Groundwater and climate dynamics derived from noble gas, 14 C and stable isotope data

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1 Klump et al., p. 1 Data Repository Item Groundwater and climate dynamics derived from noble gas, 14 C and stable isotope data Klump, S., Grundl, T., Purtschert, R., and Kipfer, R. Noble Gases as Environmental Tracers Noble gases are widely used as conservative tracers in aquatic systems (e.g., Cook and Herczeg, 2000; Kipfer et al., 2002). Atmospheric noble gases are incorporated into groundwater by exchange between recharge water and soil air within the unsaturated zone. As soon as the water enters the saturated zone and moves away from the water table, gas exchange ceases and the noble gas concentrations are kept by the water as a tracer for the environmental conditions prevailing during infiltration. Because noble gases are conservative and (bio)chemically inert, the atmospheric noble gas concentrations are well preserved in groundwater. Because the solubility of the noble gases, especially those of the heavy noble gases Ar, Kr, and Xe significantly depend on the water temperature during gas exchange, dissolved noble gases in groundwater can be used for (soil) temperature reconstruction. The noble gas temperature (NGT), i.e., the temperature deduced from the noble gas concentrations in the groundwater sample, is usually identical to the past soil temperature prevailing during infiltration (e.g., Mazor, 1972; Stute and Schlosser, 1993; Klump et al., 2007). The lower limit of soil temperature which can be reconstructed using NGTs is 0 C because no recharge can occur at soil temperatures below freezing point (permafrost conditions). Therefore, NGTs of Pleistocene groundwater usually provide an upper limit on the past minimum soil temperature. Noble-gas paleo-thermometry is a powerful tool to reconstruct paleotemperatures. Compared to temperature estimates from δ 18 O values, noble gas concentrations provide a more direct measure of the soil temperature prevailing during recharge. Further information on the physical conditions prevailing during groundwater recharge is recorded in the dissolved atmospheric gas excess which is typical for groundwater samples, and which is called excess air (e.g., Heaton and Vogel, 1981). Excess air is formed by the partial or complete dissolution of entrapped air bubbles within the quasi-saturated zone under an

2 Klump et al., p. 2 increased hydrostatic pressure and/or due to capillary pressure driven processes (Holocher et al., 2002, 2003; Mercury et al., 2004; Klump et al., 2007). The amount and fractionation of excess air, i.e., its elemental and isotopic deviation from atmospheric air, strongly depend on the hydrostatic pressure acting on the trapped gas bubbles and thus, for example, on the magnitude of groundwater table fluctuations. This in turn depends on recharge dynamics and climatic conditions. In glacial ice air is also entrapped as small bubbles. Therefore, basal melting of the ice under high ambient pressure due to glacial overburden can lead to the formation of exceptionally large amounts of dissolved excess air (Vaikmäe et al., 2001). Exceptionally large amounts of excess air can thus be viewed - in conjunction with other evidence of Pleistocene groundwater - as an indicator for subglacial meltwater recharge. Both environmental parameters, NGT, and amount and fractionation of excess air, were calculated using the closed-system equilibrium (CE) model (Aeschbach-Hertig et al., 2000) which takes into account fractionated excess air, and yielded the best fit to the measured noble gas concentrations. In the CE model, the fractionation of the excess air is described by the fractionation factor F, which ranges from 0 to 1. Complete dissolution of entrapped air, i.e., unfractionated excess air, is represented by F = 0, whereas fractionated excess air is represented by 0 <F <1, and no dissolution of entrapped air by F = 1. The CE model parameter q (pressure factor) is the ratio of the dry gas pressure in the entrapped gas phase to that in the free atmosphere and thus, q is a measure of the hydrostatic pressure exerted on the entrapped air. The good correlation between ΔNe and q (Fig. DR2) indicates that ΔNe is an appropriate proxy for the pressure conditions that prevailed during groundwater recharge. The measured dissolved noble gas concentrations were modeled using NO- BLE90, an error-weighted, least-squares fitting, inverse modeling program (Aeschbach-Hertig et al., 1999). NOBLE90 solves for parameter combinations (e.g., temperature, initially entrapped air, and fractionation factor) for the selected interpretive model, the CE model in this case, that match measured data within experimental error by minimizing χ 2, the sum of the weighted squared deviations between the modeled and measured concentrations.

