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

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.

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).

Carbonate-¹⁸O:

Phosphate-¹⁸O:

Organic-¹⁸O:

Silicate-¹⁸O:

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.