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Periodic Table--Oxygen

Oxygen has three stable isotopes: 16O (99.63%), 17O (0.0375%) and 18O (0.1995). Ratios of 18O to 16O in waters, rocks, and most solutes are reported in (permil) relative to Vienna Standard Mean Ocean Water (VSMOW).

Craig (1961a) observed that the d18O and d2H (dD) values of precipitation that has not been evaporated are linearly related by:

    dD = 8 d18O + 10
This equation, known as the "Global Meteoric Water Line" (GMWL), is based on precipitation data from locations around the globe, and has an r2 > 0.95. This high correlation coefficient reflects the fact that the oxygen and hydrogen stable isotopes in water molecules are intimately associated; consequently, the isotopic ratios and fractionations of the two elements are usually discussed together. The slope and intercept of any "Local Meteoric Water Line" (LMWL), which is the line derived from precipitation collected from a single site or set of "local" sites, can be significantly different from the GMWL. In general, most of these local lines have slopes of 8 +/- 0.5, but slopes in the range of 5 and 9 are not uncommon.

Several processes cause waters to plot off the GMWL. Water that has evaporated or has mixed with evaporated water typically plots below the meteoric water line along lines that intersect the MWL at the location of the original un-evaporated composition of the water; slopes in the range of 2 to 5 are common. Geothermal exchange also increases the 18O content of waters and decreases the 18O content of rocks as the waters and rocks attempt to reach a new state of isotopic equilibrium at the elevated temperature. This causes a shift in the d18O values, but not the dD values of geothermal waters. Low temperature diagenetic reactions involving silicate hydrolysis can sometimes cause increases in the d18O and dD values of waters.

The two main factors that control the isotopic character of precipitation at a given location are the temperature of condensation of the precipitation and the degree of rainout of the air mass (the ratio of water vapor that has already condensed into precipitation to the initial amount of water vapor in the air mass). Most water vapor in the atmosphere is derived from evaporation of low-latitude oceans. Precipitation derived from this vapor is always enriched in D and 18O relative to the vapor, with the fractionation between the rain and vapor a function of condensation temperature. Therefore, progressive rain-out as clouds move across the continent causes successive rain storms to become increasingly lighter. For example, non-equilibrium evaporation from the ocean with a d18O = 0 produces vapor of -12. Later equilibrium condensation of rain from this vapor results in water with a d18O = -3 and residual vapor with a d18O = -21.

Precipitation is the ultimate source of ground water in virtually all systems. Hence, knowledge of the factors that control the isotopic compositions of precipitation before and after recharge allows the use of oxygen and hydrogen isotopes as tracers of water sources and processes. On a regional scale, the distribution of isotopic compositions are controlled by several factors:

  • Altitude effect: On the windward side of a mountain, the d18O and dD values of precipitation decrease with increasing altitude. Typical gradients are -0.15 to -0.5 per 100m for 18O, and -1.5 to -4 per 100m for D. This pattern is often not observed in interior mountains, for snow, or on the leeward side of mountains.
  • Latitude effect: The d18O and dD values decrease with increasing latitude because of the increasing degree of "rain-out".
  • Continental effect: The ratios decrease inland from the coast.
  • Amount effect: The greater the amount of rainfall, the lower the d18O and dD values of the rainfall; this effect is not seen in snow.
At a given location, the seasonal variations in d18O and dD values of precipitation and the weighted average annual d18O and dD values of precipitation remain fairly constant from year to year. This happens because the annual range and sequence of climatic conditions (temperatures, vapor source, direction of air mass movement, etc.) remain fairly constant from year to year. In general, rain in the summer is isotopically heavier than rain in the winter. This change in average isotopic composition is principally caused by seasonal temperature differences but is also affected by seasonal changes in moisture sources and storm tracks. Shallow ground-water d18O and dD values reflect the local average precipitation values but are modified to some extent by selective recharge and fractionation processes that may alter the d18O and dD values of the precipitation before the water reaches the saturated zone (Gat and Tzur, 1967). Some of these processes include: evaporation of rain during infiltration, selective recharge, interception of precipitation by the tree canopy, and exchange of infiltrating water with atmospheric vapor. In the case of snow, various post-depositional processes, such as melting and subsequent infiltration of surface layers and evaporation, may alter the isotopic content of the snowpack, often leading to meltwater d values that become progressively enriched (Stichler, 1987).