3 Klump et al., p. 3 Experimental Methods The water samples for noble gas analyses were taken from the wellhead tap, filled in copper tubes and pinched to make a gas-tight seal. Care was taken to insure that no entrained bubbles remained in the sample. Noble gas abundances of He, Ne, Ar, Kr, Xe and the isotope ratios 3 He/ 4 He, 20 Ne/ 22 Ne, 36 Ar/ 40 Ar were measured by noble gas mass spectrometry and standard experimental protocols (Beyerle et al., 2000). The samples for stable isotope analysis were collected in polyethylene bottles and analyzed by mass spectrometry at the Stable Isotope Laboratory of the University of Bern. 14 C samples were collected in 50 L bottles, adjusted to ph 10 with NaOH and precipitated by addition of BaCl 2. Excess supernatant was decanted off to a volume of 1 L. The 1 L samples were prepared and measured by the Radiocarbon Laboratory of the University of Bern. 14 C data for four wells were obtained from D. I. Siegel (unpublished data, pers. comm.).

4 Klump et al., p. 4 References Aeschbach-Hertig, W., Peeters, F., Beyerle, U., and Kipfer, R. (1999). Interpretation of dissolved atmospheric noble gases in natural waters. Water Resour. Res., 35(9): Aeschbach-Hertig, W., Peeters, F., Beyerle, U., and Kipfer, R. (2000). Palaeotemperature reconstruction from noble gases in ground water taking into account equilibration with entrapped air. Nature, 405: Beyerle, U., Aeschbach-Hertig, W., Imboden, D. M., Baur, H., Graf, T., and Kipfer, R. (2000). A mass spectrometric system for the analysis of noble gases and tritium from water samples. Environ. Sci. Technol., 34(10): Cook, P. G. and Herczeg, A. L., editors (2000). Environmental tracers in subsurface hydrology. Kluwer Academic Publishers, Boston. Craig, H. (1961). Isotopic variations in meteoric waters. Science, 133(3465): Fontes, J.-C. and Garnier, J.-M. (1979). Determination of the initial 14 C activity of the total dissolved carbon: A review of the existing models and a new approach. Water Resour. Res., 15(2): Heaton, T. H. E. and Vogel, J. C. (1981). Excess air in groundwater. J. Hydrol., 50: Holocher, J., Peeters, F., Aeschbach-Hertig, W., Hofer, M., Brennwald, M., Kinzelbach, W., and Kipfer, R. (2002). Experimental investigations on the formation of excess air in quasi-saturated porous media. Geochim. Cosmochim. Acta, 66(23): Holocher, J., Peeters, F., Aeschbach-Hertig, W., Kinzelbach, W., and Kipfer, R. (2003). Kinetic model of gas bubble dissolution in groundwater and its implications for the dissolved gas composition. Environ. Sci. Technol., 37(7): Kipfer, R., Aeschbach-Hertig, W., Peeters, F., and Stute, M. (2002). Noble gases in lakes and ground waters. In Porcelli, D., Ballentine, C., and

5 Klump et al., p. 5 Wieler, R., editors, Noble gases in geochemistry and cosmochemistry, volume 47 of Rev. Mineral. Geochem., pages Mineralogical Society of America, Geochemical Society. Klump, S., Tomonaga, Y., Kienzler, P., Kinzelbach, W. Baumann, T., Imboden, D. M., and Kipfer, R. (2007). Field experiments yield new insights into gas exchange and excess air formation in natural porous media. Geochim. Cosmochim. Acta, 71(6): Mazor, E. (1972). Paleotemperatures and other hydrological parameters deduced from gases dissolved in groundwaters, Jordan Rift Valley, Israel. Geochim. Cosmochim. Acta, 36(12): Mercury, L., Pinti, D. L., and Zeyen, H. (2004). The effect of the negative pressure of capillary water on atmospheric noble gas solubility in ground water and palaeotemperature reconstruction. Earth Planet. Sci. Lett., 223: Pearson, F. J. and White, D. E. (1967). Carbon-14 ages and flow rates of water in Carrizo Sand, Atascosa County, Texas. Water. Resour. Res., 3(1): Stute, M. and Schlosser, P. (1993). Principles and applications of the noble gas paleothermometer. In Swart, P. K., Lohmann, K. C., McKenzie, J., and Savin, S., editors, Climate Change in Continental Isotopic Records, volume 78 of AGU Geophysical Monograph Series, pages American Geophysical Union, Washington, DC. Tamers, M. A. (1967). Radiocarbon ages of groundwater in an arid zone unconfined aquifer. In Isotope Techniques in the Hydrological Cycle, AGU Monograph 11, pages Vaikmäe, R., Vallner, L., Loosli, H. H., Blaser, P. C., and Juillard, M. (2001). Palaeogroundwater of glacial origin in the Cambrian-Vendian aquifer of northern Estonia. In Edmunds, W. M. and Milne, C. J., editors, Palaeowaters in Coastal Europe: evolution of groundwater since the late Pleistocene, volume 189 of Geol. Soc. Special Publication, pages Geological Society of London, London.

