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3He and 4He concentrations in excess of those in water in solubility equilibrium with the atmosphere by up to two and three orders of magnitude are observed in the shallow Glacial Drift and Saginaw aquifers in the Michigan Basin. A simplified He transport model shows that in situ production is negligible and that most He excesses have a source external to the aquifer. Simulated results show that 3He and 4He fluxes entering the bottom of the Saginaw aquifer are 7.5 × 10−14 and 6.1 × 10−7 cm3 STPcm−2 yr−1, both of which are lower than fluxes entering the underlying Marshall aquifer, 1.0 × 10−13 and 1.6 × 10−6 cm3 STPcm−2 yr−1 for 3He and 4He, respectively. In contrast, He fluxes entering the Saginaw aquifer are higher than fluxes entering the overlying Glacial Drift aquifer of 5.2 × 10−14 and 1.5 × 10−7 cm3 STPcm−2 yr−1 for 3He and 4He, respectively. The unusually high He fluxes and their decreasing values from the lower Marshall to the upper Glacial Drift aquifer strongly suggest the presence of an upward cross-formational flow, with increasing He dilution toward the surface by recharge water. These fluxes are either comparable to or far greater than He fluxes in deeper aquifers around the world. Model simulations also suggest an exponential decrease in the horizontal groundwater velocity with recharge distance. Horizontal velocities vary from 13 to 2 myr−1 for the Saginaw aquifer and from 18 to 6 myr−1 for the Marshall aquifer. The highly permeable Glacial Drift aquifer displays a greater velocity range, from 250 to 5 myr−1. While Saginaw 4He ages estimated based on the simulated velocity field display an overall agreement with 14C ages, 14C and 4He ages in the Glacial Drift and Marshall aquifers deviate significantly, possibly due to simplifications introduced in the He transport model leading to calculation of first-order approximation He ages and high uncertainties in Glacial Drift 14C ages. [ABSTRACT FROM AUTHOR]

RATIONALE: Mass-47 CO2 clumped isotope thermometry requires relatively large (∼20 mg) samples of carbonate minerals due to detection limits and shot noise in gas source isotope ratio mass spectrometry (IRMS). However, it is unreasonable to assume that natural geologic materials are homogenous on the scale required for sampling. We show that sample heterogeneities can cause offsets from equilibrium Δ47 values that are controlled solely by end member mixing and are independent of equilibrium temperatures. METHODS: A numerical model was built to simulate and quantify the effects of end member mixing on Δ47. The model was run in multiple possible configurations to produce a dataset of mixing effects. We verified that the model accurately simulated real phenomena by comparing two artificial laboratory mixtures measured using IRMS to model output. RESULTS: Mixing effects were found to be dependent on end member isotopic composition in δ13C and δ18O values, and independent of end member Δ47 values. Both positive and negative offsets from equilibrium Δ47 can occur, and the sign is dependent on the interaction between end member isotopic compositions. The overall magnitude of mixing offsets is controlled by the amount of variability within a sample; the larger the disparity between end member compositions, the larger the mixing offset. CONCLUSIONS: Samples varying by less than 2 ‰ in both δ13C and δ18O values have mixing offsets below current IRMS detection limits. We recommend the use of isotopic subsampling for δ13C and δ18O values to determine sample heterogeneity, and to evaluate any potential mixing effects in samples suspected of being heterogonous. [ABSTRACT FROM AUTHOR]