RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES5004, doi:10.2205/2005ES000180, 2005

Background

Paleoclimate.
[4]  The Northwest Pacific Rim is remarkable for its low present-day snowline. On the northern Kurile Islands and southern Kamchatka Peninsula that snowline is at an altitude of 600-800 m a.s.l. [Braitseva et al., 1968; Vlasov, 1958], on the Iskaten' and Pekul'ney Ranges of Chukchi Peninsula at elevations of 650-750 m a.s.l., in a more maritime area of the peninsula, near the Provideniya Harbour at 450-550 m a.s.l. [Sedov, 1997] and on a mountain of the northern Okhotsk-Sea coast, near Magadan, at about 1000 m a.s.l. [Sedov, 2000]. The equilibrium line rises to 2300 m only on the present-day glaciers of Suntar-Khayata Range [Koreisha 1991]. These observations all strongly suggest that the Ice-Age snowline on the northern Kurile Islands, eastern Kamchatka and Chukchi Peninsula dropped to sea level (which is consistent with the fjord-type geomorphology of the east Chukchi, Koryak and Kamchatka coasts), and was only a few hundred meters above that level in more continental mountains of Pacific Siberia.

[5]  With such low present-day snowlines, all the climate-based computer simulations indicate that climate perturbations of the Ice Ages had inevitably led to inception and growth of continental-sized ice sheets on the Northwest Pacific Rim, and that the past ice sheets had covered not only mountains and coastal plains of the region, but also its continental shelves.

[6]  The modeling experiments by Verbitsky and Oglesby [1992] were first to demonstrate that the last glaciation of Northeast Siberia had been about as extensive as the Scandinavian and North American ice sheets and, in addition, that it incepted easier and earlier than those ice sheets. Later, the computer models of Budd et al. [1998] more specifically implied that, first, a continuous ice sheet had developed over the highlands of Northeast Siberia and, second, the ice sheet expanded to the southeast and invaded the Sea of Okhotsk. The same extent was obtained in the paleo-ice sheet simulations by Greve et al. [1999] and Bintanja et al. [2002]. Also, according to a modeling of the former glaciation of the Northwest Pacific Rim, produced by Fastook [1997, unpublished], a broad Okhotsk-Sea ice stream not only existed, but it moved at the rate of 2 km yr -1, despite its lower (floating) part being buttressed by the Kurile Island Arc.

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Figure 1
[7]  If we place the Sea of Okhotsk and adjacent Beringia into a broader context of the Northern Hemisphere's marine paleoglacial systems with their indispensable geological attributes, such as glaciated continental shelves, fjord-type coasts, and debris-filled deep-sea trenches, we see that they fall into the huge zone north of 48oN, to which all marine paleoglacial systems belong in the Northern Hemisphere. Thus from purely geographic and paleoclimatic considerations, it is highly probable that both the regions were glaciated by former marine ice sheets (Figure 1).

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Figure 2
Results of deep-sea drilling.
[8]  In 1992, the inferences based on paleoclimatic modeling and geographic considerations became strongly supported by the results of deep-sea drilling in the northwest Pacific Ocean [Roberts, 1993]. The drilling was conducted during the ODP Leg 145 of the R/V JOIDES Resolution; its drilling sites were located 700-800 km east of the southern Kamchatka Peninsula tip, near Detroit Seamount in water depth exceeding 3 km. Based on their preliminary analyses, the on-board scientific party arrived at the solid conclusion that, 2.6 million years ago, the sedimentary environment of the North Pacific Ocean had experienced dramatic change: deep-sea circulation had abruptly become vigorous; rates of sedimentation increased many-fold, and the composition, density, and magnetic characteristics of the bottom sediments also suddenly changed (Figure 2). Abyssal redistribution and reworking of the sediments had intensified, the amount and concentration of fine-grain terrigeneous debris (relative to the in situ siliceous oozes) had increased as well. Notably, at that time, 2.6 million yr ago, the influx of all kinds of continental materials enormously accelerated, and dropstones of Siberian provenance, up to 5 cm across, became abundant in sites seaward from the Sea of Okhotsk. Ash layers also became quite abundant in the bottom sequences, attesting to enormous increase in volcanic activity unmatched by any found earlier in the Miocene through Pliocene record.

