An assessment has been made of an electromagnetic induction (EM) technique for reconnaissance surveys of soil salinity. The instrument used provides values of apparent electrical conductivity to depths ranging from 7.5 to 60 m. A comparison of EM values with actual profile salinities at 19 sites of widely differing geological and geomorphic origin showed that approximately 65% of the variance of EM values is explained in terms of salinity alone. An area of 10000 km2 in the mid-Lachlan River Valley of New South Wales was surveyed with a grid spacing of approximately 5 km at a rate of 30-40 sites per day. This permitted the definition of areas of high subsoil apparent conductivity and hence, by inference, of high soil salinity. The EM technique was demonstrated to have a potential for providing a significant contribution to land use planning at both a regional and local scale, by defining areas of possible salinity hazard.
A seasonal active layer associated with subsea permafrost was found in the sediments near the seabed of the Beaufort Sea near Prudhoe Bay, Alaska. The active layer existed where sea ice was frozen to the seabed and also in shallow water where the under-ice seawater salinities exceeded open-water values. Initial freezing of the active layer was about coincident with the formation of new sea ice. Within 400 m of shore, it appeared to freeze to an underlying ice-bonded permafrost table (IBPT). Farther offshore, where this table is deeper, the active layer thickness decreased with distance offshore, and the layer was underlain by a talik. Relative ice contents in the active layer generally decreased with distance offshore, were a few hundred parts per thousand (ppt) in the fall, and ranged to more than 800 ppt in the spring. Seasonal changes in the bulk soil solution salinity showed that the partially frozen active layer redistributed salts during freezing, was infiltrated by concentrated brines derived from the growth of sea ice, and affected the timing of brine drainage to lower depths in the sediments. These brines provide the salts required for thawing the underlying subsea permafrost in the presence of negative sediment temperatures. The region near shore had a partially frozen transition layer just above the IBPT where the ice content increased and the brine content decreased with depth. Temperature at the IBPT is nearly constant beyond about 412 m to at least 3.5 km offshore at about -2.41øC, implying relatively constant soil solution salinities there. INTRODUCTION Subsea permafrost exists in the continental shelves of theArctic Ocean and associated seas, where it is a product of changing sea levels, shoreline erosion, and past cold climates r, ._ _, ...... ,, ....... , Lewetten, t•v•acnay, 1972; 1973]. Whea sea Icvc• mc low, cold climates cause permafrost to aggrade in those parts of the shelves where the ground surface is exposed to the atmosphere. When sea levels rise, the permafrost is covered by relatively warm and salty seawater. After submergence, this subsea permafrost degrades, thawing from the seabed downward by the influx of salt and heat as a result of the new boundary conditions, even in the presence of negative mean seabed temperatures [Harrison and Osterkamp, 1978]. Subsea permafrost also thaws from the bottom by geothermal heat flow. Northwest of Prudhoe Bay at distances up to several kilometers or more offshore, thawing rates at both the top and bottom are slow, of the order of a centimeter per year [Lachenbruch et al., 1982]. Consequently, the time after submergence required to thaw the subsea permafrost completely may approach tens of millennia. Thawing at the seabed results in a talik or thawed layer, which thickens seaward, and ice-bearing subsea permafrost which generally thins seaward [Hunter et al., 1976; Osterkamp and Harrison, 1977; Lachenbruch et al., 1982; Neave and Sellmann, 1984; Osterkamp et al., 1985]. Development of a talik below the seabed after submergence is complicated by s...
Columns of sand with grain sizes from about 250 to 600/am were saturated with a sodium chloride solution with a concentration of 35 parts per thousand (ppt) and frozen at constant rates ranging from 1 to 20 mm d -•. Electrical conductivity of the soil solution, water content, and temperature were measured during the experiments. The interface between the frozen and unfrozen regions was sharply defined and mechanically hard with no evidence for a soft, ice-bearing transition region thicker than a millimeter. Ice content near the interface was less than 50%. Substantial salt redistribution occurred with downward freezing but not with upward freezing. For downward freezing, the amount of salt rejected near the interface decreased with increasing freezing rate and increased with the salinity of the soil solution in the unfrozen region. Salt redistribution consisted of salt rejection and brine drainage from an extended partially frozen region just above the interface. Brine drained from this partially frozen region until the ice content reached about 900-950 ppt. Brine drained through the interface and mixed rapidly in the unfrozen region, apparently by salt fingering. No evidence could be found for a salt-enriched layer just below the interface. There was a layer just above the interface, moving with it, which was undersaturated at low and oversaturated at high freezing rates. An effective distribution coefficient may be useful in describing salt redistribution during freezing of saline sands.
Salt fingering appears to be a viable mechanism for salt transport in the thawing layer (talik) above ice‐bonded permafrost near the seabed in the Arctic shelves [Baker and Osterkamp, 1988]. These saltwater infusions occur where concentrated brines overlie less concentrated or less dense brines. At least two mechanisms exist which cause this condition near the seabed: first, salts rejected during sea ice growth result in the formation of a concentrated brine layer on the seabed, and second, salts rejected during sediment freezing near the seabed can result in a concentrated brine layer forming within the deeper and yet unfrozen sediments. Unstable salt fingering in a vertical tube containing a porous media is known to develop whenever the density gradient exceeds the critical value: ∂ρ/∂z = 3.390 μk/gkb2 [Wooding, 1959]. This article presents a simple derivation and interpretation of this critical condition in terms of competing time scales. A convective time scale represents the time required for formation of a vertical wedge‐shaped flow due to density differences, and a diffusive time scale represents the time required to diffuse the density difference horizontally across the layer. The critical density gradient is determined from a ratio of these time scales. The stability criterion suggests downward salt fingering whenever the density gradient at Prudhoe Bay exceeds 6.2×10−5 g cm−4 in thawed subsea permafrost sediments. Maximum predicted velocity of the saltwater fingering is about 2 m d−1, and this is consistent with the direction and magnitude indicated by measurements of pressure gradients and numerical modeling in the thawing permafrost. The energy dissipated by viscous forces in the thawed layer balances the potential energy added by the salt fingers caused by concentrated brines on the seabed.
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