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2 Marine Chemistry 153 (2013) Contents lists available at SciVerse ScienceDirect Marine Chemistry journal homepage: Kinetic isotopic fractionation of argon and neon during air water gas transfer Kevin E. Tempest, Steven Emerson School of Oceanography, University of Washington, Seattle, WA 98195, USA article info abstract Article history: Received 11 September 2012 Received in revised form 5 February 2013 Accepted 11 April 2013 Available online 23 April 2013 Keywords: Isotope fractionation Argon Neon Argon isotopes Neon isotopes Gas exchange The use of the noble gas isotope ratios 22 Ne/ 20 Ne and 40 Ar/ 36 Ar as geochemical tracers of environmental conditions (i.e. continental paleotemperatures from groundwater) and processes (i.e. air sea gas exchange) requires accurate values for kinetic and equilibrium isotopic fractionations (α k and α eq ). Until now, these values have been approximated using different theoretical models. Here we evaluated both the kinetic and equilibrium fractionation factors experimentally by measuring the relative mass transfer rates of the major and secondary isotopes of argon and neon in laboratory experiments at 20 C during exchange across the air water interface as gas saturations evolved from 0 to 100%. The kinetic isotopic fractionation factors, α k, for argon (40/36) and neon (22/20) are determined to be ± (ε k = 3.9 ± 0.1 ) and ± (ε k = 6.9 ± 0.4 ), respectively. The kinetic isotopic fractionation factors are much closer in agreement to theoretical values determined by molecular dynamics simulations than to values calculated using the square root of molecular reduced masses. The equilibrium fractionation factor for neon (22/20) at 20 C is determined to be ± (ε eq = 1.04 ± 0.05 ). An argon equilibrium fractionation factor of ± (ε eq = 1.07 ± 0.02 ) determined at 20 C agrees with a linear interpolation of previous measured values at 2 C and 25 C (Nicholson et al., 2010). Published by Elsevier B.V. 1. Introduction The measurement of noble gas isotope ratios in environmental samples provides the potential to distinguish diffusion-controlled processes resulting in large isotopic fractionations from processes controlled by solubility equilibrium, which result in smaller isotopic fractionations. Precise gas isotope ratio measurements of the major and secondary isotopes of argon ( 40 Ar, 36 Ar) and neon ( 20 Ne, 22 Ne) have been used to test competing models of noble gas paleotemperature reconstruction from groundwater reservoirs (Aeschbach-Hertig et al., 2000; Peeters et al., 2003), determine past firn thickness and abrupt climate change in ice core studies (Severinghaus et al., 2003), trace bubble release from lake sediments (Brennwald et al., 2005), investigate subsurface gas/ groundwater interaction in coalbed systems (Zhou et al., 2005), and constrain ventilation processes during North Atlantic deep water formation (Nicholson et al., 2010). These studies have all been limited by uncertainty in the kinetic isotopic fractionation factors. At solubility equilibrium between the water and air, small differences in the relative free energies of isotopic species result in enhanced concentrations of heavy to light isotopes, yielding a slightly heavier dissolved gas ratio (r d ) than gas phase ratio (r g )(Benson and Krause, Corresponding author. Tel.: address: ket5@u.washington.edu (K.E. Tempest). 1980; Fuex, 1980; Knox et al., 1992). The equilibrium isotopic fractionation, α eq,isdefined as: α eq ¼ r d =r g : Kinetic isotopic fractionation, α k, arises from slight differences in the molecular transfer rates of the faster light isotope and the slower heavy isotope, and is defined as the ratio of mass transfer rates of the heavy isotope (k h ) to the light isotope (k l ): α k ¼ k h =k l : Isotopic fractionations are often expressed as per mil ( ) deviations from unity (ε), such that: ε ¼ 1000ðα 1Þ: ð3þ Despite the growing importance of noble gas isotope measurements in natural systems, there has been only one experimental measurement of the kinetic isotopic fractionation factor for a noble gas prior to this work. Jähne et al. (1987a) determined the kinetic isotopic fractionation factor, α k, for helium in water to be ± Bourg and Sposito (2008) conducted molecular dynamics (MD) simulations relying on estimates of water water and gas water intermolecular potentials to calculate the ratios of self-diffusion coefficients, D (m 2 s 1 ), in water for isotopes of Ne (D 22 /D 20 ), Ar (D 40 /D 36 ), ð1þ ð2þ /$ see front matter. Published by Elsevier B.V.

