The last interglacial as recorded in the greenland ice sheet and Canadian Arctic ice caps

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  • Quaternary International, Vol. 10-12, pp. 123-142, 1991. 1040-45182/91 $0.00 + .50 Printed in Great Britain. All rights reserved. 1992 INQUA/Pergamon Press Ltd

    THE LAST INTERGLACIAL AS RECORDED IN THE GREENLAND ICE SHEET AND CANADIAN ARCTIC ICE CAPS

    Niels Reeh Danish Polar Center, c/o The Geological Survey of Greenland, Oster Voldgade 10, DK-1350 Copenhagen K,

    Denmark

    The Greenland ice-sheet and Canadian Arctic ice-cap record of the last interglacial (understood broadly as marine Isotopic Stage 5) is critically reviewed. Lack of age control on the deep ice-core records leaves open two largely different interpretations of the climate in marine Isotopic Stage 5: either most of Stage 5 (with the exception of Substage 5e) experienced a cold climate, and the main change from interglacial to glacial conditions took place at the boundary between Substages 5e and 5d; or, during much of Stage 5 the climate was of interglacial type, i.e. as warm as, or warmer than at present, with Substage 5d as the only period with a glacial type climate. It is concluded that most of the 'hard' evidence is in favour of the latter scenario.

    Ice-sheet dynamic model studies, using the 'warm' marine Isotopic Stage 5 scenario, indicate that the Greenland ice sheet in the warmest periods of Stage 5 had split up into a main ice sheet covering central and northern Greenland, and a much smaller ice cap over the southeastern highlands.

    The Greenland record of marine Isotopic Stage 5 is then discussed in relation to other paleoclimatic and paleoenvironmental records. The possibility of major inter-hemispheric differences of the climate in mid and late marine Isotopic Stage 5 is discussed. It is concluded that it is important to study the entire marine Isotopic Stage 5 (not only Substage 5e) in order to understand possible differences in the global climatic responses to nearly identical peaks in orbitally driven summer insolation changes in the northern hemisphere.

    INTRODUCTION

    In this contribution, information about the last interglacial (understood broadly as marine Isotopic Stage 5) as recorded in the Greenland ice sheet and Canadian Arctic ice caps will be discussed. The discussion will primarily be based on the deep ice-core records from the central areas of the ice masses, and on surface-ice records from the ice margins. However, also ice-sheet-dynamic model studies provide information about past extent, thickness, and flow pattern of ice masses, and thereby contribute to a better understand- ing of past climatic and environmental conditions. Consequently, ice-dynamic model studies of the Green- land ice sheet during the last glacial-interglacial cycle will also be discussed in the following.

    On the other hand, glacial geological records from Greenland and the Canadian Arctic, and marine sediment records from the adjacent seas will only be peripherally discussed since these records are dealt with in other contributions to this volume (Funder, 1991; de Vernal, 1991; Sejrup, 1991).

    ICE SHEET RECORDS

    The ice sheets in Greenland and Antarctica, and the ice caps in the Canadian high Arctic, are rich sources of information about past climatic and environmental conditions because all types of fallout, airborne terres- trial dust and biological material, volcanic debris, sea

    Contribution to the NATO Advanced Research Workshop on the "Climate and Environment of the Last Interglacial in the Arctic and Subarctic'. Hanstholm, Denmark, October 19-22, 1990.

    salts, cosmic particles, isotopes produced by cosmic radiation, and naturally and artificially produced che- micals are incorporated in the snow. Layers of snow containing these contaminants are gradually compres- sed and transformed into solid ice which also includes small cavities containing samples of the atmosphere. The layers are buried by subsequent snow falls, and sink into the ice sheet under continuous vertical thinning, initially as a result of densification, by which the snow is transformed into ice, but then mainly due to flow-induced vertical compressive strain. In this pro- cess, the layers are stretched horizontally until they are advected by the ice motion into the ablation zone, where the ice either melts away or is removed by calving of icebergs. The flow pattern is illustrated in Fig. 1 which shows particle paths in a cross section of an ice sheet. In principle, a complete sequence of all the deposited layers can be obtained either by deep drilling in the accumulation zone (the region of positive mass input) or by surface sampling in the ablation zone from the equilibrium line (the line separating the accumula- tion and ablation zones) to the ice margin. By analysis of deep ice cores from the central ice-sheet regions, or surface-ice samples collected near the ice margin, records of climatic and environmental parameters can be established (e.g. Robin, 1983; Langway et al., 1985; Oeschger and Langway, 1989; Reeh et al., 1987; Reeh et al., 1991).

