Oxygen isotope variations in clay aquitards in the western glaciated plains of North America

In areas where low hydraulic gradients exist, flow of groundwater in the thick clay aquitards deposited by Glacial Lake Agassiz in North Dakota and Manitoba is expected to be very slow, of the order of 1 m per 1000 years. These very slow rates of flow suggest that original glacial lake water may be present in the porewater deep in the clays. Three field sites have been identified where aquitard thicknesses are in the range of 28 to 48 m. At the Manvel site, 28 m of silt and clay overlie the artesian Dakota sandstone aquifer. The Montcalm site comprises 35 m of clay overlying 35 m of till which in turns overlies and unused brackish bedrock aquifer. At the Drayton site there are 76 m of Lake Agassiz deposits which overlie shale bedrock unused for a water supply. At each site, a meteoric signature of -14 per mille near ground surface declines to a minimum deep in the clay and then increases under the influence of the underlying deposit. At Manvel the most depleted signature is -19.6 per mille, which has been influenced by mixing with water from the underlying Dakota aquifer. At Drayton, two sampling points spaced vertically 4.3 m apart have yielded δ18O signatures of -24.6 per mille. Similarly, at Montcalm, located 60 km north of Drayton, two sampling points 8.7 m apart have the same highly depleted δ18O signatures of -24.8 per mille. This suggests that δ18O of -24 to -25 per mille may be widespread and may be representative of water in Glacial Lake Agassiz, perhaps yielding the best estimates of δ18O in precipitation during the last glaciation. This finding will have major implications for calibrating global water balance and climate models. (author). 14 refs, 5 figs