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

The Marine "Back-Arc" Ice Sheet, Its Specificity

[47]  Reconstructing the Okhotsk Ice Sheet presents challenges not encountered in previous reconstructions of Pleistocene ice sheets using the geometrical force balance [e.g., Grosswald and Hughes, 1995, 2002; Hughes, 1995, 1998; Hughes and Hughes, 1994]. As we envision it, based on evidence provided here, the Okhotsk Ice Sheet is a marine ice sheet in which flow begins along a highland arc that rings the Sea of Okhotsk on the east and north, transgresses onto the Asian mainland to the west, and ends along the Kuril Island Arc to the south. Did the ice sheet form as glaciers from these highlands advanced onto the shallow continental shelf in the Sea of Okhotsk during lowering sea level at the beginning of Quaternary glaciation cycles? Or did sea ice thicken and ground in the Sea of Okhotsk and cause snowlines to lower to sea level in surrounding mountains, thereby allowing the mountain glaciers to advance and merge with the grounded sea ice to produce the marine ice sheet that ended as a floating ice shelf? How far did the mountain glaciation extend to the north of the highland arc? Did the precipitation shadow cast by the Okhotsk Ice Sheet confine these highland glaciers to the northern mountain slopes, or did they merge with a marine ice sheet in the East Siberian Sea that transgressed onto the Siberian coastal plain? Was there a connection to the east with a largely marine Beringian Ice Sheet that originated in the Chukchi Sea, poured through Bering Strait into the Bering Sea, and ended as an ice shelf calving along the Aleutian Island Arc? If any of these connections existed, when did they exist? If none of these ice sheets existed, what are we to make of the evidence presented here?

[48]  In the geometrical force balance [Hughes, 1992, 1998], the surface slope along an ice-sheet flowband of width wI is given by the expression:

eq001.gif(1)

Surface slope Dhast/Dx is the incremental increase in ice elevation Dhast in incremental horizontal distance Dx measured upslope along an ice flowline from x = 0 at the ice margin. Densities are rI for ice, rW for water, and rR for Earth's mantle. Isostatic sinking beneath the ice load lowers ice elevation h on an undepressed bed of height or depth hR relative to sea level. Lowered ice elevation hast lies on a depressed bed lowered from hR to hRast, with r defined as r = (hR - hRast)/(hast - hR ) such that hR = hG remains constant at an ice-shelf grounding line. For grounded ice, r = r0 [1 - exp (-t/t0)] during time t of ice-sheet advance and r = ra exp (-t/t0) during time t of ice sheet retreat, where t0 is the relaxation time of Earth's mantle to the ice load, r = r0 = rI/(rR - rI) for isostatic equilibrium at t= infty, and r = ra for t = 0 at the glacial maximum. Assuming present-day isostatic equilibrium, present-day bed topographies can be used for hR along flowlines of a former ice sheet. Transitions from sheet flow to stream flow to shelf flow downslope along ice flowlines are controlled by basal buoyancy factor PW/PI, where PW = rW g hW is the basal water pressure that supports water of height hW above the bed, PI = rI g hI is the ice overburden pressure for ice of thickness hI, g is gravity acceleration, PW/PIapprox 0 for sheet flow in which hW = 0 for a frozen bed and hWll hI for a thawed bed, PW/PI = 1 for shelf flow because hI = (rW/rI) hW for floating ice, and PW/PI increases from nearly zero to nearly unity downslope along ice streams, which are fast currents of ice that drain most of a marine ice sheet. Accumulation rate a along an ice flowline is taken as constant, and ice thickness changes at a rate dhI/dt that is positive for a thickening ice sheet and negative for a thinning ice sheet. Ice thickness is hastI = hast - hG across an ice-shelf grounding line, where hastI = hI and ice velocity uG is negative for x positive upslope. Stresses in equation (1) are tS for side shear in stream flow and shelf flow, t0 for basal shear in sheet flow and stream flow, and sT for longitudinal tension and its gradient DsT/Dx, expressed in terms of PW/PI and D(PW/PI)/Dx, and linked to the mass balance through longitudinal extending strain rate eT, ice hardness parameter A, and ice viscoplastic parameter n in the flow law eT (sT/A)n for creep in ice. The quantity tS(PW/PI) allows a reduction in resistance from side shear as basal shear increases due to decreasing PW/PI.