    Once the climatic and environmental information has been extracted by physical or chemical analysis, the next step is to establish a historical record (a time series). This step, which, among other things involves dating and correction for advective transport due to

    123

  • 124 N. Reeh

    ~ 0 100 200 300 400 500

    4 ICE

    ABLA ION ACCUMULATION

    (FLOW LINE)

    6 16o 2oo a6o 4oo soo DISTANCE (km)

    FIG. 1. Flow in a cross section of an ice sheet. Particle paths (flow lines) are indicated, connecting points with same 6(180) values in the accumulation and ablation zone,

    motion within the ice sheet, constitutes one of the major problems for the interpretation of climatic and environmental records from ice sheets (see e.g. discus- sion by Reeh, 1989a,b, 1990). At present, there are only a few well-established methods for absolute, experimental dating of ice in ice sheets, and none of these methods allow dating more than a few tens of thousands of years back in time (Hammer, 1989; Stauffer, 1989). Consequently, dating of ice older than this age depends on theoretical flow-model calculations (Lorius et al . , 1985), which becomes an increasingly difficult and uncertain task to perform the further back in time the dating is extended (see discussion by Reeh, 1989a), or on correlation with other dated records (Dansgaard et al., 1971, 1982; Reeh et al., 1991). These dating methods suggest that probably no more than one glacial cycle is found in the Canadian Arctic ice caps, that at least one glacial cycle is found in the Greenland ice sheet, and that several glacial cycles can be found in the Antarctic ice sheet.

    Whereas the full marine isotopic Stage 5 has been identified in the Vostok ice core from central East Antarctica (even the equivalents of Substages 5a,b,c,d,e can be easily identified (Lorius et al., 1985; Jouzel et al., 1987; see also Fig. 3)), the situation for the existing Greenland and Canadian Arctic ice cores is more complicated. In these cores, ice interpreted as originating from marine Isotopic Stage 5 has been retrieved from the deepest few metres (or tens of metres) i.e. immediately above the base of the ice sheets (Dansgaard et al., 1982; Paterson et al., 1977; Koerner et al., 1987; Fisher, 1987, see also Fig. 3). In none of the cores has ice been identified, which unambiguously can be referred to marine Isotopic Stage 6 (the previous glacial). These facts, combined with the poor absolute age control, complicate the

    interpretation of the records from marine Isotopic Stage 5 in the existing Greenland and Canadian Arctic ice cores. However, ice sheet records also have clear advantages in comparison with other paleo-records. For instance, they contain a continuous sequence of layers, except possibly for the near-bottom section where discontinuities may occur (Fisher, 1979), and, in general, they have a high resolution.

    The missing ice from marine Isotopic Stage 6 led Koerner (1989) to suggest that extensive melting of the Greenland ice sheet and the Canadian Arctic ice caps occurred in the Eemian (marine Isotopic Stage 5e). Moreover, Koerner (1989) mentions that massive (and possibly complete) retreat of the Greenland ice sheet would account for a large part (or all) of the 6 metre sea-level rise in the Eemian. However, complete melting away of the Greenland ice sheet, is contra- dicted by the 6(~SO) record from P~kitsoq on the West Greenland ice-sheet margin (Fig. 3) which seems to cover the full marine Isotopic Stage 5, and even a part of marine Isotopic Stage 6 (Reeh et al., 1991). If this interpretation is correct, a large part of the Greenland ice sheet must have survived the warm periods of marine Isotopic Stage 5. Consequently, the ongoing deep drilling efforts in the Summit area of the Green- land ice sheet (for location see Fig. 2) should provide a unique, high resolution record through marine Isotopic Stage 5 with an expected average annual layer thickness on the order of one centimetre. Moreover, the core section covering marine Isotopic Stage 5 is likely to be found so high above the base of the ice sheet (Dahl- Jensen, 1989) that one would not expect the record to be too much influenced by disturbances from the bottom.

    Even if the problem of dating the deep ice core records could be solved, this would not mean that the

  • The Last Interglacial 125

    AGASSIZ . ICE CAP

    DEVON

    ICE CAP CAMP CENTURY

    AFF IN .

    BAY

    BARNES ICE CAP

    GREENLAND

    SUMMIT

    | I /

    4**

    ' PAAKITSOQ

    DYE 3

    o

    ODP SITE 646 0 500 km

    FIG. 2. Map showing the locations of Greenland ice sheet and Canadian Arctic ice cap b(~80) records discussed in the text. The flow line from Summit to the ice margin at P~kitsoq is drawn to illustrate that Pleistocene ice now