Isotopic Composition of Modern Precipitation in Longyearbyen, Svalbard

A project funded by the University Courses on Svalbard (UNIS) 2000-2005

Ole Humlum, UNIS, Department of Geology, Svalbard, Norway

Collaborating institution: the Niels Bohr Institute, Univ. of Copenhagen

Background

Water molecules can contain different isotopes of hydrogen and oxygen. The most abundant hydrogen isotope of mass 1 is simply designated as H. The heavy isotopes of hydrogen of masses 2 and 3 have acquired the individual names of deuterium (D) and tritium (T). Tritium is unstable and decays by beta emission. For the isotopes of oxygen, there is no individual nomenclature; they are designated ¹⁶O, ¹⁷O and ¹⁸O, respectively. The most abundant oxygen isotope is ¹⁶O, followed by ¹⁸O (often called the heavy oxygen isotope), while ¹⁷O is less frequent.

In the atmosphere, the isotopic components of water are fractionated because the vapor pressure of the heaviest component is slightly lower than that of the light component; about 1% lower for H₂¹⁸O compared to H₂¹⁶O. At equilibrium, the atmospheric water vapor thus contains 1% less ¹⁸O than standard ocean water, which is the main source for water vapor in the atmosphere. Especially oceans in warm and temperate regions are the main source for atmospheric water vapor. However, if such a vapor is cooled, giving rise to raindrops in a cloud, the first small amount of precipitation will have the same composition as the ocean water because the vapor condenses with 1% preference to ¹⁸O.

As the moist air masses subsequently move toward colder regions the remaining water vapor will gradually be more and more depleted with respect to the heavy oxygen isotope ¹⁸O (see figure below). Therefore, precipitation (snow) in the polar regions far from the most important water vapor source areas will usually have a low content of ¹⁸O, while the light isotope ¹⁶O is relatively frequent.

In the commonly used delta scale for oxygen isotopes, data on natural water or precipitation are usually reported in terms of the ratio between the concentrations of heavy and light isotopes (¹⁸O/¹⁶O):

delta¹⁸O = 10³ x (((¹⁸O/¹⁶O)_SAMPLE - (¹⁸O/¹⁶O)_SMOW )/(¹⁸O/¹⁶O)_SMOW)

where SMOW stands for Standard Mean Ocean Water. In general, delta¹⁸O is high for precipitation, but low for vapor, since ¹⁸O is heavier than ¹⁶O and therefore preferentially will remain in liquid form.

Simplified circulation model showing the oxygen isotope fractionation during the evaporation of ocean water (to the left) and subsequent precipitation (to the right), as moist air masses move toward higher latitudes or altitudes. By this process precipitation is progressively becoming depleted in the heavy oxygen isotope ¹⁸O as indicated by the delta18O values (see above). Upper panel illustrates summer (or warm) conditions, while the lower panel illustrates winter (or cold) conditions. The uppermost numbers indicate delta¹⁸O values for atmospheric water vapor, while the lower numbers indicate delta¹⁸O values in the resulting precipitation. Delta¹⁸O values are lower in winter than in summer. 

A main control on the delta¹⁸O value of precipitation (rain or snow) is the temperature of formation. The relationship between the mean annual values of delta¹⁸O and surface air temperature T_s (^oC) is often seen expressed as

delta¹⁸O =  T_s(0.67±0.02) - (13.7±0.05)‰

(Dansgaard et al., 1971, 1973; Johnsen et al., 1989; Johnsen et al., 1992). This relation is primarily based on investigations on the Greenland Ice Sheet and on a smaller ice cap in East Greenland, and have been the basis for a very well-known attempt towards reconstruction of past climates from ice-core isotope-stratigraphy in the Greenland Ice Sheet  (Dansgaard et al., 1971, 1993; Fisher et al., 1996).

The link between long-term changes in the isotopic composition of precipitation and surface air temperature at a given location is probably the most important relationship as far as paleoclimatic applications are concerned. Today, a semi-empirical temperature/stable isotope relationship was established for coastal stations in the mid and high northern latitudes. 

