Heat and salt flow in subsea permafrost modelled with CryoGRID2
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Degradation of sub-aquatic permafrost can impact offshore infrastructure, affect coastal erosion and release large quantities of methane, which may reach the atmosphere and function as a positive feedback to climate warming. The degradation rate depends on the duration of inundation, warming rate, sediment characteristics, the coupling of the bottom to the atmosphere through bottom-fast ice, and brine injections into the sediment. We apply the Cryo-GRID2 model, coupled to a salt diffusion model, to near-shore subsea permafrost thawing offshore of the Bykovsky Peninsula in Siberia. We model permafrost through multiple settings, including 1) terrestrial permafrost, 2) shallow sea with ice grounding, and 3) shallow offshore sea (<= 5.3m depth) without ice grounding. The model uses a terrestrial permafrost temperature of -10 °C at the depth of zero annual amplitude, based on borehole observations, and a coastal erosion rate of 0.5 m/year, based on historical remote sensing imagery dating back to 1951. The seawater salinity prior to ice formation is based on a series of conductivity, temperature, and depth (CTD) measurements from summer 2017, as well as from Soil Moisture and Ocean Salinity (SMOS) satellite data. Water depth is available from echo-sounding surveys made in parallel with floating electrode electrical resistivity surveys in summer 2017. The model outputs are compared to the depth of the ice-bearing permafrost table (IBPT) determined from an electrical resistivity survey perpendicular to the shoreline. The floating electrode survey was combined with a terrestrial resistivity survey to show the transition from undisturbed terrestrial permafrost to submerged permafrost. The geoelectric surveys show a gently inclining IBPT table perpendicular to the coastline, which can be explained by a decreasing rate of degradation with increasing period of inundation. As the inundation period increases, the diffusive (heat and salt) gradients become less steep. The IBPT is located 20 m below the seabed 300 m offshore, which corresponds to 600 years of coastal erosion and an average IBPT degradation rate of 0.33 m per decade. The modelling results show an IBPT 18 m below the seabed and salty sediment up to 14 m below the seabed 300 m offshore. Therefore, the modelling results agree, at least qualitatively, with the sediment
state inferred from the geoelectric data. Coupled heat and salt diffusion produces profiles of temperature and salt concentration in sediment as a function of time. The inclusion of salt flow in thermal models is particularly important in shallow waters where cryotic sediments form due to negative benthic water temperatures or ice grounding, because the depressed freezing point produced by salt diffusion can delay or prevent ice formation in the sediments and enhance the IBPT degradation rate.Cite
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Ice-rich permafrost coasts in the Arctic are susceptible to a variety of changing environmental factors, all of which currently point to increasing coastal erosion rates and mass fluxes of sediment and carbon to the shallow arctic shelf seas. Coastal erosion and flooding inundate terrestrial permafrost with seawater and create submarine permafrost. Permafrost begins to warm under marine conditions, which can destabilize the sea floor and may release greenhouse gases. The rate and spatial distribution of subsea permafrost degradation in the Laptev, East Siberian and Chukchi seas, which together comprise more than half of the Arctic Ocean continental shelf, remain poorly explored. We report on the transition of terrestrial to subsea permafrost at four coastal sites in the Laptev Sea: Cape Mamontov Klyk in the western Laptev Sea, and Buor Khaya Peninsula, Muostakh Island and the Bykovsky Peninsula in the central Laptev Sea. We use coastal erosion rates from about the last 70 years to estimate the period of inundation at these sites. Combined with direct (drilling and temperature) and indirect (geophysical) observations of thaw depths of ice-bonded permafrost, we estimate recent degradation rates of permafrost over the past centuries. Based on these observations, the unfrozen sediment layer overlying ice-bonded permafrost increased from less than a meter at the shoreline to over 30 m below seabed with increasing distance from the shoreline at our study sites, with high spatial variability between and within sites. Observed temperatures of the sediment ranged from -5 °C to positive temperatures. In coastal sediments, it is difficult to establish an age-depth model, making corroboration of estimated degradation rates a challenge. Nonetheless, as the thickness of the unfrozen sediment layer increases over time, the vertical thermal and salt concentration gradients decrease, slowing the downward heat and mass fluxes responsible for degradation. High sedimentation rates and ice contents probably stabilize subsea permafrost. We suggest that permafrost degradation relevant to gas flow is likely to have occurred where permafrost warmed prior to inundation.
