HR: 12:05h
AN: PP22A-08    [Abstracts]
TI: Long Term Field Calibration of the Noble Gas Temperature System in a Shallow Unconfined Pleistocene Aquifer
AU: * Hall, C M
EM: cmhall@umich.edu
AF: U. of Michigan, Dept. of Geol. Sci., Ann Arbor, MI 48109-1005 United States
AU: Castro, M C
EM: mccastro@umich.edu
AF: U. of Michigan, Dept. of Geol. Sci., Ann Arbor, MI 48109-1005 United States
AU: Lohmann, K C
EM: kacey@umich.edu
AF: U. of Michigan, Dept. of Geol. Sci., Ann Arbor, MI 48109-1005 United States
AU: Ma, L
EM: linma@umich.edu
AF: U. of Michigan, Dept. of Geol. Sci., Ann Arbor, MI 48109-1005 United States
AB: Noble gas temperatures (NGTs) are a potentially powerful tool for paleoclimate reconstruction on continents, but the assumptions underlying the technique have not been widely tested for aquifers in colder climates. In a recent study of NGTs from S. Michigan groundwater, Ma et al. (2004) found significant NGT variation over time, but apparently modern water yielded NGTs that were ~ 3 ° C or more below that expected from the local mean annual air temperature (MAAT). For nearly a year, we have monitored NGTs and stable isotopes at a shallow water well in the glacial drift that recharges the aquifer studied by Ma et al. (2004). Initial results are in press (Hall et al., 2005) and they confirm that modern NGTs measured from Oct. to Jul. are typically 5-6°C, instead of the MAAT of 9.1°C. Measured water temperatures during sampling are within 1-2°C of MAAT. Stable isotopic analyses of local snow and rain water suggest that the groundwater is a mixture of these two end member components. Short term variations in well pH and stable isotope ratios confirm that the groundwater is indeed modern, but initial noble gas results did not show similar variations. Ground ice noble gas concentrations are very low, with the exception of He, which is enriched, with ³He/⁴He equal to the air value. All early noble gas concentrations in groundwater were too high for the MAAT, including both He isotopes which had ³He/⁴He about 1.3 times air. Measured groundwater excess He values are compatible with ⁴He influx from below and tritium decay for ~ 30 yr. None of the standard NGT interpretational models adequately account for the complete data set, without modifications to normal assumptions. The most recent Aug. 2005 data after a very rainy Jul. does show a dramatic rise in NGT, with typical excess He, suggesting that noble gas concentrations acquired high in the unsaturated zone can persist and mix with those below the water table. This implies that dynamic effects may be very important in generating NGTs and the He data also demonstrate that the soil noble gas composition is persistently out of equilibrium with the atmosphere. Ma, L., Castro, M.C. and Hall, C.M. (2004), Geophys. Res. Lett., 31, L23204. Hall, CM, Castro, MC, Lohmann, KC and Ma, L. (2005), Geophys. Res. Lett., in press.
DE: 1637 Regional climate change
DE: 4914 Continental climate records
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005