First “in situ” determination of gas transport coefficients (, , and ) from bulk gas concentration measurements (O2, N2, Ar) in natural sea ice (original) (raw)
Related papers
Journal Of Geophysical Research: Oceans, 2014
We report bulk gas concentrations of O 2 , N 2 , and Ar, as well as their transport coefficients, in natural landfast subarctic sea ice in southwest Greenland. The observed bulk ice gas composition was 27.5% O 2 , 71.4% N 2 , and 1.09% Ar. Most previous studies suggest that convective transport is the main driver of gas displacement in sea ice and have neglected diffusion processes. According to our data, brines were stratified within the ice, so that no convective transport could occur within the brine system. Therefore, diffusive transport was the main driver of gas migration. By analyzing the temporal evolution of an internal gas peak within the ice, we deduced the bulk gas transport coefficients for oxygen (D O2), argon (D Ar), and nitrogen (D N2). The values fit to the few existing estimates from experimental work, and are close to the diffusivity values in water (10 25 cm 2 s 21). We suggest that gas bubbles escaping from the brine to the atmosphere-as the ice gets more permeable during melt-could be responsible for the previously reported high transport coefficients. These results underline that when there is no convective transport within the sea ice, the transport of gas by diffusion through the brines, either in the liquid or gaseous phases, is a major factor in controlling the ocean-atmosphere exchange.
Journal of Geophysical Research: Oceans, 2015
Sea ice is a defining feature of the polar marine environment. It is a critical domain for marine biota and it regulates ocean-atmosphere exchange, including the exchange of greenhouse gases such as CO 2 and CH 4. In this study, we determined the rates and pathways that govern gas transport through a mixed sea ice cover. N 2 O, SF 6 , 3 He, 4 He, and Ne were used as gas tracers of the exchange processes that take place at the ice-water and air-water interfaces in a laboratory sea ice experiment. Observation of the changes in gas concentrations during freezing revealed that He is indeed more soluble in ice than in water; Ne is less soluble in ice, and the larger gases (N 2 O and SF 6) are mostly excluded during the freezing process. Model estimates of gas diffusion through ice were calibrated using measurements of bulk gas content in ice cores, yielding gas transfer velocity through ice (k ice) of $5 3 10 24 m d 21. In comparison, the effective airsea gas transfer velocities (k eff) ranged up to 0.33 m d 21 providing further evidence that very little mixed-layer ventilation takes place via gas diffusion through columnar sea ice. However, this ventilation is distinct from air-ice gas fluxes driven by sea ice biogeochemistry. The magnitude of k eff showed a clear increasing trend with wind speed and current velocity beneath the ice, as well as the combination of the two. This result indicates that gas transfer cannot be uniquely predicted by wind speed alone in the presence of sea ice.
2014
We present and compare the dynamics (i.e., changes in standing stocks, saturation levels and concentrations) of O 2 , Ar and N 2 in landfast sea ice, collected in Barrow (Alaska), from February through June 2009. The comparison suggests that the dynamic of O 2 in sea ice strongly depends on physical processes (gas incorporation and subsequent transport). Since Ar and N 2 are only sensitive to the physical processes in the present study, we then discuss the use of O 2 / Ar and O 2 / N 2 to correct for the physical contribution to O 2 supersaturations, and to determine the net community production (NCP). We conclude that O 2 / Ar suits better than O 2 / N 2 , due to the relative abundance of O 2 , N 2 and Ar, and the lower biases when gas bubble formation and gas diffusion are maximized. We further estimate NCP in the impermeable layers during ice growth, which ranged from −6.6 to 3.6 µmol O 2 L −1 d −1 , and the concentrations of O 2 due to biological activity in the permeable layers during ice decay (3.8 to 122 µmol O 2 L −1). We finally highlight the key issues to solve for more accurate NCP estimates in the future.
