Sulfur isotope evidence for regional recharge of saline water during continental glaciation, north-central United States

Sulfate concentration in groundwater from the Cambrian-Ordovician aquifer of southeastern Wisconsin and northern Illinois increase up to hundreds of times where the aquifer is confined beneath the Maquoketa Shale. There is no sulfate source in the aquifer or overyling rocks except for minor amounts of finely disseminated pyrite. Coinciding with increasing sulfate concentrations, δ ³⁴ S of the dissolved sulfate increases from less than -5‰ in the unconfined part of the aquifer to a nearly constant value of 20‰ where the aquifer is confined and where sulfate reduction is minimal. The most likely source for this isotopically heavy sulfate is groundwater associated with Silurian evaporites under Lake Michigan. It is uncertain if the sulfate-rich water was emplaced in pulses or mostly during the last glaciation. -Author