3 40 K.E. Tempest, S. Emerson / Marine Chemistry 153 (2013) and He (D 4 /D 3 ). These simulations predicted the ratio of helium isotope diffusivities to be ± 0.007, corresponding to an α k closer to unity than the value experimentally determined by Jähne et al. (1987a), which Bourg and Sposito (2008) attribute to quantum isotope effects not captured by their MD model. In the absence of experimental determinations of neon and argon kinetic isotopic fractionation factors, the ratio of their molecular diffusion coefficients has often been assumed to be related to the ratio of isotope masses or reduced masses, as is the case for the kinetic-theory model of gases in air (Ballentine et al., 2002; Lippmann et al., 2003; Peeters et al., 2003; Brennwald et al., 2005; Strassmann et al., 2005; Zhou et al., 2005; Hall et al., 2006; Klump et al., 2007): D h =D l ¼ ðm l =m h Þ β : ð4þ where m is the mass of the isotope, with subscripts denoting the heavier (h) and lighter (l) isotope, and β is assumed to be equal to 1 2. Experimental measurements by Knox et al. (1992) showed the squareroot relationship (β = 1 2) to be inaccurate for multiatomic species in water, while the measurements of Jähne et al. (1987a) for helium followed the kinetic-theory model within the accuracy of the data. The model simulations of Bourg and Sposito (2008) predicted the departure of the kinetic-theory model of gases for all monoatomic gases (He, Ne, Ar, and Xe), a finding they applied to previous environmental measurements of argon and neon isotopes in hydrologic studies to highlight the importance of accurate values for kinetic isotopic fractionations. Nicholson et al. (2010) did report an argon α k of (ε k = 5.0 ) as part of their attempt to constrain mechanisms of deep-sea ventilation. This value was taken from the preliminary attempts of this work, but was derived from inaccurate model treatment of the data and is revised by the experiments described here. Equilibrium fractionation factors of 1.3 ± 0.2 for argon and 2.0 ± 0.2 for neon were reported by Beyerle et al. (2000) from 70 samples of river and lake surface waters, although no temperature associated with these values was indicated. In this study we update the preliminary laboratory determination of argon α k while expanding the scope to include α k and α eq measurements for neon. 2. Isotope gas transfer model Since air water gas exchange in the absence of bubbles is ultimately controlled by molecular diffusion, this process can be used to experimentally determine kinetic isotopic fractionation factors (Jähne et al., 1987a; Knox et al., 1992). The rate of transfer of a non-reactive gas into water, F G (mol s 1 ), across the air water interface without bubble mediated exchange depends on the gas concentration gradient between the air and water, the area of the interface, A (cm 2 ), and the mass transfer coefficient, k (cm s 1 ), such that: F G ¼ dn=dt ¼ kaðpk H CÞ ð5þ where dn is the change in moles of dissolved gas, p (atm) is the partial pressure of the gas above the water which is substituted for fugacity using the ideal gas assumption for argon and neon, K H (mol cm 3 atm 1 ) is Henry's law solubility of the gas (neon and argon as reported by Hamme and Emerson, 2004), and C (mol cm 3 ) is the dissolved gas concentration in the water. The rate of transfer for both heavy and light isotopes of a given element can be expressed in terms of the heavy isotope (h) by use of gas ratios and the isotopic fractionation factors: F h ¼ k h A p h K H;h C h : ð6þ F l ¼ k h A=α K p h K H;h r g =α eq C h r d : ð7þ The mass transfer coefficient, k, for gases during air water transfer is proportional to the Schmidt number of the gas (Sc) raised to some power, n. The Schmidt number equals the ratio of kinematic viscosity of the transfer medium, water (ν), to the molecular diffusivity of the gas (D), allowing k to be expressed as, k ¼ k ðν=dþ n ð8þ where k* is the mass transfer coefficient normalized to a specific gas and temperature and depends only on the physical conditions affecting the rate of transfer. When gas transport occurs under the same physical conditions, the ratio of mass transfer coefficients for two gases or isotopes can be expressed solely in terms of the ratio of diffusivities raised to the power n. α k ¼ ðd H =D L Þ n : ð9þ The exponent, n, has been evaluated in both field and wind tunnel experiments and shown to range between 2/3 for a calm interface and 1/2 with the onset of waves (Jähne et al., 1984; Wanninkhof, 1985; Ledwell, 1984). The value of n in this range has been shown to correlate with the total mean wave slope or degree of turbulence at the interface (Jähne et al., 1987b; Bock et al., 1999) and the fractional area coverage of microscale breaking waves (Zappa et al., 2001). The model we use to interpret the experimental results is derived from Eqs. (5) through (7) as in Knox et al. (1992). A mass balance was used to account for the partitioning of the gas introduced into the system (N g,0, moles) of each isotope between dissolved (d) and gas (g) phases along with removal through sampling (s): N g;0 ¼ N g þ N d þ N s ð10þ where the moles of gas in the headspace (N g ) and removed by sampling (N s ), are: N g ¼ p g V G = ðrtþ ð11þ h i N s ¼ p g V s = ðrtþ ð12þ with V G the volume of gas