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

The Sea of Okhotsk

[17]  What is known of that sea? The Sea of Okhotsk is a large embayment of the North Pacific, it deeply penetrates the continental mass of Eurasia between Kamchatka Peninsula and Sakhalin Island and is separated from the rest of the ocean by the Kurile Island Arc. The areal extent of the sea is 1,590,000 km2, and the average depth is 860 m. It opens to the southeast, and is semi-surrounded by a highland amphitheater formed by mountain ranges of the Dzhugdzhur, Suntar-Khayata, Chersky, Kolymsky, and Kamchatka's Sredinny Ranges, which include many active volcanoes. Dozens of glacial U-shaped valleys descend into the sea down the amphitheater, as well as from the Koryak Mountains. In some places, like in Gulfs of Penzhina and Gizhiga, the glacial valleys converge to form large fjord-type inlets.

Climate and water masses.
[18]  The present-day Okhotsk-Sea region is characterized by the coldest air temperatures, and heaviest sea-ice conditions within the entire North Pacific Ocean. Yet, its climate is temperate, with monsoon-type atmosphere circulation bringing in warm air masses from the southeast in summers and cold Siberian air from the northwest in winters. Summer temperatures are +11o to +18oC while winter temperatures quite often drop to - 25oC. The annual atmospheric precipitation grades from 230 mm in the north to 1250 mm in the south. In the bordering mountains, a considerable proportion of the precipitation comes as snowfall.

[19]  Oceanographically, the Okhotsk Sea is a major source of the present-day "Intermediate Water" flow into the North Pacific. The Okhotsk-Sea surface water also interacts with the open Pacific Ocean and contributes to the formation of the cold Oyashio Current. On merging with the southward flowing East Kamchatka Current, Okhotsk surface water forms a core of the Oyashio. During glaciations, the Sea of Okhotsk was source area of the Ice-Age Pacific deep water, controlling the extent of the deep water ventilation and nutrient distribution [Duplessy et al., 1988; Keigwin, 1995b].

Sea floor. The evidence.
[20]  The Sea of Okhotsk, in particular its bottom topography and geologic structure, is rather well studied. The floor of the Sea of Okhotsk is tectonically young and active. It was affected by the India-Eurasia collision tectonics, which is believed to have produced normal faults, half-grabens, shearing, folding, and thrusting during the Eocene through Pliocene. These structures often followed the course of the older Mesozoic structural lines of the Mongol-Okhotsk-Chukchi active margin [Worall et al., 1996]. With all this in mind, it seems obvious, at least to us, that the sea floor has a glacial geomorphology superimposed on this relatively passive tectonic background, the geomorphology of the kind typical for glaciated continental shelves in general [Holtedahl, 1970; Shepard, 1973]. In addition, a reconstruction by a LGM-sedimentation model, as well as present-day geomorphic processes operating on that floor, appear typical for glaciated shelves, with their steep-sloped channels and sills [Wong et al., 2000].

[21]  As early as in 1949-1955, expeditions of the Institute of Oceanology, an oceanographic branch of the USSR Academy of Sciences, charted a system of submarine troughs, basins and ridges, as well as the "back-arc" deep-sea Kurile Basin. The depth of the Shelikhov Deep was fathomed to 445 m, of the TINRO Deep - to 991 m and of the Deryugin Deep - to 1795 m. Further, it was found that all the deeps are interconnected by submarine troughs which, in turn, are subdivided by transverse ridges and turned into alternations of basins and sills. The minimum depths in the troughs were located on inter-basin divides, and were found to be 369 m in Shelikhov Bay, 539 m in Lebed' Trough, 1354 m in Makarov Trough, and 1315 m in Pioter-Schmidt Trough. The deepest part of the sea was found in its southern sector, the Kurile back-arc Basin, which is characteristically 3000 m to 3200 m deep, with a maximum depth of 3372 m [Udintsev, 1957].

[22]  Several broad submarine sills, or "uplands", also became known. The two largest sills, the Academy and the Institute of Oceanology Sills, with their tops being deeper than 900 m, tower above the adjacent troughs and basins by 400 m to 800 m. The straits crossing the Kurile Island Arc were also explored and found to be characterized by the same U-shaped profiles, as the intra-sea troughs. The major Bussol and Kruzenstern Straits were 2318 m and 1929 m deep, while four other straits were deeper than 500 m [Udintsev, 1957; Udintsev et al., 1981].

