Maria Clara Castro

Cross-formational flow and salinity sources inferred from a combined study of helium concentrations, isotopic ratios, and major elements in the Marshall aquifer, southern Michigan

Helium data and major ion chemistry are presented for the shallow Marshall aquifer in southern Michigan. This data set is subsequently analyzed in conjunction with major element data sets from deeper and shallower water levels previously collected in this area. He excesses and isotopic ratios suggest the presence of tritiogenic 3He in young waters in the Marshall aquifer. He excesses in old groundwater samples are mostly of crustal origin although the presence of a significant mantle He component in some samples cannot be ruled out. He excesses in the Marshall aquifer are unusually high for such shallow depths (≤300 m) and reach over two and three orders of magnitude above those of air-saturated water (ASW) for 3He and 4He, respectively. He isotopes require a source external to the aquifer, partly supplied by underlying formations within the sedimentary sequence, partly from the crystalline basement. Calibration of He concentrations observed in the Marshall aquifer requires He fluxes of 1 × 10−13 and 1.6 × 10−6 cm3 STP cm−2 yr−1 for 3He and 4He, respectively. These He fluxes are far greater than those reported in other sedimentary basins around the world (e.g., Paris Basin, Gulf Coast Basin) at similar and far greater depths. Such high He fluxes present at such shallow depths within the Michigan Basin strongly suggest the presence of a dominant vertical water flow component and further indicate that impact of recharge water at depth is minor. Upward cross-formational flow is also likely responsible for the extremely high salinities present in the shallow subsurface of the Michigan Basin. The observed positive correlation between helium and bromide strongly suggests that these two very distinct conservative tracers both originate at greater depths and further suggests that advection is the dominant transport mechanism within the basin. The occurrence of large-scale cross-formational flow is also consistent with the evolution displayed by the major ion chemistry throughout most of the sedimentary sequence, indicating that solutes from shallow levels carry the signature of deep formation brines.

Crustal noble gases in deep brines as natural tracers of vertical transport processes in the Michigan Basin

4 He, 21 Ne, 40 Ar, and 136 Xe. Both 40 Arcrust and 136 Xecrust display the presence of a strong vertical gradient along the sedimentary strata of the basin. We show that the in situ production for these two gases within the sedimentary strata is insufficient to account for the observed crustal component in the Michigan brines. These point to the presence of a deep, external source for crustal noble gases, likely the Precambrian crystalline basement beneath the Michigan Basin. Furthermore, observed elemental ratios of crustal noble gases ( 4 He/ 40 Ar, 21 Ne/ 40 Ar, 4 He/ 136 Xe, and 21 Ne/ 136 Xe) in these brines vary over several orders of magnitude with respect to the expected production ratios from the crystalline basement rocks and display a systematic pattern within the basin. Specifically, samples above the Salina Group (shallow formations) are relatively enriched in 4 Hecrust and 21 Necrust with respect to 40 Arcrust and 136 Xecrust, as opposed to those below the massive Salina evaporite layer (deeper formations) which exhibit complementary patterns. We show that such a general trend is best explained by a Rayleigh-type elemental fractionation model involving upward transport of crustal noble gases and associated elemental fractionation processes, controlled by both diffusion- and solubility-related mechanisms. As previously indicated by the mantle and atmospheric noble gas signatures in these same Michigan brine samples, release of deep crustal noble gases into the basin is yet another independent indicator pointing to the occurrence of a past thermal event in the basin. We suggest that recent reactivation of the ancient midcontinent rift system underneath the Michigan Basin is likely responsible for the upward transport of heat and loss of the atmospheric noble gas component, as well as release of crustal (still ongoing) and mantle noble gases into the basin via deepseated faults and fracture zones. Such a model also supports an internal heat source hypothesis as being largely responsible for the existence of past high temperatures in the basin without involvement of largescale brine migration from peripheral forming orogenic fold belts.

Reply to comment by Klump et al. on "Noble gases and stable isotopes in a shallow aquifer in southern Michigan: Implications for noble gas paleotemperature reconstructions for cool climates"

[1] In their comment, Klump et al. [2006], hereinafter referred to as KBK, make three main points concerning Hall et al. [2005]: (1) that the model of excess air that accounts for fractionation via the mechanism of diffusive loss should have produced significant changes from normal isotopic ratios; (2) the suggestion that the presence of excess He in the gas phase in the unsaturated zone is incorrect; and (3) the observed offset in noble gas temperatures (NGTs) from measured ground temperature can be explained by locking in noble gas concentrations during the annual snow melt. Point 3, which is by far the most relevant portion of the comment, deserves most of our attention in this reply. It is important to note up front that the model, as suggested in KBK is completely unworkable for the site studied in the work of Hall et al. [2005]. KBK note that more data would resolve some of the unresolved issues from Hall et al. [2005], but this is true for any preliminary study.