The δ¹⁸O record along the Greenland Ice Core Project ice core and the problem of possible Eemian climatic instability

Journal of Geophysical Research, Vol. 102, No. C12, p. 26.397-26.410, 1997

S.J. Johnsen
Science Institute, University of Iceland, Reykjavik, also at Departement of Geophysics, The Niels Bohr Institute of Astronomy, Physics and Geophysics, University of Copenhagen.
H.B. Clausen, W. Dansgaard, N.S. Gundestrup, C.U. Hammer, U. Andersen, K.K. Andersen C.S. Hvidberg, D. Dahl-Jensen, J.P Steffensen
Departement of Geophysics, The Niels Bohr Institute of Astronomy, Physics and Geophysics, University of Copenhagen.
H. Shoji
Dep. of Civil Engineering, Kitami Institute of Technology, University of Kitami, Kitami, Japan.
A.E. Sveinbjörnsdottir
Science Institute, University of Iceland, Reykjavik, Iceland.
J. White
Institute of Alpine and Arctic Research and Department of Geological Sciences, University of Colorado, Boulder.
J. Jouzel
Laboratoire de Modélisation du Climat et de l'Environnement, Commissariat à l'Energie Atomique/Sciences de la Matière, Centre d'Etudes de Saclay, Gif-Sur-Yvette, France.
D. Fisher
Glaciology Section, Terrain Sciences Division, Geological Survey of Canada, Ottawa, Canada.

ABSTRACT.
Over 70.000 samples from the 3029-m-long Greenland Ice Core Project (GRIP) ice core drilled on the top of the Greenland Ice Sheet (Summit) have been analyzed for δ¹⁸O. A highly detailed and continuous δ¹⁸O profile has thus been obtained and is discussed in terms of past temperatures in Greenland. We also discuss a three-core stacked annual δ¹⁸O profile for the past 917 years. The short-term (<50 years) variability of the annual δ¹⁸O signal is found to be 1 ‰ in the Holocene, and estimates for the coldest parts of the last glacial are 3 ‰ or higher. These data also provide insights into possible disturbances of the stratigraphic layering in the core which seems to be sound down to the onset of the Eemian. Spectral analysis of highly detailed sequences of the profile helps determine the smoothing of the δ¹⁸O signal, which for the Holocene ice is found to be considerably stronger than expected. We suggest this is due to a process involving diffusion of water molecules along crystal boundaries in the recrystallizing ice matrix. Deconvolution techniques were employed for restoring with great confidence the highly attenuated annual δ¹⁸O signal in the Holocene. We confirm earlier findings of dramatic temperature changes in Greenland during the last glacial cycle. Abrupt and strong climatic shifts are also found within the Eem/Sangamon Interglaciation, which is normally recorded as a period of warm and stable climate in lower latitudes. The stratigraphic continuity of the Eemian layers is consequently discussed in section 3 of this paper in terms of all pertinent data which we are not able to reconcile.