Salt fingering in subsea permafrost: Some stability and energy considerations

Abstract

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.

Seawater circulation in sediments driven by interactions between seabed topography and fluid density

“…Of the many parameters affecting circulation, the groundwater upwelling rate (or average seepage velocity) appears to be the most important, and it is primarily controlled by the head difference (as governed by the assumed head in the underlying aquifer in the model analyses) and by the hydraulic conductivity. Our results can be related to a finger velocity number presented by Gosink and Baker [], who analyzed salt fingering. They present the term, w , for the vertical velocity in a finger as …”

“…The magnitude of the seawater circulation flux driven by this mechanism is small but can be similar to that resulting from other small‐scale driving forces. Like any small‐scale surface water‐groundwater exchange mechanisms [ Riedl et al ., ; Gosink and Baker , ; Cardenas and Jiang , ], this mechanism would be difficult to detect (or to identify as a separate process) using standard SGD measurement methods. Seepage meters are the primary method for direct measurement, but their use may be problematic because typical surface areas of collection may straddle more than one local circulation cell and the discharge flux can also vary greatly over very short distances—even in a homogeneous porous medium.…”

Geoelectric observations of the degradation of nearshore submarine permafrost at Barrow (Alaskan Beaufort Sea)

“…Salt water penetration continues, since the relatively thin water layer between winter sea ice and seabed continues to be enriched in salt for most of the year. In addition to diffusive migration of salt along a concentration gradient, salt fingering due to density differences (based on differences in salt concentration and temperature) in sediment pore water may result, speeding the rate of chemical degradation at the freezing front [ Gosink and Baker , 1990]. Thermal degradation of permafrost begins as well, since the seabed temperature is warmer [ Lewellen , 1973] than the mean annual ground temperature on land, and the long‐term fate of the permafrost is sealed, as long as it remains submerged.…”

Sediment characteristics of a thermokarst lagoon in the northeastern Siberian Arctic (Ivashkina Lagoon, Bykovsky Peninsula)

“…Deeper, down to 2.8 m, the core shows relatively uniform EC values around 1000 µS cm −1 , which could be explained by the downward transportation of salt solutions through the original freshwater sediment of a thermokarst lake. This kind of salt diffusion in thawed deposits has been described for subsea permafrost deposits by Harrison and Osterkamp [19], Gosink and Baker [15] and Osterkamp [31].…”