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

Sorting Through the Rubble

After 10 years of concerted effort, investigators working on the PONAM and QUEEN programs have collected a great deal of geological information about the former ice sheets that from time to time blanketed vast areas of the Eurasian Arctic. In a nutshell, our work provided three important findings. First, it documented that a large, marine-based ice sheet formed on the continental shelf in the Barents Sea during the LGM, some 20,000 years ago. At that time, the Barents Ice Sheet coalesced with the Scandinavian Ice Sheet, forming a continuous ice cover that extended from Germany and the United Kingdom, across Scandinavia and the Barents Sea shelf, and east to the Kara Sea. Second, this work showed that fast-flowing ice streams transported large volumes of glacial sediment to the continental margin. Third, the research demonstrated that the ice sheet disintegrated quickly at the end of the Ice Age. How exactly did we come to these conclusions, and why are they significant?

We know that the Barents Sea was glaciated during the recent geologic past because, in general, the seafloor is underlain by only a few meters of soft mud: The sediment found underneath this thin layer of mud is full of glacially derived debris and has been heavily compacted by the enormous weight of the last ice sheet. This sediment layer, which is a till (a characteristic glacial deposit), is widely distributed on the continental shelf. In central parts of the Barents Sea, the ice sheet also left behind a series of long parallel furrows, which reflect former ice movements. To evaluate the dimensions and age of this ice sheet, we and our many colleagues undertook various geological investigations in the Barents Sea region and along the northern margin of the Eur-asian continent.

The geological field studies started on Svalbard, a group of islands situated 600 kilometers north of the Scandinavian mainland. The pattern of raised shorelines on this archipelago and on Franz Josef Land farther east told us that the northern Barents Sea most likely had been weighed down by a major load of ice during the LGM. (Such raised shorelines form because breaking waves carve flat zones along the strand. Then after the ice sheet melts, the crust springs upward, transforming these beaches into raised terraces.) Along the western coast of Svalbard there are also several uplifted beaches with shells and whale bones, which were found to be more than 40,000 years old using radiocarbon dating. Most of the scientists involved believed that the presence of organic material this old proved that glaciers could not have reached the coast after these uplifted beaches were formed. Jan Mangerud, a Norwegian geologist from the University of Bergen, then made a ground-breaking discovery. He found that at least some of the old raised beaches had been overridden by ice, implying that during the LGM glaciers covered much more of Svalbard than anyone had imagined.

Subsequent work revealed that the main fjords were in fact filled by ice at this time and that the entire archipelago was covered by an ice sheet that was centered on the seafloor to the east of Svalbard. To determine the western boundary of the ice sheet, many investigators together made a major effort to map the sedimentary characteristics of the seafloor across the western Eurasian continental margin using various kinds of seismic and acoustic soundings as well as sediment coring. This work revealed large volumes of glacial sediments arrayed in huge fan complexes along the continental slope. The top layer of these sediments originates from the last extensive glaciation, so these fans clearly place the edge of the former ice sheet at the shelf margin. The vast amount of material transported to these fans demonstrates that fast-flowing ice streams must have been active along the western side of the ice sheet.

Establishing the southern and eastern margin of the former ice sheet has been a more difficult task. In the late 1970s, Mikhail Grosswald, a well-known geographer from the Russian Academy of Sciences in Moscow, hypothesized that a 3.5-kilometer thick, pan-Arctic ice sheet covered vast areas of the European Arctic and Siberia during the LGM. His hypothesis, questioned by many Russian scientists, was soon adopted by the majority of the scientific community in the west. Meanwhile, Valery Astakhov, a geologist from the National Institute of Remote Sensing Methods for Geology in St. Petersburg, was digging in the frozen earth of western Siberia searching for the hidden remnants of the last ice sheet. He found that a former ice sheet centered on the Kara Sea shelf did indeed advance south across the Yenissei river valley—but that this episode took place much earlier than the LGM. Thus, it was clear that something was wrong with Grosswald's hypothesis.

Research undertaken in the QUEEN program showed that a fresh-looking belt of moraines in the European part of the Russian Arctic (to the east of the White Sea) were, in fact, deposited as long as 60,000 years ago—some 40,000 years before the LGM. To locate the ice margin of the LGM, it was therefore once again necessary to turn our eyes towards the sea.

Examination of the seafloor sediments offshore of the Russian mainland has confirmed that the most recent ice sheet terminated on the continental shelf. Cores recovered from the Pechora Sea show that marine sedimentation in this area has gone on throughout the last 40,000 years, whereas inside the inferred margin of the ice sheet the oldest marine deposits above the upper surface of the till are less then 14,000 years old. Geological investigations on the Yamal Peninsula, which juts into the Kara Sea, have shown that the ice sheet did not reach that area either. Thus the southern limit of the ice sheet must have been somewhere to the north, within the shallows of the Kara Sea.

The position of the eastern margin of the ice sheet is similarly hard to pin down precisely. One avenue of investigation has been to study sedimentary deposits within the many lake basins of the Taymyr Peninsula, east of the Kara Sea. The sediments that accumulated at the base of these lakes comprise fine-grained muds. If an ice sheet had been located over this peninsula, these sediments would have been replaced or covered by coarse, glacially derived material, which is easily distinguishable. However, the sediment sequences in these lakes reflect a continuous accumulation of nonglacial sediments throughout the LGM, which means that the central parts of Taymyr Peninsula were free of ice when the ice sheet existed to the west across the Barents and Kara seas.

Interestingly, a team of geologists led by Christian Hjort at the Lund University, Sweden, concluded that the northwestern fringe of the Taymyr Peninsula was indeed touched by glacial ice after 20,000 years ago. However, this group of investigators believe that this ice was not an extension of the large marine ice sheet that covered the Kara Sea. More likely, it was part of a more restricted glacier located on the shallow sea floor just off the coast.

On Severnaya Zemlya, an island group to the north of the Taymyr Peninsula, Russian scientists have reported finding mammoth tusks dating from the period between 25,000 and 19,000 years ago, suggesting that the glaciers on this archipelago during the LGM were, surprisingly, even smaller than they are today. So the eastern boundary of the former ice sheet remains rather difficult to delineate.