in the headspace (L), p g the partial pressure of the noble gas (atm), R the universal gas constant ( L atm mol 1 K 1 ), T the absolute temperature (K), and V s the volume of the aliquoting chamber for sampling (L). Noble gas partial pressure, p g, in the model is a function of the initial pressure and the hydrostatic pressure from the manometer water column as gas transfer evolves (see experimental setup): p g ¼ ðp 0 p H2O Þþ ðδzþ 0:000987atm cm 1 ð13þ where P 0 is the initial pressure of the headspace gas (atm), p H2 O the water vapor partial pressure, and Δz (cm) the total change in manometer water column height as net gas dissolution occurs. The degree of gas saturation and the isotopic composition relative to the initial headspace composition are calculated as a function of time using 5-min time steps. In this way, the model predicts changes in the isotopic composition of headspace as the gas exchanges across the air water interface for prescribed α K values using the gas transfer calculation for each isotope (Eqs. (6) and (7)). Sampling is simulated in the model by removal of an aliquot volume of gas with the modeled headspace gas ratios. The model generated gas isotope ratios as a function of gas saturation are insensitive to the surface area of the interface, the initial isotopic ratios, the initial headspace pressure, and the absolute values of mass transfer velocities. The headspace gas isotopic composition is most sensitive to the kinetic isotopic fractionation factor, α K, with other relevant variables being the final equilibrium fractionation

4 K.E. Tempest, S. Emerson / Marine Chemistry 153 (2013) value, α eq, and volumes of water and gas. We use ± 0.02 for the ε eq of argon at 20 C based on linear interpolation of those determined at 25 C (1.05 ± ) and 2 C (1.21 ± 0.01 ) during previous gas solubility studies, which agrees within error to our only experimental determination (1.07 ± 0.02, Ar2011 discussed below). The argon equilibrium fractionations were first reported in Nicholson et al. (2010), although the values for the two temperatures were mistakenly reversed. The neon ε eq used in the model, 1.04 ± 0.05, averages endpoint gas and water isotopic ratios from three experimental runs, including the two from 2010 reported here (Table 1). 3. Experimental methods We report on five experimental runs: Ar2009, Ar2011, Ne2010a, Ne2010b, and Ne All experiments were conducted following the methodology of Knox et al. (1992) in a ~4.6 L Kimax cylindrical flask with a modified neck system (Fig. 1). One neck was used for the initial introduction of pure argon or neon gas into the flask to form a headspace above degassed water, and for subsequent sampling of headspace gas aliquots between two 9 mm single O-ring Louwers Hapert valves. A second, central neck extending approximately two-thirds of the way to the bottom of the flask was connected to a water manometer (radius = 1.03 cm) to maintain headspace pressure near one atmosphere as the gas volume decreases during the course of the experiment. Prior to the experiment, deionized water in the experimental flask was degassed for 2 h by rapid boiling then cooled to 20 C in a constant temperature bath without air exposure. Each experiment was initiated by introduction of a pure, single gas headspace into the experimental flask from a tank of ultra high purity compressed gas. The increased water height within the manometer as the introduction of gas forced water out of the flask provided a precise measure of the initial headspace gas volume. Immediately following gas introduction, the top of the manometer was connected with a large air reservoir (~12.5 L), isolating the system from atmospheric pressure fluctuations. Constant temperature in the system was maintained by submerging the experimental flask and air reservoir in a constant temperature bath. The initial pressure of the gas headspace was a combination of the atmospheric pressure when the air reservoir was isolated from the room and the hydrostatic pressure from the manometer water column above the air water interface in the flask. As gas dissolved across the air water interface and was removed for sampling, water flowed from the manometer into the flask, maintaining the gas pressure near one atmosphere. The drop in height of water in the manometer indicated the volume of net gas transfer into the water. The pressure of the gas headspace dropped slightly throughout because of decreasing hydrostatic pressure. Hydrostatic pressure decreases ranged between 5.0% to 6.0% for argon experiments and 2.0% to 3.0% for neon experiments (see supplementary material in appendix EA-1). Experiments were conducted at only one temperature. The results of Knox et al. (1992) displayed no discernable temperature dependence of α k for N 2 and O 2 over a 10 C range. A 5 cm stir bar was used to maintain uniform dissolved gas concentration in the water. We tested the impact of stirring rates on α k by repeating one of the experiments at different rates in which the surface conditions ranged from no visible surface deformation to slight rippling of the surface. Contamination by atmospheric gas, originating from incompletely degassed (>99.8% degassed) flask water, was evaluated by analyzing the nitrogen and oxygen content of headspace gas samples