6 Klump et al., p. 6 δ 2 H [ ] MUS5 FR5 DO1 NB7 WK9 SUS1 NB3 BR15 PEW4 WK5 GMWL : δ 2 H = 8 δ 18 O + 10 WK δ 18 O [ ] Figure DR 1: Stable isotopic composition of the groundwater samples. All samples plot close to the global meteoric water line (GMWL; Craig, 1961). Squares: pre-ice cover samples q [ ] ΔNe [%] Figure DR 2: ΔNe vs. pressure factor q. The good correlation indicates that ΔNe provides an appropriate proxy for the pressure conditions prevailing during groundwater recharge.

7 Klump et al., p. 7 Table DR 1: Noble gas concentrations and isotope ratios of the groundwater samples. All concentrations are given in [cm 3 STP /g]. Overall measurement errors of the noble gas concentrations are about ±1 % for He, Ne, and Ar, ±2% for Kr, and Xe, and <0.5 % for the isotope ratios. Sample He Ne Ar Kr Xe 3 He/ 4 He 22 Ne/ 20 Ne 40 Ar/ 36 Ar [10 6 ] [10 7 ] [10 4 ] [10 7 ] [10 8 ] [10 8 ] BR DO FR MUS NB NB PEW SUS WK WK WK WK1 n.d. n.d. not determined

8 Klump et al., p. 8 Table DR 2: Sampling locations and tracer data of the groundwater samples from Wisconsin. The model parameters NGT (noble gas temperature), A (initial amount of entrapped air), F (fractionation factor), and q (pressure factor) were calculated using the closed-system equilibration model of (Aeschbach-Hertig et al., 2000). The uncertainties of these model parameters are determined from the covariance matrix in the least squares fitting algorithm (Aeschbach-Hertig et al., 1999). The parameters derived from the inverse modeling were then used to calculate the Ne excess and hence the ΔNe as the difference between the measured total Ne and the calculated equilibrium fraction. Sample Well depth Flow dist. NGT ΔNe A F q [m] [km] [ C] [%] [cm 3 STP /g] [-] [-] BR ± ± ± ± DO ± ± ± ± FR ± ± ± ± MUS ± ± ± ± NB ± ± ± ± NB ± ± ± ± PEW ± ± ± ± SUS ± ± ± ± WK ± ± ± ± WK ± ± ± ± WK ± ± ± ± WK n.d. n.d. not determined

9 Klump et al., p. 9 Table DR 3: Stable isotope and radiocarbon data for the groundwater samples. The 14 Cagesanderrorsareaveragevaluesand1-σ errors determined making use of three different correction models for calculating the initial activity A 0 of radiocarbon (Fontes and Garnier, 1979; Pearson and White, 1967; Tamers, 1967). It is important to note that all models yield very similar values for A 0. Sample δ 18 O δ 2 H 14 C δ 13 C A 0 14 C age [ ] [pmc] [ ] [pmc] [yr] BR n.d. DO ± ±0.2 49±5 2065±828 FR ± ±0.2 52± ±478 MUS n.d. NB *± ±0.2 54± ±23619 NB ± ±0.2 54± ±583 PEW n.d. SUS n.d. WK ± ±0.2 52± ±245 WK *± ±0.2 52± ±371 WK *± ±0.2 54±1 6277±150 WK1 n.d. 18.9*± ±0.2 51±5 8274±780 *unpublished data from D. I. Siegel (pers. comm.) n.d. not determined