[9]  Members of the on-board party had little doubt that all these changes clearly indicated that a large-scale glaciation had incepted in the North Pacific Ocean at 2.6 million yr ago - at the same time as in the North Atlantic Ocean.

[10]  This was further developed by detailed studies of Kotilainen and Shackleton [1995], which demonstrated that the rate of the ice rafting, reflected in densities of deep-sea cores, had experienced periodic variations. A curve depicting these variations for the last 95,000 years exhibited some 20 cycles of ups and downs in ice rafting, thus implying about 20 cycles of climate coolings and warmings. These researchers stated that the cycles were similar to and likely synchronous with the Dansgaard-Oeschger climate events revealed by the Greenland ice cores.

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Figure 3
[11]  Actually, extensive glaciation of the Northern Pacific Rim was suggested much earlier, over 30 years ago, when abundant ice-rafted erratics were discovered in deep-sea sediments of the North Pacific Ocean [Conolly and Ewing 1970], and 15 years later the onset of the ice-rafting was dated to about 2.5 million years ago [Rea and Shrader, 1985]. In this context, it is noteworthy that a multitude of dropstones were located in the Upper Pliocene through Upper Pleistocene deep-sea cores (to the very tops of them!) recovered by drilling east of Honshu Island, Japan [Okada, 1980]. Importantly, one of the drilling sites was situated east of the Japanese Trench, which implied that the dropstones could have only come from icebergs transported by the Oyshio Current from far north, and not from nearby Honshu Island (Figure 3).

[12]  The consistent LGM oceanographic changes in NW Pacific Ocean were, in fact, an outright "thermal revolution". In particular, extending of a cold, iceberg-infested Oyshio Current to the shores of southern Japan [Oba et al., 1991], and tremendous cooling of the Sea of Japan due to isolation, infilling by the Amur-River water and freezing [Grosswald, 1998b], inevitably resulted in cooling, much deeper than it is generally thought, and glaciation of the Islands of Japan much greater than presently reconstructed [Grosswald, 2002].

[13]  Still, a big problem remains: from where did all these erratics come, and what were the path tracks of North Pacific icebergs that transported the erratics?

Search for the source of North Pacific icebergs.
[14]  The issue of this source is not resolved; yet the number of options cannot be great. Keigwin [1995a], who was the first to pose this problem, conceived only two probable alternatives; first, that the icebergs came from the Alaskan and Siberian coasts after calving from tidewater glaciers, and second, that they had been released by a nearby full-sized marine ice sheet of unknown size and location.

[15]  From our perspective, only the second option is realistic. The ODP Leg 145 drilling site and Honshu Island, on the one hand, and the coastal zones of Alaska and Siberia, on the other, are thousands of miles apart, and such a long-distance rafting of erratics appears hardly possible. By contrast, a marine ice sheet, or ice sheets, on the Northwest Pacific Rim would explain not only the distribution of erratics, but also the entire complex of paleoceanographic changes established by the deep-sea drilling. In particular, it would account for the abrupt acceleration of abyssal circulation in the North Pacific, and for the sudden growth in influx of all kinds of continental materials (including the erratics). Being located in marginal seas of the ocean, the ice sheet or ice sheets would be much closer to the depositional sites than the coasts of Alaska and Siberia and also would be a much more productive source of icebergs and meltwater than any mountain glaciers. They would even account for the enormous increase in volcanic activity, as, according to computations by Nakada and Yokose [1992], there are direct links between glacial and volcanic phenomena. Specifically, these geophysicists have found that ice sheets placed upon the Northwest Pacific Rim would have been a sufficiently heavy load to push the hydrostatic pressure in deep magma centers over critical thresholds and trigger massive volcanic eruptions.

[16]  Where were those hypothetical ice sheets located? Members of the Roberts [1993] speculated that the Sea of Okhotsk had been the most probable source of the icebergs. Our analyses of the Okhotsk Sea and its environ's geomorphology, of the pattern and causes for Amur River reorganizations, coupled with new climate-based modeling experiments, strongly suggest that this assumption was correct.


RJES

Citation: Grosswald, M. G., and T. J. Hughes (2005), "Back-arc" marine ice sheet in the Sea of Okhotsk, Russ. J. Earth Sci., 7, ES5004, doi:10.2205/2005ES000180.

Copyright 2005 by the Russian Journal of Earth Sciences

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