An overlooked terrestrial archive: Freshwater cements and hydroclimate signals in Quaternary glaciogenic deposits

Quaternary glaciogenic deposits, which serve as vital groundwater aquifers, are often partially cemented with carbonate minerals, but the timing of emplacement of these cements and their geochemical conditions of formation remain understudied. Here, we integrated petrography, δ18O, Mg/Ca, and clumped isotope (Δ47) data from two calcite-cemented outcrops in western Michigan (USA) to investigate the origin of glacial sediment cements. The two sites have contrasting topographic and textural settings reflecting distinct formation conditions. The elevated Wabasis Lake deposit records sequential phreatic-to-vadose cementation, while the Cherry Valley Sands site, very close to the modern water table, shows a mixed phreatic and vadose overprinted cement fabric. The Cherry Valley Sands geochemical data are essentially as expected for cement formation under modern conditions. In contrast, Wabasis Lake phreatic cements have lower calcite δ18O (−8.3‰ ± 0.5‰ relative to Vienna Peedee belemnite), show a clear correlation between δ18O and Mg/Ca, and, via a Δ47 temperature of 9 ± 2 °C, yield a precipitation water δ18O value of −9.7‰ ± 0.5‰ relative to Vienna standard mean ocean water, i.e., ∼2‰ more negative than modern meteoric water. Rather than near-modern, we conclude these data are instead consistent with preserved ancient conditions in phreatic cements, reflecting late glacial to early Holocene recharge. These results highlight coarse, well-cemented glaciogenic deposits as valuable and overlooked potential paleohydroclimate proxies. They also place constraints on the conditions of cement formation in glacial aquifers.