The slope of 0.69 (or sometimes 0.67; see relation above) per mil per °C has been used in numerous climate studies to reconstruct past temperatures. However, from what is empirically know today, this relation may vary over a broad range since local temperature is not always the best measure for the isotopic composition of precipitation at a given site. In a temperature dominated regime, three different isotope/temperature relationships can be established; first the spatially coherent relation between long-term annual averages of stable isotopes and of temperature, secondly a more local temporal relationship between the two and finally, a short term temporal linkage between seasonal changes in stable isotopes and temperature either on a local or more regional scale.

At some investigation sites, observational data have displayed a lack of correlation with air temperature or a strong variation in the delta¹⁸O/T gradient, this, however, does not invalidate the isotope approach. In such cases it may well be that the isotope-thermometer has measured precipitation, volume, source, or continentality instead.

The International Atomic Energy Agency (IAEA), in co-operation with the World Meteorological Organization (WMO), is now conducting a worldwide survey of oxygen and hydrogen isotope content in precipitation. This programme was initiated in 1958 and became fully operational in 1961. The main initial objective is the systematic collection on a global scale of basic data on isotope content of precipitation to determine temporal and spatial variations of environmental isotopes in modern precipitation and, consequently, to provide basic isotope data for the use of environmental isotopes in hydrological investigations within the scope of water resources inventory, planning and development.

Although this primary objective remains as an important factor, in recent years, two other objectives have attracted increasing interest: (i) providing input data to verify and further improve atmospheric circulation models, and (ii) the study of climate changes, past as well as present.

Actually, the use of isotopes derived from old ice to reconstruct the local palaeoenvironment remains among the most important reason for Investigating the present day behavior of stable isotopes in precipitation.

For these reasons, a sampling scheme of modern precipitation has been initiated at UNIS in 1999 in order to investigate the local relation between air temperatures and the isotopic composition of local precipitation, especially expressed by the the delta¹⁸O value. Sampling of precipitation is done during each precipitation event, along with measurements of air- and cloud base temperature simultaneously. The precipitation samples are transported in a frozen state to the isotope laboratory in the Department of Geophysics (the Glaciology Group), the Niels Bohr Institute (University of Copenhagen), where the analyses are done. 

The figure below exemplifies the relation between air temperature and the delta¹⁸O-values for a group of  precipitation samples collected in Longyearbyen November-December 1999.

This sampling scheme will continue in the years to come and will gradually improve understanding between meteorology and the local isotopic composition of precipitation in central Spitsbergen. Eventually, this knowledge will form a basis for deriving palaeoenvironmental information from various forms of ice accumulations such as represented by glaciers, rock glaciers (Humlum, 1999), ice wedges, etc. 

Before using this knowledge in an operative way, however, all delta¹⁸O-values will be statistical analyzed with respect to their correlation with a number of meteorological parameters such as, e.g. air temperature, cloud base temperature, regional and hemispheric air flow and the most likely source region. In this context the on-line availability of both local meteorological observations as well as hemispherical meteorological maps and satellite images represent valuable tools.  

Post-depositional isotopic changes

In polar regions especially isotopic changes occurring in the snow cover has implications for interpreting the isotope stratigraphy found in various types of ice accumulations.

Several post-depositional factors may modify the isotopic signature of the snow cover. As an example, Ambach et al. (1968) explained the ¹⁸O content in snow on the Austrian glacier Kesselwandferner as being controlled by wind drifting of isotopically light winter snow. So knowledge on the local importance of drifting snow may also be important.

Significant summer surface melting may lead to an enrichment of the heavy isotope of oxygen as the snow ages (MacPherson and Krouse, 1967; Arnason, 1969; Stichler, 1987; Theakstone, 1991; Theakstone and Knudsen, 1996a, 1996b; Raben and Theakstone, 1998). As the upper part of the winter snow cover melts the following summer, percolating water warms the pack, accompanied by isotropic homogenization of the remaining snow (Raben and Theakstone, 1994; Humlum, 1999). As a result of isotopic fractionation, the water which drains from a glacier is relatively depleted of the heavy isotope ¹⁸O, while the remaining snow is enriched (Theakstone and Knudsen, 1996b). At the end of the summer, therefore, the mean d¹⁸O value of the residual snow pack is higher than at the end of the preceding winter, and the resulting firn is isotopically more homogeneous (Raben and Theakstone, 1998).