Coastal erosion
Thermokarst
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Melt pond
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To better understand the response of the western Arctic upper ocean to late summer ice-ocean interactions, a range of surface, interior, and basal sea ice conditions were simulated in a 1-D turbulent boundary layer model. In-ice and under-ice autonomous observations from the 2014 Marginal Ice Zone Experiment provided a complete characterization of the late melt-season sea ice and were used to set initial conditions, update boundary conditions, and conduct model validation studies. Results show that underestimates of open water and melt pond fraction at the sea ice surface had the largest influence on ocean-to-ice turbulent heat fluxes reducing basal melt rates by as much as 32%. This substantial reduction in latent heat loss was attributed to underestimates of open water areas and the exclusion of melt ponds by low-resolution synthetic aperture radar imagery. However, the greatest overall effect on the ice-ocean boundary layer came from mischaracterizations of basal roughness, with smooth ice scenarios resulting in 7 m of summer halocline shoaling and preservation of the near-surface temperature maximum. Rough ice conditions showed a 23% deepening of the mixed layer and erosion of heat storage above 40 m. Adjustments of conductive heat fluxes had little effect on the near-interface heat budget due to small internal thermal gradients within the late summer sea ice. Results from the 1-D boundary layer simulations highlight the most influential components of sea ice structure during late summer conditions and provide the magnitude of errors expected when ice conditions are mischaracterized.
Melt pond
Sea ice concentration
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Abstract A new halodynamic scheme is coupled with the Los Alamos sea ice model to simulate western Weddell Sea ice during the winter‐spring transition. One‐dimensional temperature and salinity profiles are consistent with the warming and melt stages exhibited in first‐year ice cores from the 2004 Ice Station POLarstern (ISPOL) expedition. Results are highly sensitive to snowfall. Simulations which use reanalysis precipitation data do not retain a snow cover beyond mid‐December, and the warming transition occurs too rapidly. Model performance is greatly improved by prescribing a snowfall rate based on reported snow thicknesses. During ice growth prior to ISPOL, simulations indicate a period of thick snow and upper ice salinity enrichment. Gravity drainage model parameters impact the simulation immediately, while effects from the flushing parameter (snow porosity at the ice top) appear as the freeboard becomes negative. Simulations using a snow porosity of 0.3, consistent with that of wet snow, agree with salinity observations. The model does not include lateral sources of sea‐water flooding, but vertical transport processes account for the high upper‐ice salinities observed in ice cores at the start of the expedition. As the ice warms, a fresh upper‐ice layer forms, and the high salinity layer migrates downward. This pattern is consistent with the early spring development stages of high‐porosity layers observed in Antarctic sea ice that are associated with rich biological production. Future extensions of the model may be valuable in Antarctic ice‐biogeochemical applications.
Fast ice
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Abstract Although the Canadian polar shelf is dominated by thick fast ice in winter, areas of young ice or open water do recur annually at locations within and adjacent to the fast ice. These polynyas are detectable by eye and sustained by wind or tide‐driven ice divergence and ocean heat flux. Our ice‐thickness surveys by drilling and towed electromagnetic sounder reveal that visible polynyas comprise only a subset of thin‐ice coverage. Additional area in the coastal zone, in shallow channels and in fjords is covered by thin ice which is too thick to be discerned by eye. Our concurrent surveys by CTD reveal correlation between thin fast ice and above‐freezing seawater beneath it. We use winter time series of air and ocean temperatures and ice and snow thicknesses to calculate the ocean‐to‐ice heat flux as 15 and 22 W/m 2 at locations with thin ice in Penny Strait and South Cape Fjord, respectively. Near‐surface seawater above freezing is not a sufficient condition for ocean heat to reach the ice; kinetic energy is needed to overcome density stratification. The ocean's isolation from wind under fast ice in winter leaves tides as the only source. Two tidal mechanisms driving ocean heat flux are discussed: diffusion via turbulence generated by shear at the under‐ice and benthic boundaries, and the internal hydraulics of flow over topography. The former appears dominant in channels and the coastal zone and the latter in some silled fjords where and when the layering of seawater density permits hydraulically critical flow.