Modelling argon dynamics in first-year sea ice
Ocean Modelling, 2014
Focusing on physical processes, we aim at constraining the dynamics of argon (Ar), a biogeochemically inert gas, within first year sea ice, using observation data and a onedimensional halo-thermodynamic sea ice model, including parameterization of gas physics. The incorporation and transport of dissolved Ar within sea ice and its rejection via gas-enriched brine drainage to the ocean, are modeled following fluid transport equations through sea ice. Gas bubbles nucleate within sea ice when Ar is above saturation and when the total partial pressure of all three major atmospheric gases (N 2 , O 2 and Ar) is above the brine hydrostatic pressure. The uplift of gas bubbles due to buoyancy is allowed when the brine network is connected with a brine volume above a given threshold. Ice-atmosphere Ar fluxes are formulated as a diffusive process proportional to the differential partial pressure of Ar between brine inclusions and the atmosphere. Two simulations corresponding to two case studies that took place at Point Barrow (Alaska, 2009) and during an ice-tank experiment (INTERICE IV, Hamburg, Germany, 2009) are presented. Basal entrapment and vertical transport due to brine motion enable a qualitatively sound representation of the vertical profile of the total Ar (i.e. the Ar dissolved in brine inclusions and contained in gas bubbles; TAr). Sensitivity analyses suggest that gas bubble nucleation and rise are of most importance to describe gas dynamics within sea ice. Ice-atmosphere Ar fluxes and the associated parameters do not drastically change the simulated TAr. Ar dynamics are dominated by uptake, transport by brine dynamics and bubble nucleation in winter and early spring; and by an intense and rapid release of gas bubbles to the atmosphere in spring. Important physical processes driving gas dynamics in sea ice are identified, pointing to the need for further field and experimental studies.
Journal of Geophysical Research C: Oceans, 2013
The impacts of the seasonal evolution of sea-ice physical properties on ice-ocean biogeochemical exchanges were investigated in landfast ice at Barrow (Alaska) from January through June 2009. Three stages of brine dynamics across the annual cycle have been identified based on brine salinity, brine volume fraction, and porous medium Rayleigh number (Ra). These are sea-ice bottom-layer convection, full-depth convection, and brine stratification. We further discuss the impact of brine dynamics on biogeochemical compounds in sea ice: stable isotopes of water (D, 18 O), nutrients (NO 3 À , PO 4 3À , NH 4 þ), microalgae (chlorophyll-a), and inert gas (argon). In general, full-depth convection events favor exchanges between sea ice and seawater, while brine stratification limits these exchanges. However, argon responds differently to brine dynamics than the other biogeochemical compounds analyzed in this study. This contrast is attributed to the impact of bubble nucleation on inert gas transport compared to the other biogeochemical compounds. We present a scenario for argon bubble formation and evolution in sea ice and suggest that a brine volume fraction approaching 7.5-10% is required for inert gas bubbles to escape from sea ice to the atmosphere.
A parameter model of gas exchange for the seasonal sea ice zone
Ocean Science, 2014
Carbon budgets for the polar oceans require better constraint on air-sea gas exchange in the sea ice zone (SIZ). Here, we utilize advances in the theory of turbulence, mixing and air-sea flux in the ice-ocean boundary layer (IOBL) to formulate a simple model for gas exchange when the surface ocean is partially covered by sea ice. The gas transfer velocity (k) is related to shear-driven and convection-driven turbulence in the aqueous mass boundary layer, and to the meansquared wave slope at the air-sea interface. We use the model to estimate k along the drift track of ice-tethered profilers (ITPs) in the Arctic. Individual estimates of daily-averaged k from ITP drifts ranged between 1.1 and 22 m d −1 , and the fraction of open water (f) ranged from 0 to 0.83. Converted to area-weighted effective transfer velocities (k eff), the minimum value of k eff was 10 −5 m d −1 near f = 0 with values exceeding k eff = 5 m d −1 at f = 0.4. The model indicates that effects from shear and convection in the sea ice zone contribute an additional 40 % to the magnitude of k eff , beyond what would be predicted from an estimate of k eff based solely upon a wind speed parameterization. Although the ultimate scaling relationship for gas exchange in the sea ice zone will require validation in laboratory and field studies, the basic parameter model described here demonstrates that it is feasible to formulate estimates of k based upon properties of the IOBL using data sources that presently exist.