[23]  A number of smaller-scale seafloor forms were also uncovered during the expeditions. Among them were submarine canyons incised into the slopes of the "uplands", flat platforms on upland tops, erosional terraces on their slopes, and giant grooves and ridges of the TINRO Deep. Some other troughs aligned parallel to their long axes were also charted, as well as swarms of hills and ridges scattered over the bottom of the Deryugin Deep. On the submarine slopes of Sakhalin Island and Kamchatka Peninsula, within the depth range of 200 m to 1000 m, staircases of ridge-and-channel pairs were also recorded. The pairs had a relief of 5 m to 6 m, and were aligned subhorizontally facing each other across the Sea of Okhotsk [Udintsev, 1957; Udintsev et al., 1981]. The longitudinal ridges of the TINRO Deep were found to be made up of silts and fine sands, and characterized by gradational bedding and frequent occurrence of scattered pebbles [Volnev, 1983]. These ridges were up to 200 km long and 1.5 km to 0.6-0.8 km wide, their relief changes from 100 m in the north to 25-30 m and less in the south, and they diverge southward into systems of subparallel ridges.

[24]  It was also found that bare rock outcrops, patches of nonsorted gravel, and glacial erratics were widespread on the Sea of Okhotsk floor and on the bottoms of the Kurile straits [Bezrukov, 1960]. The erratic boulders were typically faceted and striated, and their number and size appeared totally independent of the distance from shores; as to their abundance, it was so great that, as a rule, the overwhelming ubiquity of erratics totally obscured the results of dragging for local bedrock fragments (B. V. Baranov, pers. comm.). As for the deep Kurile Basin, it is infilled by a sequence of terrigenous sediments of a thickness measured in kilometers [Zonenshain et al., 1990].

[25]  There are some indications of past glaciation. So far it has been reported only in two places: from the northeastern coast of the sea, and from an offshore area of southwestern Kamchatka Peninsula. The first area included the Gulfs of Penzhina and Gizhiga where Bondarenko [1931] and Udintsev [1957] described extensive fields of glacial and glaciofluvial deposits. Both the researchers visualized big valley glaciers that formerly debouched from near-shore mountains into the sea and filled in Shelikhov Bay. In the second area, on the shallow shelf of Kamchatka, a 3 to 8 m-thick sheet of unsorted solid boulder clays, interpreted as lodgement till, was encountered in the process of drilling; this till sheet occurred beneath a 15-m thick sequence of late-glacial and postglacial sediments [Kuz'mina and Eremeeva, 1990].

[26]  However, the glaciation responsible for these landforms and deposits was considered local and insignificant, and its role in the geological history of the sea was thought to have been negligible. Glacial erosion and accumulation was never taken into account when explaining the seafloor geomorphology [e.g., Antipov et al., 1997; Savostin et al., 1983]. The range of environmental change in the Okhotsk-Sea region over the last glacial-interglacial cycle has been discussed only in terms of sea-ice expansions and shrinkages [Gorbarenko et al., 2003; Shiga and Koizumi, 2000; Wong et al., 2003].

Sea floor. Interpretation of the data.
[27]  Until recently, the origin of all described submarine landforms of both larger and smaller scale was either explained by operation of diverse nonglacial processes or considered enigmatic. In particular, the seafloor troughs, basins, and sills were taken for products of tectonic faulting and folding [Udintsev et al., 1981]; the longitudinal ridges of TINRO Deep were results of downslope turbidity currents and near-bottom sediment transport [Volnev, 1983]; the submarine terraces and subhorizontal ridge-and-channel pairs were ascribed to near-shore impact of waves during the stages of lower sea levels; and the submarine canyons were attributed to normal river erosion operating on dry land before the land became the seafloor [Udintsev, 1957]. As for the bottom erratics, their spreading was (and still is) commonly accounted for by rafting activity of sea ice and icebergs [Bezrukov, 1960].

[28]  Today, the situation has changed and Quaternary glaciations of the Sea of Okhotsk appear probable. What is more, glaciations are strongly suggested by the concerted data coming from paleoclimatic, geomorphologic, and marine geological studies [Grosswald, 1998b; Grosswald and Hughes, 1998]. Actually, the best documentation of glacial invasion of the Sea of Okhotsk comes from the geomorphology of its bottom, first of all from the system of deep U-shaped valleys and dividing sills that dominate the Okhotsk sea floor topography, and also from the smaller seafloor landforms. Previously, as we pointed out above, there had been a tendency to account for all the landforms by operation of only nonglacial processes, ascribing a specific mechanism for each kind of landforms.

[29]  We repeat, however, that the entire submarine geomorphology of the Sea of Okhotsk is typically glacial, having resulted from the impact of a grounded ice sheet and subglacial meltwater activity. The marine Okhotsk Ice Sheet was grounded in the larger northern province of the sea, lying north of 47-49oN. From geological and geophysical standpoints this regions is a genuine continental shelf (B. V. Baranov, L. P. Zonenshain, personal communication). A Kurile ice shelf, an extension of the grounded ice sheet, was floating in the second (smaller) province, the Kurile Basin, confined between the above continental shelf and the Kurile Island Arc. Tectonically, that basin is a typical "back-arc" part of the deep ocean; during the Quaternary Ice Age, it was turned into a basin of intense glaciomarine deposition.