throughout the course of the experiments. Maximum atmospheric gas contamination did not exceed 0.25% of the initial headspace gas. Table 1 summarizes the run conditions for each experiment. Samples of headspace gas were collected at intervals of 5 10% of the approach to equilibrium saturation. The sample aliquot chamber was allowed to equilibrate with the headspace gas for 10 min after which it was purified of water vapor using a liquid nitrogen trap before being frozen into a stainless steel finger submerged in liquid helium. The stainless steel fingers were thawed for at least 4 h and attached directly to a Finnigan MAT 253 mass spectrometer where the isotope ratios were determined against a working standard consisting of the same ultra high purity gas as that in the headspace. Drift in the isotopic composition of the working standard was checked by comparison with a second tank reference gas at least every fifth sample. Isotopic ratios are reported in standard del (δ) notation relative to the working standard, where δ ð Þ ¼ 1000 r sample =r standard 1 : ð14þ At the conclusion of most experiments, headspace and water samples were collected in duplicate for measurement of α eq. Water samples were collected in approximately 160 ml evacuated glass flasks fitted with Louwers Hapert double O-ring valves and dissolved gases were extracted from the water samples following procedures described in Emerson et al. (1999) and analyzed in the same manner as the headspace samples. 4. Results Time evolutions of gas saturation are presented for the argon (Fig. 2) and neon (Fig. 3) experiments. Experimental data are included in the supporting material (Appendix A). Gas saturations are reported relative to their Henry s Law solubility (Hamme and Emerson, 2004) for the headspace pressure at time of measurement, including a correction to the headspace gas partial pressure to account for the presence of oxygen and nitrogen in the headspace (EA-1 tables, column 6). Initial gas dissolution in both cases is very rapid, with two-thirds saturation reached in less than 24 h for argon experiments and less than 10 h for neon experiments. As the smaller molecule, neon diffuses more quickly resulting in the more rapid approach to saturation. The isotopic composition of the argon and neon headspace gas as a function of the degree of saturation is presented in Figs. 4 and 5,respectively. All isotope ratios are reported relative to the initial headspace gas Table 1 Experimental run conditions. Ar2009 Ar2011 Ne20l0a Ne20l0b Ne2011 Headspace pressure, t = 0 (atm) V gas added (ml) V gas, final (ml) V gas sampled (ml) Aliquot volume 1.76 ± ± ± ± ± 0.12 Experiment run time (hours) Stir rate Slower Slower Slower Faster Faster δ 0 headspace gas ± ± ± ± ± ε eq, 1.07 ± ± ± 0.03

5 42 K.E. Tempest, S. Emerson / Marine Chemistry 153 (2013) Fig. 1. Schematic of laboratory setup for fractionation experiments. Gas transfer takes place in a ~4.6 L experimental flask fully submerged in a temperature controlled water bath and connected by an extended central neck to a ~12.5 L air reservoir via a water manometer. A pure argon or neon headspace is introduced into the flask via the gas extraction line. Sampling aliquots are frozen from the aliquoting chamber through the gas extraction line and into a stainless steel finger submerged in liquid helium. isotopic composition. Combined sampling and measurement error was evaluated by replicate sampling at least once during most experiments. The standard error of the means for replicate samples was ±0.014 for δ 40 Ar and ±0.004 for δ 22 Ne and is treated as the uncertainty in the headspace gas isotopic ratio measurements. The solid and dashed lines in Figs. 4 and 5 are model-generated curves described by the best-fit α k values and uncertainty bounds in α k, and are described in detail in the discussion section. A positive δ indicates enrichment of the heavy isotope in the gas headspace relative to the initial gas ratio with an associated depletion of the heavy isotope in the dissolved gas. The lighter isotopic species initially diffused more rapidly across the air water interface, enriching the isotopic composition of the headspace gas and indicating an α k of less than one (k h b k l ). After maximum enrichment, the headspace isotopic composition became progressively depleted until an equilibrium distribution of isotopic species was achieved. The final isotopic composition of the headspace is depleted in the heavier isotope relative to the initial isotopic composition due to the greater solubility of the heavier isotopic species (α eq >1). 5. Discussion Before describing the model fit to the data and the assignment of uncertainties, we briefly elaborate on the methods for determining the initial value of the isotope ratio of the headspace gas Initial headspace gas isotope composition The initial composition of the headspace gas relative to the working standard was calculated as in Knox et al. (1992) using the following Fig. 2. Degree of saturation (%) versus time for Ar2009 (circles) and Ar2011 (squares) as measured by the change in water height of manometer. The black line marks 100% saturation based on gas solubility from Hamme and Emerson, The error in degree of saturation estimates is smaller than the height of the symbols.