Once the rain or snowmelt passes into the saturated zone, the d values of the subsurface water change only by mixing with waters that have different isotopic contents. The homogenizing effects of recharge and dispersive processes produce ground water with d values that approach uniformity in time and space, and that approximate a damped reflection of the precipitation over a period of years. It is important to remember that although an individual storm may be large and isotopically very different from the old water in the catchment, the amount of precipitation that infiltrates will likely be small compared to the amount of old water in storage. Although there may be significant storm-to-storm and seasonal variations in precipitation d18O and dD values, baseflow d values remain relatively uniform in most streams in humid, temperate areas.

In small catchments, temporal and spatial variability in ground water and baseflow d values may reflect seasonal variability in precipitation d values. However, these variations are less extreme and usually delayed relative to the temporal variations that occur in precipitation (Turner et al., 1987). These variations can be utilized for water residence time calculations. Superimposed on the seasonal cycles in precipitation d values are storm-to-storm and intra-storm variations in the d18O and dD values of precipitation. These variations may be as large as the seasonal variations. It is this potential difference in d values between the relatively uniform old water and variable new water that permits isotope hydrologists to determine the contributions of old and new water to a stream during periods of high runoff (Sklash et al., 1976).

Oxygen isotope ratios can be determined for many O-bearing compounds, including organics, silicates, carbonates,
phosphates, nitrates, sulfates, and gases. The d18O values of all these compounds are reported relative to VSMOW with the exception of carbonates, which are often reported relative to Vienna Pee Dee Belemnite(VPDB). Oxygen isotopic ratios can be determined for many O-bearing compounds, including organics, silicates, carbonates, phosphates, nitrates, sulfates, and gases. For a brief review of the compositions of these materials and the processes that affect their compositions, see Hoefs (1987). A "dual" or "multi-isotope" approach to determining sources or geochemical processes takes advantage of the fact that compounds consist of several elements, many of which can be easily analyzed for their isotopic ratios. For example, organic matter can be analyzed for the major elements C, H, N, O, S -- and for trace elements.

Nitrate-18O: The recent development of methods to analyze nitrate for d18O should expand the use of isotope techniques to trace nitrate sources and transformations. Nitrate d18O values show a wide range of values, ranging from about -10 to +80 . Many nitrate sources have distinctive d18O and d15N values; in particular, nitrate derived from atmospheric or nitrate fertilizer sources is easily distinguished from nitrified ammonium and urea. A figure showing the approximate ranges of compositions of many compounds is available. A useful application of the "dual isotope approach" is the determination of the relative contributions of atmospheric and microbial-derived sources of nitrate in shallow ground water (Durka et al, 1994) and stream water (Kendall et al., 1995). Amberger and Schmidt (1987) showed that denitrification results in enrichment in 18O of the residual nitrate, as well as enrichment in 15N. Therefore, analysis of both d15N and d18O of nitrate should allow denitrification effects to be distinguished from mixing of sources. This dual isotope approach takes advantage of the observation that the ratio of the enrichment in 15N to the enrichment in 18O in residual nitrate during denitrification appears to be about 2:1 (Amberger and Schmidt, 1987). For more information about nitrate-d18O, see Kendall (1998).

Sulfate-18O: Use of a dual isotope approach to tracing sources of sulfur (i.e. measurement of d18O and d34S of sulfate) will often provide better separation of potential sources of sulfur and, under favorable conditions, provide information on the processes responsible for sulfur cycling in the ecosystem. The rate of oxygen isotope exchange between sulfate and water is very slow at normal pH levels. Even in acidic waters, the "half-life" of exchange is on the order of 1000 years. Depending on the reaction responsible for sulfate formation, between 12.5 to 100% of the oxygen in sulfate is derived from the oxygen in the environmental water; the remaining oxygen comes from O2 (Taylor et al., 1984). Reviews of applications of d18O of sulfate include Holt and Kumar (1991) and Pearson and Rightmire (1980).





Source of text: This review was assembled by Carol Kendall, Eric Caldwell and Dan Snyder, primarily from Kendall et al. (1995) but also from the references below.