[30]  Major glaciological agents of that process were the Okhotsk-Sea ice streams that unloaded debris as they exited their troughs and became afloat. In particular, large ice streams exited from Pioter-Schmidt and Makarov Troughs, as well as from Golyginsk Channel, entering Kurile Basin northwest of Paramushir Island. The ice stream in Golyginsk Channel appears to have drained the ice masses of the Central Depression of Kamchatka Peninsula. The depression of Golyginsk Channel is similar to the Bering Trough on the Alaskan continental shelf, which may have been also occupied by an Ice Age ice stream [Grosswald and Vozovik, 1984a].

2005ES000180-fig04
Figure 4
2005ES000180-fig05
Figure 5
[31]  The Okhotsk-Sea assemblage of basins and sills, including Deryugin and TINRO Deeps, Lebed', Makarov and Pioter-Shmidt Troughs, and Institute of Oceanology and Academy "Uplands", conform to the geomorphology that is typical of a continental shelf deeply eroded by an ice sheet (Figures 4 and 5). That assemblage is a regular combination of deep U-shaped troughs, both longitudinal and transverse, and huge bedrock sills, that is similar both in scale and pattern to what was revealed by surveys of the world's glaciated continental shelves [Holtedahl, 1970; Shepard, 1973], notably in Antarctica [Anderson, 1999].

[32]  The deepest troughs of the assemblage have depths of 1.5 km to 2 km as do their analogues from elsewhere, for instance from the Weddell Sea floor in Antarctica, and are as closely tied to the sources of ice inflow as are the submarine troughs of Antarctica [Vaughan et al., 1994]. The mean depth of glacial scour of the Okhotsk-Sea floor implied by its geomorphology amounts to many hundred meters, which is commensurate with the depth of glacial erosion of other glaciated continental shelves, e.g., of the Barents-Sea shelf, where the depth of scour was found to be in the range of 500 m to 1500 m [Solheim et al., 1996], and beneath the expanded marine ice sheet in West Antarctica [Anderson, 1999].

[33]  We have also found the Okhotsk ice-sheet hypothesis provides the best clues to the origin of the smaller-scale forms of the sea floor in question. Specifically:

[34]  (a) V-shaped submarine canyons incised into sill-slopes are similar to the forms typically occurring on all glaciated continental shelves. They are commonly accounted for by erosional activity of high-pressure subglacial meltwater [Shepard, 1973];

[35]  (b) the terrace-like ledges of the sill-slopes, which are believed to have formed through wave erosion, could be as readily produced by glacial scour. The "terraced" slopes of this kind were found to be common features of glaciated terrains in Antarctica [Evteev, 1964];

[36]  (c) the longitudinal sea-floor ridges and grooves of the TINRO and other submarine troughs, currently thought to be produced by turbidity flows or to be of "unknown" origin, can be simply taken for the products of ice and meltwater activity at the base of large paleo-ice streams. This glacial interpretation comes from the mechanisms operating at the base of ice sheets and from studies of formerly glaciated troughs in Antarctica [Anderson, 1999] and, in Arctic Siberia, in Saint Anna Trough of the Kara Sea, at depths between 300 m and 550 m, where side-scan sonar records have revealed a system of longitudinal furrows and ridges made up of basal diamictons, or lodgement till [Polyak et al., 1997]. The Saint Anna furrows are up to 20 m deep and 200 m wide, and the ridges are up to 30 m high and 5 km wide, thus similar in shape and size to the TINRO-Deep forms. In that Siberian trough, numerous smaller ridges and hummocks with prevailing heights of 5 m to 10 m were also located, which are similar to the respective features of the Deryugin-Deep;

[37]  (d) finally, the staircases of ridge-and-channel pairs occurring on submarine slopes of Sakhalin Island and Kamchatka Peninsula, now believed to have formed in coastal environments when sea-level was 200 m to 1000 m below present, can be naturally explained in terms of glacial and meltwater erosion and accumulation operating beneath the moving ice-sheet margins. These features may have formed along the grounding line of an ice shelf, where bottom crevasses would open and be filled with glacially eroded material, somewhat like the ribbed moraines that form where a frozen subglacial bed becomes thawed [Hättestrand, 1997], but on a larger scale for larger crevasses.

[38]  (e) sea-floor terraces could also form during deglaciation as rising sea level forced the ice-shelf grounding line to retreat.

[39]  All in all, based on the premise of the Okhotsk marine ice sheet, we have gained new insights into the genesis of all the sea's submarine forms, both of large and small scale. In this new light, they can be explained by operation of only one process - the glacial one, instead of resorting to a multitude of unrelated diverse mechanisms.


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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