Nuclear Instruments and Methods in Physics Research B75 (1993) 428-434 North-Holland NOM zyxwvutsrqpo B zyxwvutsrq Beam Interactions with Materials&Atoms PIXE analysis as a tool for dating of ice cores from the Greenland ice sheet H.-C. Hansson, E. Swietlicki and N.P.-0. Larsson zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM Department of Nuclear Physics, Lund University, W vegatan 14, S-223 62 Lund, Sw eden S.J. Johnsen Science Institute, University of Iceland, Dunshaga 3, IS-107, Reykjavik, Iceland Sections from the 2037 m long Dye 3 ice core drilled in 1979-1981 in the ice sheet of Southern Greenland were analysed with PIXE. The seven selected sections were from depths between 1778 and 1813 m, which corresponds to a time interval between about 8500 and 10000 years B.C. at the end of the last Ice Age. During this time period, fast climatic changes of several degrees centigrade per century are known to have taken place. The exact time scales of these changes need yet to be verified by renewed measurements using nonconventional stratigraphic dating techniques such as PIXE. The problem is highly relevant for the prediction of climatic changes in our present age. A new sample preparation technique was developed which enables the determination of annual thicknesses of the parts of the ice core representing 10000-40000 years before present, where the thickness of the annual ice layers are believed to be less than 2.5 cm. More commonly used techniques of dating, such as measurements of oxygen and hydrogen isotopes 6]*0 and SD, nitrate, acidity or conductivity all have difficulties in resolving annual cycles in thicknesses of less than about 2 cm. The new technique involves sublimation of 18 cm long ice sections, after which the material contained in the ice is deposited on a thin backing. In this way, the material to be analysed is preconcentrated through the removal of the H,O, while still retaining the spatial distribution pattern of the various water soluble and insoluble components along the ice core. The resulting spatial resolution of the sublimation technique is estimated to be & 1 mm. A PIXE analysis was performed in contiguous millimeter steps across the sublimated ice sections. Estimations of annual ice layer thicknesses were based on the patterns of seasonal variation along the ice sections for several major and minor elements quantified with PIXE. For the seven ice sections studied, annual ice layer thicknesses between 1.2 and 2.5 cm were found. 1. Introduction Human activities, such as the burning of fossil fuels and farming, has now reached a level that they affect the composition of the atmosphere and thereby have an impact on global climate. Even more important is the fact that this human impact on climate has been induced in a relatively short time scale of about 200 years, or since the advent of the industrial revolution. The evidence gathered so far indicates an increase in global mean surface air temperature by 0.3-0.6”C over the last 100 years [1,2], mostly due to increased concen- trations of greenhouse gases such as CO,, CH, and N,O. Water vapour, the most important of the green- house gases, will increase due to the global warming and further enhance it. Over the same period, the global sea level has increased by lo-20 cm. Predictions of future changes in climate can be made from ex- pected concentration levels of the components believed to be involved in the global heat exchange. Such mod- els predict a continued global warming of 1°C until 2025 and 3°C until the end of next century [1,2]. Climatic changes of this speed and magnitude were not experienced over the last 10000 years of the ongoing interglacial period, Holocene, which climatically has been fairly stable. However, from the existing records of previous climatic change, such as deep ice cores and deep sea sediments, it can be clearly seen that rapid changes in climate have indeed occurred in the past, especially during the last glaciation (Wisconsin; 120 000 to 10000 years B.P.) of the Pleistocene era preceding Holocene. At the Holocene/ Pleistocene transition marking the end of the intermediate Younger Dryas cold period at 8770 B.C. [3], the temperature is esti- mated to have risen 7°C in a mere 50 years [4]. This estimation was, however, not based on stratigraphic dating, but on estimated annual thicknesses during the time interval. During the last glaciation, there are indications that the transitions from cold to mild cli- mate proceeded more abruptly than the reverse tem- perature shift [5]. There are several methods available for observing 0168-583X/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved