USGS Open-File Report 94-588

Introduction: Pliocene high-latitude climate records

Scott E. Ishman

U.S. Geological Survey, Reston, VA

The Pliocene represents a period in the recent geologic past when global average climatic conditions were significantly warmer than present. Associated with this time period are distinct differences from present vegetation distributions, sea-level, sea-surface temperatures, and deep ocean circulation. The high latitudes are the most sensitive to these changes, and their response to warmer climates is important for predicting their response to future climate change. Polar regions play important roles in controlling the earth's climate because they essentially drive oceanic circulation by being the loci of deep water formation. The Arctic and Antarctic regions make up the majority of the Earth's cryosphere, with ice volumes representing a sea-level equivalent in excess of 50 meters.

A workshop sponsored by the U.S. Geological Survey, from July 26 through July 28, 1994, in Herndon, VA, focused on the Pliocene climatic and oceanographic conditions in the Arctic and Antarctic regions, and the relationship of these records to the global Pliocene environment. The objective of the workshop was to gather scientists conducting Pliocene paleoclimate research from the polar regions in order to compare polar records, initiate a dialog between polar workers, and to highlight major polar paleoclimate issues related to the Pliocene. Important and somewhat controversial issues discussed throughout the workshop included:

• The Arctic land and sea ice record. It is clear that many issues regarding the Arctic paleoceanographic and paleoclimatic histories remain unresolved (Y. Gladenkov; D. Clark). Marine sediments from the Canada Basin indicate the presence of sea ice in the latest Pliocene and increasing throughout the Pleistocene (R.Z. Poore). In addition, the ice rafted record suggests increased presence of continental ice occurring just prior to the Gauss-Matuyama boundary (R.Z. Poore). Near-shore marine records indicate late Pliocene-early Pleistocene high stands with little associated continental ice (J. Brigham-Grette). Vegetation records from the Russian Arctic indicate conditions warmer in the Pliocene than present.

• Antarctic ice volume. The stability of the Antarctic ice sheet has been a focus of considerable scientific debate over the past decade. Paleontologic and sedimentologic evidence for warm atmospheric temperatures and rapidly fluctuating ice volume throughout the Pliocene (D.M. Harwood, P.N. Webb, S.E. Ishman, G.A. Wilson, F. Fleming, R.S. Hill, A.C. Ashworth) have been argued based on the interpretations of geomorphologic (D.A. Marchant), deep sea stable isotopic (M.A. Prentice), and ice-rafted detritus data (D.A. Warnke).

• Antarctic sea ice. The extent of Antarctic sea ice in the Pliocene is an important factor in estimating the possible dimensions of the Antarctic ice sheet. Faunal and floral data from the Southern Ocean indicate a southerly shift in the Polar Frontal Zone (J.A. Barron, J.J. Morely). Pliocene sea surface temperatures in the South Atlantic warmer than today indicate a southerly shift in surface isotherms (H.J. Dowsett). These conditions indicate greatly reduced sea ice conditions in the Southern Ocean during the mid-Pliocene.

• Global Pliocene data and ice volume. Sea level records from various localities around the globe reflect sea level greater than 25 meters over present day for the Pliocene (G.A. Wilson). Fluctuations in stratigraphic sequences from New Zealand reflect a high frequency of sea level increase and decrease, related to the waxing and waning of the Antarctic ice sheet (G.A. Wilson). Current interpretation of the deep sea stable oxygen isotopic record allow for some reduction of the Antarctic ice sheet (M. Prentice). However, debate still persists over the contribution of ice volume versus temperature to the delta 18 O record. New information, resulting from trace element analyses of deep water ostracodes, records deep sea temperatures and provides a direct measure of temperature versus sea level (G. Dwyer).