halite and anhydrite, cation exchange, microbial processes, and mixing with basinal brines. Carbon isotope values of dissolved inorganic carbon (DIC) range from -10 to -2‰, 87Sr/86Sr ratios range from 0.7080 to 0.7090, and δ 34 S s(-) SO4 values range from +10 to 30‰. A few waters have elevated δ13CDIC values (>15‰) from microbial methanogenesis in adjacent organic-rich Upper Devonian shales. Radiocarbon ages and δ18O and δD values of confined groundwaters indicate they originated as subglacial recharge beneath the Laurentide Ice Sheet (14-50 ka BP, -15 to -13‰ δ18O). These paleowaters are isolated from shallow flow systems in overlying glacial drift aquifers by lake-bed clays and/or shales. The presence of isotopically depleted waters in Paleozoic aquifers at relatively shallow depths illustrates the importance of continental glaciation on regional-scale groundwater flow. Modern groundwater flow in the Great Lakes region is primarily restricted to shallow unconfined glacial drift aquifers. Recharge waters in Silurian-Devonian and unconfined drift aquifers have δ18O values within the range of Holocene precipitation: -11 to -8‰ and -7 to -4.5‰ for northern Michigan and northern Indiana/Ohio, respectively. Carbon and Sr isotope systematics indicate shallow groundwaters evolved through congruent dissolution of carbonate minerals under open and closed system conditions (δ13CDIC = -14.7 to-11.1‰ and 87Sr/86Sr = 0.7080-0.7103). The distinct elemental and isotope geochemistry of Pleistocene- versus Holocene-age waters further confirms that surficial flow systems are out of contact with the deeper basinal-scale flow systems. These results provide improved understanding of the effects of past climate change on groundwater flow and geochemical processes, which are important for determining the sustainability of present-day water resources and stability of saline fluids in sedimentary basins.
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U2 - 10.1016/j.gca.2006.02.002
DO - 10.1016/j.gca.2006.02.002
M3 - Article
AN - SCOPUS:33748640816
SN - 0016-7037
VL - 70
SP - 2454
EP - 2479
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 10
ER -