A model for stable isotopes of residual liquid water and ground ice in permafrost soils using arbitrary water chemistries and soil‐specific empirical residual water functions

Abstract

We present the basic theory of stable isotopes (δ( 18 O) and δ(D)) of freezing water solutions in the environment set within a water isotope-augmented version of FREZCHEM(V15). We validate this model with a couple of examples. The isotopecapable FREZCHEM is simplified to run much faster using set-piece initial chemistries to calculate the freezing temperature of the remaining water. The fast version is embedded in a semi-empirical model for residual liquid water in sub-zero soils. A uniform specific soil column is driven with a defined seasonal temperature wave. The co-isotopes of the residual water and ice are calculated as a function of time and depth. The model is applied to clayey soils of the sort sampled in detail in a 10 (10 to 20 m) core suite from the Mackenzie Delta, Canada. The stable isotopes are compared to the model ones and the match is quite good except in the uppermost 2.5 m, where an upturn in the model δ( 18 O) curve is smaller. If the upper 2.5 m of icy soil is populated with "modern" water, and below by late glacial meltwater the size of the upturn in δ( 18 O) starting at 2.5 m is reproduced by the model.

Cryostratigraphy of mid-Miocene permafrost at Friis Hills, McMurdo Dry Valleys of Antarctica

“…The evolution of an initial ground-ice profile in the 50 m sediment sequence was assessed using the REGO model and its WATERREGO subroutine (Fisher et al 2020a(Fisher et al , 2020b. The model allows for evolving residual unfrozen water, including its chemistry and its freezing-point temperature, under changing soil temperature and water-ice phase changes, and the unfrozen water can migrate in the icy soils by diffusive and advective transport driven by stress fields over diurnal and seasonal temperature cycles.…”

Hugh French memorial for Permafrost and Periglacial Processes

“…However, the stable‐isotope composition of ground ice needs to be interpreted with caution, because various environmental factors (e.g., soil texture, temperature gradients) and processes (e.g., diffusion, refreezing of percolating meltwater) may modify the composition over historical or geological timescales. To this end, Fisher et al (2021, this issue) 40 develop and apply a model to predict the stable‐isotope composition of ice and residual liquid water in frozen soils that experience annual temperature cycles. Model results compare reasonably well to observations from permafrost cores in clay soils in northwest Canada, except within the upper 2.5 m of the cores.…”

Modeling δD-δ18O Steady-State of Well-Sealed Perennially Ice-Covered Lakes and Their Recharge Source: Examples From Lake Untersee and Lake Vostok, Antarctica

“…The core of the FREEZCH5 model is FREZCHEMv15, an equilibrium chemical thermodynamic model over the temperature range of −73 to 25 • C (Marion and Kargel, 2008;Fisher et al, 2020). FREZCHEM simulates the freezing of water by decreasing the temperature in fixed steps and determines the presence of residual water if the water activity calculated from the Pitzer equations is less than the equilibrium constant for water-ice.…”

“…where δ 18 Ocations are functions of the remaining liquid water molality, which are calculated at each temperature step. The FREEZCH5 model is described in detail in Supplementary Appendix A of Fisher et al (2020).…”

“…The objective of this study is to develop a model of δDδ 18 O evolution of well-sealed perennial ice-covered lakes that accounts for changing chemistry in the residual water and annual recharge that mixes with residual water. This objective was achieved by using FREEZCH5, an isotope-augmented version of the FREZCHEM low temperature geochemistry model (Marion et al, 2010;Fisher et al, 2020) and making the model recursive. The model is tested against datasets from Lake Vostok and is then used to assess the δD-δ 18 O mass balance of Lake Untersee and evaluate if the lake is in isotopic steady-state.…”