Amberger, A., and Schmidt, H.-L. (1987). "Naturliche Isotopengehalte von Nitrat als Indikatoren fur dessen Herkunft" (Natural isotope abundance of nitrate as an indicator of its origin). Geochim. et Cosmochim. Acta, 51: 2699-2705.
Bottcher, J., Strebel, O., Voerkelius, S. and Schmidt, H.L. (1990). "Using isotope fractionation of nitrate-nitrogen and nitrate-oxygen for evaluation of microbial denitrification in a sandy aquifer", J. Hydrol., 114: 413-424.
Coplen, T.B. (1993) "Uses of Environmental Isotopes", In: W.M. Alley (Ed.), Regional Ground-Water Quality , Van Nostrand Reinhold, New York, pp. 227-254.
Craig, H., (1961). "Isotopic variations in meteoric waters". Science, 133: 1702-1703.
Durka, W., Schulze, E.-D., Gebauer, G., and Voerkelius, S. (1994). "Effects of forest decline on uptake and leaching of deposited nitrate determined from 15N and 18O measurements". Nature, 372: 765-767.
Gat, J.R., and Tzur, Y. (1967). "Modification of the isotopic composition of rainwater by processes which occur before groundwater recharge". Isotope Hydrology, Proc. Symp. Vienna 1966, IAEA, Vienna, 49-60.
Hoefs, J. (1987).Stable Isotope Geochemistry, Third Edition, Springer-Verlag, Berlin, 241 p.
Holt, B.D. and Kumar, R. (1991). "Oxygen isotope fractionation for understanding the sulphur cycle." In: H.R. Krouse and V.A. Grinenko (Eds.), Stable Isotopes: Natural and Anthropogenic Suphur in the Environment, Scientific Committee on Problems of the Environment (SCOPE) 43, pp. 27-41.
International Atomic Energy Agency (1981). Stable Isotope Hydrology, Deuterium and Oxygen-18 in the Water Cycle, (Eds. J.R. Gat and R. Gonfiantini) Technical Reports series No. 210., IAEA, Vienna, 337 p.
International Union of Pure and Applied Chemistry (IUPAC) (1991). "Isotopic Compositions of the Elements". Pure & Appl. Chem., 63(7): 991-1002.
Kendall, C. (1998). "Tracing nitrogen sources and cycling in catchments", In: C. Kendall and J.J. McDonnell (Eds.), Isotope Tracers in Catchment Hydrology, Elsevier, Amsterdam, pp. 519-576.
Kendall, C., Campbell, D.H., Burns, D.A., Shanley, J.B., Silva, S.R., and Chang, C.C.Y. (1995). "Tracing sources of nitrate in snowmelt runoff using the oxygen and nitrogen isotopic compositions of nitrate: pilot studies at three catchments", In: K. Tonnessen, M. Williams, and M. Tranter (Eds.), Biogeochemistry of Seasonally Snow-covered Catchments, Intern. Assoc. of Hydrol. Sci. Pub., July 3-14, 1995, Boulder CO, IAHS Publ. # 228, pp. 329-338.
Kendall, C., Sklash, M.G., Bullen, T.D. (1995). "Isotope Tracers of Water and Solute Sources in Catchments", in Solute Modelling in Catchment Systems, J. Wiley & Sons, New York, pp. 261-303.
Pearson, F.J., and Rightmire, C.T. (1980). "Sulphur and oxygen isotopes in aqueous sulfur compounds." In: P. Fritz and J. Ch. Fontes (Eds.), Handbook of Environmental Isotope Geochemistry , Elsevier, Amsterdam, pp. 179-226.
Sklash, M.G., Farvolden, R.N., and Fritz, P., (1976). "A conceptual model of watershed response to rainfall, developed through the use of oxygen-18 as a natural tracer". Can. J. Earth Sci., 13: 271-283.
Taylor, B.E., Wheeler, M.C., and Nordstrom, D.K., (1984). "Oxygen and sulfur compositions of sulphate in acid mine drainage: evidence for oxidation mechanisms". Nature, 308: 538-541.
Turner, J.V., Macpherson, D.K., and Stokes, R.A., (1987). "The mechanisms of catchment flow processes using natural variations in deuterium and oxygen-18." In: A.J. Peck and D.R. Williamson (Eds.), Hydrology and Salinity in the Collie River Basin, Western Australia. J. Hydrol., 94: 143-162.
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Fundamentals of Stable Isotope Geochemistry
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