2 Volumes Final Proceedings - Washington 1984.pdf - IARC Research

CRYOGEOTHERMAL PROBLEMS IN THE STUDY OF THE ARCTIC OCEANYa. V. Neizvestnov, V.A.Soloviev, and G.D. GinsburgAl 1-Uniorl Institute of Mineral Resources of the World OceanLeningrad, USSROne of the most notable geological-geophysical features of the Arctic Ocean is thepresence in any sea-bed section of a cryolithozone, involving frozen sediments andground ice. The stages of its evolution in shelf areas correspond to cyclic regressionsand transgressions in the Polar Arctic Basin. This involves an oscillationfrom unfrozen submarine sediments through a newly formed submarine frozen to a zonefrozen terrestrial zone, and then back through a relic submarine frozen zone to anunfrozen submarine zone. Sedimentation on the sea bed on the shelf is closely relatedto the evolution of the cryolithozone. Widespread occurrence of ice-rich lacustrinealluvialdeposits on the Arctic shelf in the Late Pleistocene is inferred on the basisof peculiarities of sedimentogenesis in the course of thermal abrasion of the sedimentsduring the transgressive phase of development of the Arctic Basin. Submarine lithogenesisis accompanied by unique changes in the fluid phase, viz., salinization ofpore water and the formation of gas hydrates. It is possible to outline possible gashydrate-bearing areas on the basis of cryogeothermal reconstructions. Submarinepermafrost also dictates a number of peculiarities in the geophysical fields, especiallythe mobility of the thermal fields; thisextremely critical in the case of formationand degradation of the frozen andlor gas hydrate-bearing strata.The cryolithozone is the most distinctive andcharacteristic geologic-geophysical feature of theArctic Ocean area. This paper deals mainly withcryogeothermal aspects related to paleo-geography,polar lithogenesis, and changes in the fluid phasemarine frozen zone. The first stage favored deepfreezing of much of the shelf, at the presentapproximately 110 m isobath. In the easternEurasian shelf, over a vast area adjacent to theNew Siberian Islands, off the southern Laptev Sea,of rocks and bottom sediments, in particular, gas and the southwestern East Siberian Sea, there washydrate formation, peculiarity of geophysical field a concurrent freezing sedimentation and the formaisdiscussed as well.A cryolithozone, submarine inclusive, may betion of thick syngenetic polygonal wedge ice. Thepresent subaqueous frozen rocks outside thedetermined as a part of the lithosyhere within the littoral zone are mainly relics of that time.negative temperature belt. Physically, it may be During the last transgression, which began 18-frozen (ice-bearing, in particular ice-bonded) and 19,000 yr B.P., the frozen zone was not completelyunfrozen (saturated by negative temperature salinewaters-cryopegs). To distinguish types of submardegradedand it fell within the subaqueous environment.This stage was marked by thermal abrasionine cryogeothermal environments it is reasonable to that resulted in the reworking of older rocks thattake into account a thermic field pattern, whichcan be stationary and nonstationary. The nonstationarystate is typical of the submarine frozencryolithozone (frozen zone, for short) both for theaggradation during the formation and the degrada-were different in genesis and age. The heavilyiced syngenetically frozen deposits from the UpperPleistocene were then thermally abraded for theentire thickness. Thermal abrasion was strongeston the eastern Eurasian Arctic (Are, 1980) andtion of relic frozen ground.Beaufort Sea shelves.Conditions favorable for deep freezing andThe above two stages of cryolithozone evolutionfrozen zone formation on land in the northern polar are based on numerous datings of the sea level inarea seem to have existed during the entire Late various tectonically stable regions of the worldCenozoic since the Eopleistocene. Within the ocean over the last 30,000 yr (Curray, 1961, andpresent shelf area, freezing could take place only others) including the Laptev Sea Shelf (Holmes andduring subaerial stages, though it may start even Creager, 1974). The average curve plotted usingunder subaqueous conditions when the water depth these data (Figure 1) is free of a tectonic condoesnot exceed the ice thickness. A transition stituent and hence reflects actual eustatic changesof the frozen zone, formed under subaerial condi- (the nature of which can be hydrocratic, particutions,into the subaqueous position is accompanied larly glacioeustatic and geocratic). The eustaticby abrasion and thermal abrasion of the frozen changes were a global factor that determined theground. Thus the evolution of the shelf cryolitho- cyclic development of the shelf cryolithozone. Inzone is subject both to subaerial and subaqueous certain cases the change of subaqueous and subconditionsand by the change of environments. aerial regimes is independent of eustatic changes,Two final stages-Late Pleistocene (pre-Holocene) though against their background. Local coastlineregressions and the last Late Pleistocene-Holocene displacements now taking place in some areas offtransgression-affected most of the present sub- the Arctic Ocean are mainly locally controlled and273