6 K.E. Tempest, S. Emerson / Marine Chemistry 153 (2013) Fig. 3. Degree of saturation (%) versus time for Ne2010a (circles), Ne2010b (squares), and Ne2011 (triangles) as measured by the change in water height of manometer. The black line marks 100% saturation based on gas solubility from Hamme and Emerson, The error in degree of saturation estimates is smaller than the height of the symbols. Fig. 4. δ 40 Ar of headspace gas, relative to initial headspace gas composition (see Section 5.1), versus degree of gas saturation (%) for Ar2009 (top) and Ar2011 (bottom). Circles show experimental data. Circle height is representative of combined sampling and analytical error (±0.014 ). The solid line is the isotope gas transfer model (Section 2) generated curve for the best fit ε K, while dashed lines are model-generated curves for ±1σ bounds for ε K as reported in Table 2.

7 44 K.E. Tempest, S. Emerson / Marine Chemistry 153 (2013) saturation. This value was adjusted for curvature in the initial sample versustimecurvebycomparisonwiththeotherneonexperiments where the initial value could be calculated both by Eq. (15) and a linear extrapolation. The adjustment was determined to be ± Degree of saturation With the exception of Ar2009 (6.8% supersaturated), averaged final gas saturations for all experiments run to equilibrium were slightly supersaturated (1.3% to 2.4%) relative to equilibrium (Figs. 2 and 3). We believe that there are several reasons for this: (1) in the case of Ar2009, we believe that an unknown bias in sample aliquot volume estimates resulted in an underestimate of sampled gas and a corresponding overestimate of dissolved gas (assuming an aliquot volume equal to that in Ar2011 (see Table 1) results in a one percent endpoint supersaturation for Ar2009); (2) diffusion of a small amount of gas upwards into the manometer system, increasing the effective volume of water and resulting in greater gas dissolution in all experiments; (3) a leak observed in the large air reservoir for experiments Ne2010a and Ne2010b, linking only headspace pressure (see table EA-4 for experimental air reservoir pressures) to ambient conditions so that a rapid pressure decrease (increase), causing temporary supersaturation before outgassing (ingassing) could re-equilibrate the dissolved and headspace gas pressures (for Ne2010b a 1.2% drop in pressure observed between hours 115 and 162 corresponded to a 0.7% degree of saturation increase). Despite any bias in the determined degree of saturation arising from any of these factors, the best estimates for the kinetic isotopic fractionation factors (as detailed in Section 5.3) do not change when gas supersaturations are proportionally adjusted to establish a final gas saturation of 100% equilibrium. This is because the model-data fit is primarily determined by the magnitude of change in headspace gas ratios during the experiment, which is large compared to and unaltered by the saturation bias Determination of kinetic isotopic fractionation factor Fig. 5. δ 22 Ne of headspace gas, relative to initial headspace gas composition (see Section 5.1), versus the degree of gas saturation (%) for Ne2010a (top), Ne2010b (middle), and Ne2011 (bottom). Circles (±0.004 )andlinesareasinfig. 