Polar Ice Cores . Reading the frozen Archives of our varied Past
by
Pieter M. Grootes
Leibniz Labor, Christian Albrecht University, Kiel, Germany
Coauthors: Marie-Josée Nadeau (Leibniz Labor, Christian Albrecht University, Kiel, Germany)

The firn fields of polar ice sheets and high altitude ice caps and glaciers collect annual precipitation and build an archive where details of, for example, temperature, windiness, chemistry and gas composition of the atmosphere, and biospheric activity, extending over hundreds of thousands of years, are preserved in great detail.

Two cores, GISP2 and GRIP, 28 km apart and each over 3000 m in length, from the Summit area of the Greenland ice sheet, allowed a direct verification of the validity of the paleoenvironmental records retrieved from each of them. Excellent agreement exists over the first 90 % of their length down to 2750 m and over the last 100,000 years. This established the many large and rapid changes in isotopic composition of the ice (used as a temperature proxie) in the Summit cores as real. The bottom 10 % of both cores shows sections with inclined layering indicative of flow deformation. The resulting loss of a clear relation between depth and age makes the environmental record of this deepest section hard to interpret and unsuitable for paleoclimate reconstruction.

Counting the annual layering in the upper 2400 m of the GISP2 core on a special light table provided a detailed, independent time scale going back about 50,000 years. This established for the first time that the strong warming at the start of an interstadial is vary rapid and may take less than a decade. After a gradual decline, the final temperature drop may also be vary rapid. The reconstructed climate changes at the Greenland Summit thus have the character of a fast switching between cold and warm climate states. Comparison with other major Greenland ice core records shows that the characteristics of the many interstadials are Greenland wide. Sediment cores from the North Atlantic and from European lakes have since linked the oceanic and ice core records and made it likely that European climate showed a similar variability. Oceanic records provide a link between conditions at the ocean surface (sea surface temperature) and the strength of the thermohaline circulation (THC) and latitudinal oceanic heat transport. Ocean-atmosphere heat exchange and atmospheric circulation modulated by the strength and position of the Azores High and the Iceland Low atmospheric pressure cells translated this into an atmospheric signal, felt and recorded in Greenland and Europe.

In Antarctica, the Vostok core reaches back over 400,000 years and four glacial-interglacial cycles. Taylor Dome and Dome Fuji both show well over one full glacial-interglacial cycle, and Byrd Station and other cores extend well into the last glacial. Low accumulation at these core sites prevents the construction of a time scale by layer counting. Stadial-interstadial climate changes in Antarctica are generally smaller, and warming is more gradual, than observed in Greenland. The strongest changes are observed in Taylor Dome, a small ice dome at the head of Taylor Valley, Southern Victorialand. Changes in the gases trapped in air bubbles in the ice of Greenlandic and Antarctic cores are used, after correction for the difference between ice age and gas age, to synchronize the paleoenvironmental records. This leads to contradictory results. Synchronisation based on methane indicates that climatic warming at Byrd Station and, possibly, Vostok preceded the sudden warming at the start of major Greenland interstadials by 1000 to 3000 years. Yet a similar methane synchronisation indicates a Younger Dryas synchrony at Taylor Dome, and d18O of O2 shows roughly synchronous isotope maxima at Vostok and GISP2. Several deep-sea sediment cores from the South Atlantic record changes in antiphase with the North Atlantic and the Greenland Summit cores for Heinrich events, and further conflicting evidence regarding the phasing of northern-southern hemisphere climate changes comes from the Indian Ocean and from the terrestrial records of New Zealand and Chile.

The strong link between North Atlantic Deep Water formation and the global thermohaline circulation on the one hand, and between the heat transport of the Gulf Stream/North Atlantic Drift and climate in Europe and Greenland on the other, indicates the climatic importance of the THC. Surface currents of the THC provide regional cooling when directed towards the equator, and warming when flowing poleward. Changing the intensity of the THC will thus modulate this cooling and warming, and lead to synchronous, but regionally different, climate changes. The sign, size, and timing of climate variations thus need to be established regionally to develop the full picture of Pole-Equator-Pole climate variability and to understand the mechanisms of global climate change.

Date received: May 23, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahr-14.