For the majority of the samples, the 20Ne/22Ne and 36Ar/40Ar ratios agree or GS7340 very close to the ASW values. As diffusion-controlled Ne and Ar isotope fractionation in not evident, the observed noble-gas depletion can conceptually be understood in terms of loss of a gas phase being equilibrated with the surrounding pore water phase.
Such gas loss can be corrected as described for the case of organic-rich lacustrine sediments in Soppensee using a 1-step-degassing model (Brennwald et al., 2005). The 1-step-degassing model calculates the noble-gas concentration in the pore water after equilibration with a gas phase as Ci = Ci∗/(1 + B·Hi/P0) using the expected atmospheric equilibrium concentration Ci∗ (with i = Ne, Ar, Kr, Xe), the STP bubble volume per unit mass of pore water B, the Henry coefficient Hi for noble gas i (corrected for the specific hydrochemistry of Lake Van, see Section 3.2), and the STP dry-gas pressure P0.
We based our degassing correction on Ne, Ar, Kr, and Xe. We estimated B (i.e., the variable parameterising gas loss) for each sample by least-square fitting of the concentrations predicted by the degassing model to the measured noble gas concentrations (Fig. 3; for further details see Brennwald et al., 2005). The error for B is mass extinction calculated from the deviations of the modelled concentrations with respect to the measured concentrations. The estimation of B is used to calculate the original He concentration in the pore water (i.e., prior to equilibration with the gas phase).
Such gas loss can be corrected as described for the case of organic-rich lacustrine sediments in Soppensee using a 1-step-degassing model (Brennwald et al., 2005). The 1-step-degassing model calculates the noble-gas concentration in the pore water after equilibration with a gas phase as Ci = Ci∗/(1 + B·Hi/P0) using the expected atmospheric equilibrium concentration Ci∗ (with i = Ne, Ar, Kr, Xe), the STP bubble volume per unit mass of pore water B, the Henry coefficient Hi for noble gas i (corrected for the specific hydrochemistry of Lake Van, see Section 3.2), and the STP dry-gas pressure P0.
We based our degassing correction on Ne, Ar, Kr, and Xe. We estimated B (i.e., the variable parameterising gas loss) for each sample by least-square fitting of the concentrations predicted by the degassing model to the measured noble gas concentrations (Fig. 3; for further details see Brennwald et al., 2005). The error for B is mass extinction calculated from the deviations of the modelled concentrations with respect to the measured concentrations. The estimation of B is used to calculate the original He concentration in the pore water (i.e., prior to equilibration with the gas phase).