Literature

Arnason, B. 1969: The exchange of hydrogen isotopes between ice and water in temperate glaciers. Earth Planetary Science Letters 6, 423-430.

Dansgaard, W.; Johnsen, S. J., Clausen, H. B. and Langway, C. C., Jr. 1971: Climatic record revealed by the Camp Century ice core. In Turekian, K. K., Editor, The Late Cenozoic Glacial Ages. Yale University Press, New Haven, 37-56.

Dansgaard, W., Johnsen, S. J., Clausen, S. J. and Gundestrup, N. 1973: Stable isotope glaciology. Meddelelser om Grønland 197, 1-53.

Dansgaard, W.; Johnsen, S. J.; Clausen, H. B.; Dahl-Jensen, D.; Gundestrup, N.; Hammer, C. U.; Hvidbjerg, C. S.; Steffensen, J. P.; Sveinbjörnsdottir, A. E.; Jouzel, J. and Bond, G. 1993: Evidence for instability of past climate from a 250-kyr ice-core record. Nature 364, 218-220.

Fisher, D. A.; Koerner, R. M.; Kuivinen, K.; Clausen, H. B.; Johnsen, S.; Steffensen, J.-P.; Gundestrup, N. and Hammer, C. 1996: Inter-comparison of ice core d(18O) and precipitation records from sites in Canada and Greenland over the last 3500 years and over the last few centuries in detail using EOF techniques. In Jones, P. D., Bradley, R. S. and Jouzel, J., editors, Climatic variations and forcing mechanisms in the last 2000 years, NATO ASI Series v. 141, Berlin, Springer-Verlag, 297-328.

Humlum, O. 1999:  Late Holocene climate in central West Greenland: meteorological data and rock glacier isotope evidence. The Holocene, 9,4: 581-594.

Johnsen, S. J.; Dansgaard, W. and White, J. W. C. 1989: The origin of Arctic precipitation under present and glacial conditions. Tellus 41B: 452-468.

Johnsen, S. J.; Clausen, H. B.; Dansgaard, W.; Gundestrup, N. S.; Hansson, M.; Johnsson, P.; Steffensen, J. P. and Sveinbjörnsdottir, A. E. 1992: A “deep” ice core from East Greenland. Meddelelser om Grønland, Geoscience 29, 3-22.

MacPherson D. and Krouse, H. R. 1967: O18/O16 ratios in snow and ice of the Hubbard and Kaskawulsh glaciers. In Stout, G. E., editor, Proceedings of the symposium on isotopes techniques in the hydrological cycle. American Geophysical Union, Washington D.C., pp. 180-194.

Raben, P. and Theakstone, W. F. 1994. Isotopic and ionic changes in a snow cover at different altitudes: observations at the glacier Austre Okstindbreen in 1991. Annals of Glaciology, 19, 85-91.

Raben, P. and Theakstone, W. F. 1998. Changes of Ionic and Oxygen Isotope Composition of the Snowpack at the Austre Okstindbreen, Norway, 1995. Nordic Hydrology, 29, 1-20.

Stichler, W. 1987. Snowcover and snowmelt processes studied by means of environmental isotopes. In Jones, H. G. and Orville-Thomas, W. J. (eds.), Seasonal Snowcovers: Physics, Chemistry, Hydrology. Springer-Verlag, Berlin, pp.673-317.

Theakstone, W. F. 1991. Stable isotope investigations at Austre Okstindbreen, 1980-1990. Okstindan Glacier Project Report 91.3, 46 pp.

Theakstone, W. F. and Knudsen, N. T. 1996a. Isotopic and ionic variations in glacier river water during three contrasting ablation seasons. Hydrological Processes, 10, 523-539.

Theakstone, W. F. and Knudsen, N. T. 1996b. Oxygen Isotope and Ionic Concentrations in Glacier River Water: Multi-Year Observations in the Austre Okstindbreen Basin, Norway. Nordic Hydrology, 27, 101-116.

Latest update: 5. April 2005.