Fjord
Fast ice
Stratification (seeds)
Iceberg
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The distribution and origin of offshore permafrost is discussed for the southern Beaufort Sea. Two types of permafrost are identified: permafrost which is in thermal equilibrium with negative sea bottom temperatures, and disequilibrium permafrost, which is not in equilibrium with either positive or negative sea bottom temperatures. The origin of permafrost is considered in terms of the Quaternary period when coastal areas were exposed to cold air temperatures and then submerged. The effect of warm river waters, primarily from the Mackenzie River, is shown to ameliorate coastal water temperatures and may be responsible for a thin active layer at some sites. Water quality and oxygen isotope ratios are given for some samples. The evidence suggests that some relic land permafrost, with ground ice, is present beneath the southern Beaufort Sea. Perforated permafrost should be present, but not extensive thermokarst depressions. By inference, permafrost probably underlies much of the Canadian Arctic waters, although ground ice is likely restricted to a relatively few shallow coastal zones.
Thermokarst
Arctic geoengineering
Beaufort sea
Active layer
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Abstract In the upper McMurdo Dry Valleys, 90% of the measured ice table depths range from 0 to 80 cm; however, numerical models predict that the ice table is not in equilibrium with current climate conditions and should be deeper than measured. This study explored the effects of boundary conditions (air versus ground surface temperature and humidity), ground temperature cycles, and their diminishing amplitude with depth and advective flows (Darcy flow and wind pumping) on water vapor fluxes in soils and ice table depths using the REGO vapor diffusion model. We conducted a series of numerical experiments that illustrated different hypothetical scenarios and estimated the water vapor flux and ice table depth using the conditions in University Valley, a small high elevation valley. In situ measurements showed that while the mean annual ground surface temperature approximates that in the air, the mean annual ground surface relative humidity (>85% ice ) was significantly higher than in the atmosphere (~50% ice ). When ground surface temperature and humidity were used as boundary conditions, along with damping diurnal and annual temperature cycles within the sandy soil, REGO predicted that measured ice table depths in the valley were in equilibrium with contemporary conditions. Based on model results, a dry soil column can become saturated with ice within centuries. Overall, the results from the new soil data and modeling have implications regarding the factors and boundary conditions that affect the stability of ground ice in cold and hyperarid regions where liquid water is rare.
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Due to difficult access to large lakes at the time of freezing their hydrodynamic regime is poorly known. In this study, we investigated the dynamics of temperature during the formation of ice cover in 78 m deep Lake Paajarvi in southern Finland. Temperature loggers and CTD were the main tools applied. In early winter, the cooling rate was highest in shallow areas and in bays where water exchange with the central basin was lowest. At the deepest point of the lake temperatures remained vertically uniform until 3 C, after which temperature was inversely distributed and at times also stratified. When open water area decreased, water mixing by wind was dramatically reduced. In accordance with that, we observed an increase in temperature and a decrease in oxygen below the depth of 50 m before the whole lake was ice covered. This means that heat and possible electrolyte flux from the sediment increased the overlaying water density so that it started to flow along the rather steep sediment slopes. Our results help to understand large scale and dynamic translocations of water masses in early winter. This can help interpretation and modeling of oxygen consumption in deep lake basins in winter.
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ABSTRACT Large areas of first-year sea ice containing disseminated loads of silt- and clay-sized materials are common off the northern coast of Alaska. Sand and coaser material is also found in this ice in the form of distinct masses of sediment-laden ice. Sediment is entrained into the sea-ice cover during fall storms when frazil, or small discs of ice in suspension, is actively forming in the supercooled water column. At this time, sea ice is unconsolidated and very mobile, making long-range ice rafting possible. Sediment-rich ice may be advected off the shelf and, upon melting, deposit material in the deep Arctic basin. The amount of sediment incorporated into the ice cover varies considerably from year to year, depending on the location and severity of storms accompanying freezeup. In 1978 the seasonal ice cover in an area off northern Alaska carried 16 times as much sediment as the total annual suspended sediment input from adjacent coastal rivers. Sea-ice sediment transport is important in the overall sediment budget in northern polar regions, particularly the transport of fine-grained material, but entrainment mechanisms are poorly understood.
Entrainment (biomusicology)
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