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OXYGEN ISOTOPE MEASUREMENTS IN GLACIAL ICE S. Epstein California Institute of Technology Due to the fact that the vapor pressure of H->O18 in equatorial waters is about 0. 8 percent less than that of H2O^, the water vapor in tropical air masses is about eight mils depleted in C*18 relative to mean water. [The depletion or enrichment in permils is computed from the following formula: (018/Ol6)samPle 5 = (018/016) ocean - 1 x 1000 (1)] As the water vapor in the air masses moves to higher latitudes, it under- goes further isotopic fractionation, since H^Ql8 precipitates preferentially to H2C)!". The degree of fractionation depends upon the thermal gradient through which the air mass moves; the thermal gradient produces a sepa- ration equivalent to a multistage distillation. The path followed by the air mass should not greatly affect the fractionation. Isotopic measurements performed on rainwater or snow obtained at different locations--at Hawaii, where 5 = -1 to - 3; at Pasadena ( 8 = - 2 to -16); Saskatchewan ( 8 = -10 to -26); Greenland (8 = -21 to -47), and the South Pole (8 = -44 to -51)-- reveal a striking latitude dependence. Hundreds of measurements were performed, and the 8 values quoted represent extremes for that location; experimental errors were just a few tenths of a mil. The large South Pole negative 6 values are probably a result not only of a steep thermal grad- ient between equator and pole but also reflects the lack of reevaporation from the cool oceanic waters over which the southward-bound air masses pass. Because the steepness of the thermal gradient determines the extent of isotope fractionation and is seasonal, it was decided to measure isotope ratios at various depths in a glacier. Figure 1 shows the 8 values found in the Greenland glacier at depths from one to five meters. These depths represent the years 1957, 1956, 1955, 1954, and 1953. A pronounced summer-winter effect is easily seen in the results, together with con- siderable fine structure representing changing isotope ratios during the course of a single storm. During the years 1956 and 1955 some surface melting is known to have taken place for a few weeks during the summer. 102
-25.0 -30.0 S -35.0 GREENLAND 0.86-5.15 METERS â¢*' 1 u / If 1.0 2.0 3.0 Figure 1. 4.0 5.0 The effect of this melting is easy to discern in the figure, inasmuch as the amplitude of both the peaks and valleys are reduced for those two years as a result of mixing. The peaks are not all the same breadth; this is due partly to differences in the density of snow with depth, and partly to variations in the amount of precipitation from year to year. A system- atic increase in density is noticed in working toward greater depths as a result of the weight of overlying layers; consequently the peaks become narrower. Distinct stratification of the glacial snow is lost at about the 1918 level, and gives way to typical glacial ice: clear, granular and appar- ently undifferentiated. Oxygen isotope measurements have been made at the hundred meter, the three hundred meter, and the four hundred and ten meter level, and these clearly show summer-winter variations akin to those seen in Figure 1. With increasing consolidation some fine structure is lost which relates to individual storms, but the summer and winter distinction remains clear even at the deepest levels measured so far. The ice has a thickness of about three years per meter, so that the hundred meter sample is about three hundred years old, and the other 103
samples are respectively about 900 and 1200 years old. There appear to be rather significant secular changes in the heights of peaks, and also some changes from year to year, suggestive of climatic changes. The data should be analyzed to see what climatic trends might be revealed. With extension of the analyses in either direction from the depths already measured it should be possible to detect climatic temperature or precipi- tation changes which might be correlated with the 11 year or 22 year sun- spot cycle. [See Meyer's discussion of the correlation between the solar cycle and cosmic rays in this volume.] Any increase in precipitation might be manifested in two ways--through an increase in the thickness of the annual contribution to the glacier, and through a change in 5 values. There is no reason, in principle, why it should be impossible to work with samples from greater depth, dating back to the last ice age. Thus far it has not been possible to take cores from such depths, but new glaciological techniques may very soon provide cores through the entire thickness of the Greenland glacier. Movement of the glacial ice may give rise to the greatest problem because it smears the isotopic profiles. Analyses at greater depths should contribute important clues regarding the climatic factors conditioning the onset of an ice age. Arnold: What would be the effect of a general world cooling on the isotopic ratios observed at Greenland? Epstein: Although the effect of a localized cold spot is obvious, it is hard to predict how the isotope ratios would be affected by a general cooling. The extent of the isotope fractionations depend on the differences of equatorial and polar temperatures. Fortunately there is some reason to hope that the glacial epochs did not produce great changes in the equatorial ocean temperatures. Broecker: In all samples taken from parts of the Greenland glacier which are calving off into the ocean, a depth effect may be expected in the oxygen isotopic ratio in the ice. The explanation, as deVries gave it, is that the older samples were formed higher up, and consequently con- tained less O18. Younger samples, which were formed nearer the sea, result from precipitation richer in O18. Epstein: I have found some variation of this type. [There followed general discussion of the possibility of using C14 dating to tie down the absolute age of a glacial sample.] I have doubts about trying to apply the C14 method to carbon dioxide dissolved in the ice. The origin of the carbon dioxide is certainly not very well understood, and the ratios of carbon dioxide to nitrogen in the ice vary enormously. I would feel much happier employing carbon from organic specimens--say from pollen in the ice. 104
Broecker: One must be careful to exclude organic material in the soil deposited on the ice by continental winds. Such material, constituting as it does the humus of the soil, may be as much as 1500 years old at the time of deposition. Epstein: It would be interesting to see whether wet periods can be de- tected through these isotopic measurements. Tree rings provide a record of the wet seasons existing in lower latitudes at least. Broecker: Records exist of the abundance of ice floes in northern waters, which provide a measure of precipitation. There were periods in the 1700s when the floes almost disappeared in waters near Greenland [suggesting either dry years or cold summers]. 105
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