Antarctic Ice Sheet Surface Oxygen Isotope Values

Abstract

Collected data on the mean annual surface values for δ¹⁸O over Antarctica have been tabulated and also presented in map form. An additional map shows contours of constant δ¹⁸O values.

Teneurs en isotope de l’oxygène à la surface de la calotte glaciaire antarctique. Les données recueillies sur les valeurs annuelles moyennes en surface pour la teneur en δ¹⁸O dans l’ensemble de l’Antarctique ont été compilées et sont également présentées sous forme cartographique. Une carte supplémentaire montre les lignes d’égales teneurs en δ¹⁸O.

Zusammenfassung

Werte für Sauerstoff-Isotope an der Oberfläche der antartischen Eisdecke. Datenmaterial über die mittleren jährlichen Oberflächenwerte von δ¹⁸O in Antarktika wurde tabuliert und auch in kartographischer Form dargestellt. Eine zusätzliche Karte enthält Linien konstanter δ¹⁸O-Werte.

Introduction

One of the diffigulties in interpreting oxygen-isotope data from deep cores is in making allowance for the different place of deposition of the lower layers. If the direction and velocity of the ice flow is known, a first-order correction to the core data can be made by considering the variations in the present-day surface δ ¹⁸O. For more sophisticated studies, however, changes in the surface isotope ratio which will have occurred with changes to the ice sheet also have to be considered and this presents considerable difficulty.

The prominent change in the δ ¹⁸O values found in the deep core data around 10000 years b.p. (see e.g. (Johnsen and others 1972)) is rather too large to be solely due to temperature and probably also reflects a general lowering of the ice sheet surface in conjunction with a retreat of the ice margin. Both the lowering and the reduction in area will reduce the depletion of ¹⁸O in the surface snow but at present it is not easy to assign magnitudes to the changes.

The main idea behind this collection and presentation of data is to give a broad-scale picture of the surface δ ¹⁸O for comparison with core δ ¹⁸O profiles. A second use is that future examinations of the compiled data will enable a better understanding of the variations in the depletion in ¹⁸O in Antarctic precipitation in terms of the temperature, elevation, continentality, etc. of the particular location.

The experimentally observed relationship between δ ¹⁸O and temperature arises because of the decrease in the saturation vapour pressure of water with temperature. A cooling air mass must precipitate water to avoid supersaturation and as this water will be enriched (c. + 8‰) in ¹⁸O with respect to the vapour, the vapour will therefore become progressively depleted in ¹⁸O. The mechanism, complicated by non-equilibrium transfers of water vapour, is affected by the path the air mass takes for instance over land, sea, or ice, its temperature and rate of temperature change, its turbulence, and other factors most of which are climatic variables and thus presumably will have varied in the past.

It is envisaged that the data presented here in Figs 1 and 2 and Table I may be used to obtain empirical values for the fractionation processes which affect precipitation falling on different parts of Antarctica and hence can be used to adjust deep core δ ¹⁸O data for such factors as changes in ice-sheet elevation and area.

Fig. 1. Mean annual surface values of δ¹⁸O in parts per thousand obtained from measurements in Antarctica.

Fig. 2. Contours of constant mean annual surface values of δ¹⁸O in parts per thousand derived from the point values in Figure 1.

Table I. Antarctic ice sheet surface data

Table I.

Table I.

Table I.

Sample and Data Collection

The large seasonal δ ¹⁸O variability means that some form of averaging is required to obtain a value which approximates the long-term mean. Where deep bore-hole analysis data are available, this is effectively done by averaging values from the top 50 m or so of core. Where only shallow cores are obtained, e.g. by a hand auger or where pit samples are used, an even number of seasons’ accumulation will probably not be collected and, due to the relatively small number of seasons represented, the δ ¹⁸O value will be biased away from the long-term mean.

We can obtain some idea of the magnitudes involved by examining a core from the Law Dome summit for which some detailed measurements have been made. (See (Budd and Morgan, 1977)). This location is probably representative of one of the more difficult places to sample because the high annual accumulation and the general lack of wind (for Antarctica) allows the large seasonal δ variation to be well preserved in amplitude. The accumulation is 0.6 Mg m⁻²a⁻¹ and the range of isotope values is 8‰ at 100 m depth, and probably about 12‰ near the surface. Assuming roughly equal amounts of summer and winter precipitation, the deviation in the δ value from the mean if an exactly odd number of seasons are sampled is:

To have a sample within 1‰ of the long-term in such a location requires that at least six seasons (i.e. three years) accumulation be sampled. This could cover a depth of 4 m in the near-coastal areas, but will be much less inland, where the accumulation rate is much smaller.

Some of the data used may be marginal in respect of sampling depth. Where possible the depth covered by the sample is noted in the listing of data.

The measurements are given as δ ¹⁸O with respect to SMOW, expressed as parts per mil deviation (see (Craig, 1961)). Where a number of measurements are closely spaced, e.g. on the Law Dome, the number of points has been reduced by averaging to give a reasonable spacing on the map (Fig. 1). The second map shows contours of constant δ ¹⁸O values (Fig. 2).