Estimation of oxygen and carbonic acid diffusion through sea ice
Izvestiya, Atmospheric and Oceanic Physics, 2012
The gas permeability of the pore visibly exceeds the gas permeability of continuous solid ice with no pores. Expressions for the diffusion coefficients of oxygen and CO 2 through sea ice at a given ice temperature and salinity are obtained. Calculations of the gas transfer for the central part of the Chukchi Sea are fulfilled. Numerical experiments have shown that gas fluxes through thin sea ice are not negligibly small. The fluxes significantly decrease only if one year ice thick ness exceeds about 100 cm.
The Cryosphere, 2014
Typically, gas transport through firn is modeled in the context of an idealized firn column. However, in natural firn, imperfections are present, which can alter transport dynamics and therefore reduce the accuracy of reconstructed climate records. For example, ice layers have been found in several firn cores collected in the polar regions. Here, we examined the effects of two ice layers found in a NEEM, Greenland firn core on gas transport through the firn. These ice layers were found to have permeability values of 3.0 and 4.0 × 10<sup>−10</sup> m<sup>2</sup>, and are therefore not impermeable layers. However, the shallower ice layer was found to be significantly less permeable than the surrounding firn, and can therefore retard gas transport. Large closed bubbles were found in the deeper ice layer, which will have an altered gas composition than that expected because they were closed near the surface after the water phase was present. The bubbles in t...
The physical basis for gas transport through polar firn: a case study at Summit, Greenland
The Cryosphere Discussions, 2013
Compared to other natural porous materials, relatively little is known about the physical nature of polar firn. This intricate network of ice and pore space that comprises the top 60-100 m of the polar ice sheets is the framework that forms the natural archive of past climate information. Despite the many implications for ice core interpretation, direct measurements of physical properties throughout the firn column are limited. Models of gas transport through firn are used to interpret in-situ chemical data which is retrieved to analyze past atmospheric composition. These traditional models treat the firn as a "black box," with gas transport parameters tuned to match gas concentrations with depth to known atmospheric histories. Though this method has been largely successful and provided very useful insights, there are still many questions and uncertainties to be addressed. This work seeks to understand the impact of firn structure on gas transport in firn from a first principles standpoint through direct measurements of permeability, gas diffusivity and microstructure. The relationships between gas transport properties and microstructure will be characterized and compared to existing relationships for general porous media. Direct measurements of gas diffusivity are compared to diffusivities deduced from models based on firn air chemical sampling. Our comparison illuminates the primary importance of including microstructural parameters, beyond just porosity or density, in mass transport modeling, and it provides insights about the nature of gas transport throughout the firn column. Guidance is provided for development of nextgeneration firn air transport models.
Frontiers in Earth Science
The increased fraction of first year ice (FYI) at the expense of old ice (second-year ice (SYI) and multi-year ice (MYI)) likely affects the permeability of the Arctic ice cover. This in turn influences the pathways of gases circulating therein and the exchange at interfaces with the atmosphere and ocean. We present sea ice temperature and salinity time series from different ice types relevant to temporal development of sea ice permeability and brine drainage efficiency from freeze-up in October to the onset of spring warming in May. Our study is based on a dataset collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) Expedition in 2019 and 2020. These physical properties were used to derive sea ice permeability and Rayleigh numbers. The main sites included FYI and SYI. The latter was composed of an upper layer of residual ice that had desalinated but survived the previous summer melt and became SYI. Below this ice a layer of new first-...