4. Corresponding ε K values are reported in Table 2. mass balance: N f gxδ f δ 0 g þ N f d xδf d þ s¼end s¼1 ðn s xδ s Þ g ¼ : ð15þ N 0 g Superscripts 0 and f indicate initial and final conditions, respectively, and subscripts are as in Eq. (10). The summation superscript s=end indicates the final headspace sample taken for each experiment. We found that the initial headspace gas isotopic ratios were depleted (lighter) relative to the working standard by for argon and for neon (Table 1), indicating fractionation of the tank gas during the initial gas introduction. In the case of Ar2009, the final dissolved isotopic ratio was not measured. We therefore use α eq of ±0.02 at 20 C (interpolated from values in Table 3) and the measured endpoint headspace gas isotopic ratio to determine the final dissolved gas isotopic ratio for use in Eq. (15). In experiment Ne2011, we were unable to measure end member headspace or dissolved gas ratios. We therefore determined the initial headspace gas ratios by linear extrapolation of the δ 22 Ne curve versus time to 0% The kinetic isotopic fractionation factor is determined by a least squares fit of the experimental isotopic δ values to the model curves (Section 2) derived for different values of α k, assuming initial headspace gas ratios as described above and saturation equilibrium at the end of the experiment. Figs. 4 and 5 display model generated curves for α k values corresponding to the best fit (solid) and uncertainty bounds of the best fit (one sigma, dashed) along with the experimental data. Error estimates for α k are derived from the uncertainties in: (i) the value of the initial gas isotope ratio (given in Table 1); (ii) combined sampling and analytical error of the headspace gas as described in the results (±0.014 for δ 40 Ar and ±0.004 for δ 22 Ne); and (iii) error estimates in the final equilibrium fractionation values (± 0.02 for ε eq,ar and ±0.05 for ε eq,ne ). The first two uncertainties are incorporated directly into the standard deviation of the experimental measurements, as all measurements are reported relative to the initial isotopic ratio of the headspace. A Monte Carlo method (15,000 simulations) was used to determine the combined error in α k stemming from these two uncertainties (Press et al., 1992). The error in α k associated with the uncertainty in equilibrium fractionation factors was evaluated separately by determining the best fit α k to the mean experimental data for model curves generated using three different α eq values: the mean and ± one standard deviation. We present the uncertainty for each experiment (Table 2) as the square root of the sum of the squares of the two separate error estimates. The overall α k for argon and neon is reported (Table 2) asthe mean of all experimental runs and the error (1σ) onα k is presented as the standard error of the means of the individual experiments.

8 K.E. Tempest, S. Emerson / Marine Chemistry 153 (2013) Table 2 Kinetic isotopic fractionation factors with one-sigma errors for each experimental run and the overall mean and standard deviation of the mean for each gas (see Discussion for details on the determination of uncertainties). ε k ( ) Ar ± 0.1 Ar ± 0.2 Ar mean 3.9 ± 0.1 Ne2010a 6.0 ± 0.3 Ne2010b 7.2 ± 0.3 Ne ± 0.4 Ne mean 6.9 ± Comparison with previous results and models All reported α k and α eq values, in per mil form (ε k and ε eq ), are presented in Table 3. In order to compare the model results for molecular diffusion coefficients (D) of Bourg and Sposito (2008) with our gas transfer rate measurements, we assume the gas transfer molecular diffusion coefficient relationship described by Eq. (9), and convert D to α k for the cases where n = 1/2 and n = 2/3. As in Knox et al. (1992), the argon and neon experiments were run with clean, deionized water under conditions of low surface perturbation, indicating a regime that should be near n = 2/3 (Jähne et al., 1987b; Zappa et al., 2001). Increased surface turbulence would be expected to push the diffusion fractionation relationship towards n = 1/2 with a corresponding decrease in the magnitude of fractionation. We determined the fractionation factor as a function