[49]  In the geometrical force balance, t0 is determined by the ice surface slope Dhast/Dx and by the ice thickness hIast - hWast (rW/rI) that is supported by the bed, not by basal water pressure:

eq002.gif(2)

and sT is determined by the ice thickness hWast (rW/rI) that is supported by basal water pressure, not by the bed, where sT appears in longitudinal tensile force FT given by:

eq003.gif(3)

so that:

eq004.gif(4)

Therefore, t0 decreases as sT increases, and vice versa. The controlling variable is PW/PI.

[50]  The decrease of PW/PI upslope from an ice-shelf grounding line is a consequence of progressive loss of hydraulic continuity as the ice overburden becomes increasingly supported by the bed, not by basal water pressure. In ice streams, this loss is accompanied by side shear and basal shear which contribute to a back-force FB that becomes progressively larger with distance x upslope from the calving front of an ice shelf. For floating ice, the back force is also due to average water pressure PW = 12 PW = 12 rW ghW exerted on cross-sectional area wI hW for a flowband of width wI at x = 0. The back stress sB for both grounded and floating ice at any distance x is given by the negative term in equation (3):

eq005.gif(5)

For right-hand terms in equation (5), the first term is the back force exerted by water at the calving front of an ice shelf where x = 0, the second term is the increase of the water back force due to ice-shelf thickening over floating length L from the calving front to the grounding line of the ice shelf, the third term is the average side shear force along x due to average side shear stress tS acting on average ice thickness hI for PW/PI veraged over x, and the fourth term is the average basal shear force along x due to average basal shear stress t0 acting on average flowband width wI.

[51]  In applying equations (1) through (5) to flowlines of the Okhotsk Ice Sheet, ice-shelf buttressing is virtually complete because floating ice over Kuril Basin would have been grounded along Kuril Island Arc. Therefore floating ice would have had a nearly constant thickness, so the second term in equation (5) can be ignored. An average side shear stress of tS = 250 kPa can be applied, taking measurements along West Antarctic ice streams [Raymond et al., 2001]. An average basal shear stress can be determined from equation (2) for average values of hIast, PW/PI, and Dhast/Dx, using an iterative procedure for successive steps of Dx = 20 km along ice flowlines. In the first iteration, PW/PI = 1 for floating ice and PW/PI = (1 - x/Ls)c for grounded ice, with Ls being the length of the grounded flowline and c ranging from 2 to 5 such that all flowlines have comparable heights at the interior ice divide. For the first Dx step, t0 = 100 kPa is taken for ice margins grounded on land or in water, assuming the ice shelf is grounded at the calving front along the Kuril Island Arc. A frozen bed is assumed for ice flowlines north of the interior ice divide, so PW/PI = 0 and t0 = 100 kPa are taken along these flowlines. In the flow law of ice, n = 3 and A = 231pm 10 kPa a1/3 were used, assuming an average ice temperature of - 20oC. In adition, r = r0, a = 0.2 m a-1 and dhI/dt = 0 were assumed, with rR = 3200 kg m-3, rI = 917 kg m-3, rW = 1020 kg m-3, and g = 980 m s-2. Present-day topography and bathymetry were used for hR, with hG = 1000 m and uG determined from mass-balance conservation.

2005ES000180-fig07
Figure 7
[52]  Figure 7 presents our reconstructed Okhotsk Ice Sheet, with ice elevations calculated by numerically integrating equation (1) along the flowlines shown. Four ice domes appear along the ice divide; over the Verkhoyansk Range, the Cherski Range, The Kolyma Range, and on Kamchatka Peninsula. A continuation of the glaciation along the Anadyr Range of Chukchi Peninsula is in dispute [Brigham-Grette et al., 2003; Grosswald and Hughes, 2004], so is not shown. Also not shown are possible mergers of the Okhotsk Ice Sheet with a postulated Beringian Ice Sheet that originated as a marine ice sheet on the broad Arctic continental shelf of northeast Siberia [Grosswald and Hughes, 1995, 2002; Grosswald, 1998a; Hughes, 1998]. Two major marine ice streams drain most of the Okhotsk Ice Sheet, one in Deryugin Deep and one in TINRO Deep, see Figure 4, and the downdrawn converging ice flowlines in Figure 7. Marine ice transgressing onto the Asian mainland causes reroutings in the Amur River system, see Figure 6. Calving that intermittently freed the ice shelf from pinning points on the Kuril Islands may have caused iceberg outbursts and cooling that Kotilainen and Shackleton [1995] reported.


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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