Ninth International Conference on Permafrost ... - IARC Research

Tides as a Possible Reason for Massive Ice Beds FormationSergey A. Sokratov, German A. RzhanitsynFaculty of Geography, Moscow State University, GSP–1, Moscow, 119991, RussiaIntroductionThe argumentations on the genesis of massive ice bedsdiscovered on wide territories of the Arctic and Subarctichave special meaning for the Russian permafrost scientificcommunity. The critical point of the ongoing discussionsturns out to be either marine or glacier origin of the massiveice beds, since this affects in whole the interpretation ofthe paleo-history of Northern Eurasia (Solomatin 2005).It should be emphasized that the massive ice beds ofinterest (plastovye l’dy) neither are a part of the Russian groundice classification (Popov et al. 1985) nor a singled-out type ofground ice in the IPA glossary (van Everdingen 2005). Froma terminology viewpoint, different genesis of the massive icebeds at different sites is the natural expectation. Recently,these massive ice beds are being classified by expectedgenetic types (Shpolyanskaya & Streletskaya 2004).However, the disagreement on either buried or in-site originof seaside massive ice beds, especially, still exists due touncertainties in explanations of the existence of salinefreemassive ice lenses incorporated into salted marinesediments, the isotopic difference between the sea waterand the massive ice lenses, and particularly the morphologyinterpreted as the deformation markings on some of themassive ice beds (Solomatin 2005).Leaving aside the fundamental question of ice sheetexistence over Northern Eurasia, the paleo-climate signalenclosed into the massive ice beds and their bearing stratais the product of the processes responsible for the formation,growth, and melt of an ice body. Since most of the knownmassive ice beds are found in the former or presentcoastal zones (Shpolyanskaya & Streletskaya 2004), it isworth considering coastal zones-specific processes as thepossible participants in the massive ice beds formation andmaintenance. The background of the reported investigationwas to test the possibility that periodic natural fluctuations ofgroundwater level due to upthrust of groundwater runoff bytides play a part in ice lens formation. Such a mechanism, withfavorable groundwater freezing climate conditions, allowsfor formation of saline-free ice lenses from groundwaterwith close to the atmospheric precipitations isotopic contentin the saline marine sediments.Tides and Groundwater RunoffThe result of field investigations at the White Seacoast (Pon’goma) was the confirmation of the dependenceof groundwater level variation on the tidal regime, at least inthe case of sandy ground and tides up to 2 m (Fig. 1).The dependence observed by measurements in dug pitsby automatic water level sensors was pronounced up to tensof meters from the shoreline. Figure 1 shows the results ofFigure 1. The correlation between sea level variation andgroundwater level variation.the groundwater level measurements in a dug pit 7 m fromwater’s edge at maximal tide, 1 m above the maximal sealevel. The variation is extracted from the overall changeof groundwater level due to variability in precipitationand snowmelt, and represents about 10% of the overallvariability. The delay between the extremums of sea andgroundwater level variations is approximately 4 hr. Suchdependence was not observed in another area of the coastalzone with clayey and peat grounds, despite high (up to 9 m)tides (Mezenskaya Bay, White Sea).The field observations also demonstrated strong seasonalvariability of the dependence of the groundwater levelvariability on tides up to disappearance of the propagationof the tides’ signal at distance from the shoreline, caused bythe amount and unevenness of the groundwater runoff. Animportant role in the tides’ signal propagation was foundto belong to the formation of an impermeable seasonallyfrozenground layer near the soil surface. Therefore, itwas concluded that the relation between the tides and thegroundwater level variation is different for territories withdifferent texture of soil, is determined by the amount of thegroundwater runoff and by the activity of tides, and variesconsiderably in dependence on the soil freezing conditions.A study of thick deposits of ground ice in exposure at theUral coast of Baidaratskaya Bay revealed their polygeneticorigin. The analysis of the structural characteristics of theice, however, suggested the possibility of participation of theproposed mechanism in ice lenses formation.Groundwater Level Variation and Ice LensGrowthThe laboratory studies included modeling of thegroundwater level change, periodically introduced upwardinto two-layer samples with unidirectional frost penetration297