of stirring rate in experiments Ne2010 (slower stirring), and Ne2010b and Ne2011 (faster stirring) (Table 1) and found the opposite effect, indicating that stirring rate was not controlling our results. Assuming n = 2/3, our results indicate smaller kinetic isotopic fractionations (α k closer to unity, ε k closer to zero) than the MD simulations of Bourg and Sposito (2008), although our results for argon fall just within the relatively large error estimates of the MD predictions. Despite similar isotope mass ratios (1.11 and 1.10), our results show the magnitude of ε k to be significantly larger for neon relative to argon ( 6.9 vs. 3.9 ), in general agreement with the MD predictions ( 9.5 vs. 5.5 for the n = 2/3 assumption). Both the MD simulations and our laboratory results indicate gas-specific β values (Eq. (4)) that are much smaller than the square root relationship (β = 0.5) of the kinetic theory of gases. To illustrate this, we plot the ε k values from Table 3 against isotopic mass ratios (Fig. 6). The lines are trends for ε k assuming a square-root relationship (β = 0.5 Fig. 6. Kinetic isotopic fractionation factor (ε k ) versus isotope mass ratio values as reported in Table 3. Experimental results from this work are shown as filled circles. Bourg and Sposito (2008) helium, neon, and argon values (Xs) are shown by for both n = 1/2 and n = 2/3 (see Eq. (9)). The solid (n = 1/2) and dotted lines (n = 2/3) show the expected α k as a function of the isotope mass ratios assuming the kinetic theory of gases. The inset expands the isotope mass ratio window to encompass the Jähne et al. (1987a) experimental measurement for helium (lower diamond). in Eq. (4)) in the cases where n = 1/2 (solid line) and 2/3 (dotted line) (Eq. (9)). Only the helium result of Jähne et al. (1987a) is within the theoretical curves generated by the β = 0.5 assumption. The ε k values for argon and neon are much closer to those of the other experimentally determined multiatomic gases than the kinetic theory model. The equilibrium fractionation factors measured here (and given by Nicholson et al., 2010 for Ar) are smaller (ε eq closer to one) than the values reported by Beyerle et al. (2000). While Beyerle reported an equilibrium fractionation factor for neon that was nearly twice as large as that for argon (2.0 ± 0.2 versus 1.2 ± 0.2 ), we observe values that are the same within the uncertainty of the determination. We suspect that the freshwater samples taken by Beyerle et al. (2000) had not achieved isotopic equilibrium. If the waters sampled were on average warming, outgassing would lead to isotopic disequilibrium created by the more rapid exchange kinetics of the lighter isotope, leaving water enriched in the heavier isotope relative to solubility equilibrium. The larger kinetic fractionation between neon isotopes would be anticipated to create disequilibrium on the order of twice as great as that observed for argon. Table 3 Compilation of experimentally determined and simulated kinetic and equilibrium isotopic fractionation factors for gases diffusing in water. Most values are reported at 20 C. Gas Isotope mass ratio ε eq ( ) ε K ( ), experimental ε K ( ) theoretical (n = 1/2) n = 2/3 H ± 1 a 18 ± 2 a He ± 10 b 23.9 ± 0.4 c 31.8 ± 5.0 c Ne ± 0.05 d 6.9 ± 0.4 d 7.1 ± 0.8 c 9.5 ± 1.1 c Ar ± 0.01 (2 C) 3.9 ± 0.1 d 4.1 ± 1.3 c 7.1 ± 0.8 c 1.05 ± (25 C) e CH 4 ( 13 C) ± 0.02 f 0.8 ± 0.2 a O ± 0.02 g 2.8 ± 0.2 a N ± 0.02 h 1.3 ± 0.1 a CO 2 ( 13 C) i 0.9 ± 0.05 b a Knox et al. (1992). b Jähne et al. (1987a). c Bourg and Sposito (2008). d This work. e First reported in Nicholson et al. (2010). f Fuex (1980). g Benson and Krause (1980). h Benson and Krause (1980). i Vogel et al. (1970).

