The Eurasian Arctic During the Last Ice Age

A vast ice sheet once covered the Barents Sea. Its sudden disappearance 100 centuries ago provides a lesson about western Antarctica today

Anders Elverhøi, Martin Siegert, Julian Dowdeswell, John-Inge Svendsen

The Ice Sheet Cometh

With a general knowledge of orbital variations and feedback mechanisms, it is quite easy to envision how the deterioration of climate results in the growth of ice on land. It is, however, not so easy to see how global cooling causes an ice sheet to form on the seafloor. The method by which a large continental shelf can become covered by an ice sheet has been debated for many years. The main problem is that the calving of icebergs at the grounded margin of an ice mass intensifies with increasing water depth. So as the margin of an ice sheet migrates into deeper water, the rate of calving will increase, and this process should act to curtail the further spread of ice.

Terence Hughes of the University of Maine proposed that an ice sheet within the Barents Sea basin could form from a pre-existing ice shelf—a solid mass of ice floating on the surface. He suggested that permanent sea ice (a few meters thick) would thicken into an ice shelf (a few hundred meters thick) if the surface accumulation of ice exceeded the basal melt rate for a few thousand years. An ice shelf within the Barents Sea would encourage the growth of an ice sheet in two ways. First, the calving of icebergs from an adjacent ice sheet would cease: Ice would simply flow into the ice shelf. Second, as the ice shelf thickened, it would eventually touch the sea floor and become part of the grounded ice sheet itself.

Another mechanism may also be at least partly responsible for the growth of an ice sheet within the Barents Sea. Several scientists have suggested that ice accumulated initially over the island archipelagos located across the northern edge of the Eurasian continental shelf. As it did so, its great weight pushed the crust downward under it, causing the crust in the shallow central regions of the Barents Sea to bulge upward—just as a downward force applied to the center of a steel beam causes it to flex upward at either side. This uplift, combined with the lowering of sea level (of up to 120 meters' worth), may have allowed ice to fill the shallows. This ice may have flowed in from adjacent ice sheets, or it may have formed in place from thickening sea ice.

Indeed, both these processes may have operated during the last ice age. In addition, evidence from the nearby Norwegian-Greenland Sea shows that open-ocean conditions reigned there during the last ice age. This relatively warm ocean water provided an ample source of moisture for snowfall over the Barents Sea. The combined influence of enhanced snowfall, uplift and thickening ice shelves probably led to rapid glaciation of the Barents Sea. So the real question for geologists is not how this ice came to be, but how it disappeared.

There are several clues to the nature of the breakup of the ice sheet that once covered this portion of the Eurasian Arctic. The oxygen-isotope content of tiny shells within sea-floor sediments across the nearby Fram Strait and continental slope records a substantial amount of "light" oxygen in the water 16,000 years ago. In this case, the oxygen isotopes do not reflect the general state of the global ocean. Rather, they indicate a massive influx of glacial meltwater in the region, which in turn reflects the disintegration of the ice mass over the Barents Sea quite early during the last deglaciation.

We and others have also charted moraines left by the retreating ice front across the floor of the Barents Sea using shipborne sonar and seismic imaging. These data indicate that the deeper-sea regions of the ice sheet broke up first. By 14,000 years ago the Bear Island Trough and several smaller local depressions were deglaciated, leaving a series of open ocean embayments surrounded by crumbling walls of ice. By 12,000 years ago, the ice sheets had decayed further such that they were limited to the northern archipelagos and the shallow seas that surround them.

The pattern of ice decay within the Barents Sea is also recorded by the uplift that has gone on around Svalbard and Franz Josef Land. Raised beaches on these islands have been dated by the radiocarbon technique on whale bones, mollusk shells and driftwood. The dates show that deglaciation of the archipelagos took place several thousand years after the decay of ice in the deeper regions of the Barents Sea.