9 46 K.E. Tempest, S. Emerson / Marine Chemistry 153 (2013) Application to field data interpretation Peeters et al. (2003) used measured argon and neon isotope ratios from a continental aquifer in Niger to test two competing models for noble gas paleotemperature reconstructions: the partial re-equilibration (PR) model of Stute et al. (1995) and the closed system equilibration (CE) model first advanced by Aeschbach-Hertig et al. (2000). Themajor difference in these models is that the PR model assumes that diffusion, and thus kinetic fractionation, is an important mechanism. The PR model application to groundwater data from Brazil suggested that tropical low-altitude continental temperatures in the last glacial maximum were over 5 C cooler than present day. However, paleotemperature reconstructions, particularly at the Brazil site, depended on correcting for large gas excesses and their fractionation in groundwater. Peeters et al. (2003) applied both the PR and CE models to data for 20 Ne/ 22 Ne and 40 Ar/ 36 ArandconcludedthatthePRmodelshouldberejectedin favor of the CE model if one assumes the isotopic fractionation factors follow the relationship in the kinetic theory of gases (Eq. (4), β =0.5). The results of Bourg and Sposito (2008) contested the square root dependency based on their calculations of the diffusion rates in water. However, it is difficult to accurately capture the highly complex nature of molecular interactions in a model. While the CE and PR models predicted similar temperature differences between present day and the last glacial maximum (Aeschbach-Hertig et al., 2000; Peeters et al., 2003), the absolute temperatures determined were colder in the CE model. Choosing between these two models for gas excess and subsequent gas fractionations has implications not only for paleotemperature reconstructions but also groundwater age estimates (Aeschbach-Hertig et al., 2000; Peeters et al., 2003; Price et al., 2003; Castro et al., 2007) and biogeochemical fluxes (Kipfer et al., 2002). A re-interpretation of argon and neon isotope ratio measurements in groundwaters from a continental aquifer in Niger (Beyerle et al., 2003) using both the CE and PR models and our equilibrium kinetic Fig. 7. Dissolved noble gas isotope ratios ( 22 Ne/ 20 Ne, top; 36 Ar/ 40 Ar, bottom) from groundwaters in the Continental Terminal aquifer (Niger) predicted by the PR model (open symbols) and the CE model (filled circles) plotted against the measured ratios (Beyerle et al., 2003). The dashed line shows where modeled values and measured values are equal. Isotopic equilibrium fractionations used for modeling isotope ratios are those reported in this paper. The isotopic mass-dependencies of Ne and Ar diffusion coefficients for the PR model were the square-root values from Eq. (4) used in Peeters et al. (2003) (unfilled circles), those given for the MD simulations of Bourg and Sposito (2008) (triangles), and those presented here, assuming n = 2/3 (Eq. (9)) (squares). Confidence intervals are not shown, but are assumed similar to those reported by Peeters et al. (2003) (2σ; 0.07 for 20 Ne/ 22 Ne and for 40 Ar/ 36 Ar).

10 K.E. Tempest, S. Emerson / Marine Chemistry 153 (2013) isotopic fractionation factors is presented in Fig. 7. The inverse fitting program Noble90 was used to determine model parameters based on measured noble gas concentrations as in Peeters et al. (2003) and Bourg and Sposito (2008). Noble gas isotope ratios were then predicted for the PR model using these model parameters with argon and neon kinetic isotopic fractionation factors based on the kinetic theory of gases (Eq. (4), β = 0.5), the results of the Bourg and Sposito (2008) MD simulations, and the experimental results of this work. In agreement with Bourg and Sposito (2008), we note that the large differences in PR modeled and measured isotope ratios originally reported by Peeters et al. (2003) largely arise from inaccurate assumptions about neon and argon kinetic isotopic fractionations. Our empirically determined ε k values indicate less fractionation than the simulations of Bourg and Sposito (2008), bringing the PR modeled isotope ratios slightly closer to measured values. 6. Conclusions We present the first experimental determinations of the kinetic isotopic fractionation factor, α k, for argon and neon and the equilibrium isotopic fractionation factor, α eq, for neon. These precise determinations will greatly inform the use of argon and neon isotope ratio measurements as geochemical tracers. Our results generally agree with the molecular dynamics simulations of Bourg and Sposito (2008) in capturing a larger magnitude ε k for neon relative to argon, despite nearly identical isotope mass ratios. However, employing the most accepted gas exchange model for calm surface conditions, we find ε k values for argon (40/36) and neon (22/20) which are smaller in magnitude than predicted by Bourg and Sposito (2008).Ourfindings extend the results of Knox et al. (1992) and Jähne et al. (1987a) by completing experimental determinations for the kinetic and equilibrium isotopic fractionation factors for the lighter noble gases and have important implications for investigations of environmental processes using noble gas isotopes as geochemical tracers. Acknowledgments We gratefully acknowledge Charles Stump for his work with the experimental setup and aid in executing the experiments. Valuable discussions and comments were provided by the Masters Advisory Committee of Dr. Paul Quay, Dr. James Murray, and Dr. Matt Alford. Mark Haught provided indispensable technical help for mass spectrometery analysis. Funding was provided by NSF OCE Appendix A. Supplementary data Supplementary data to this article can be found online at dx.doi.org/ /j.marchem References Aeschbach-Hertig, W., Peeters, F., Beyerle, U., Kipfer, R., Paleotemperature reconstruction from noble gases in ground water taking into account equilibration with entrapped air. 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