Water Isotopes (dD and d18O in the Hydrosphere)Reading:Alley and Cuffy chapter in the Valley and Cole BookReview Chapter 28 in WhiteSome great information can be found at the International Atomic Energy Agency web site:http://www.iaea.or.at/programmes/ripc/ih/volumes/volumes.htm

Motivation:Fractionation of oxygen and hydrogen isotope occurs in the atmosphere and hydrosphere andgives us some very powerful tools for tracking climate change and hydrologic phenomena.

Guide Questions:What are the standards used for H and O isotope measurements?Is isotopic equilibrium approached during precipitation of water from H2O vapor?Why is precipitation a Rayleigh process?Is isotopic equilibrium approached during evaporation of water to form H2O vapor?Roughly how large are the isotopic fractionations for the water - H2O vapor equilibrium at0˚C and 100˚C, for oxygen and hydrogen?Explain why the isotopic composition of precipitation on earth corresponds somewhat to aRayleigh distillation model?Name several ways in which actual system on earth is NOT like a simple Rayleighdistillation process.What is the relationship between d18O in average precipitation and mean annual temperaturefor the entire earth?Why is this relationship so strong, when the d18O actually depends most directly on theamount of rainout?About how much d18O change occurs, on average, per degree Celsius change in T?How much of the line plotted by Dansgaard (1964) is applicable to mid latitudes?How much scatter is there away from the best fit curve, in d18O values?What is the global meteoric water line and what is its slope?Why do some bodies of water plot to the right of the GMWL? What process is indicated bythis?What happens to the isotopic composition of groundwater during extensive water-rockinteraction? Why?What patterns appear in the O and H isotopes of ice cores from Antarctica and Greenland?How do they compare with other measures of climate?Are the O and H isotopes in ice cores perfect paleothermometers? What are the potentialproblems?Are there ice core records from lower latitude places?Are dD and d18O in precipitation different for different seasons? Why?In a given area are dD and d18O in precipitation different for different altitudes? Why?Why does a record of dD and d18O in precipitation on a mountain range tell us aboutpaleoaltitude?How is dD and d18O in groundwater that was recharged to an aquifer 20,000 years agodifferent from that of today (say, for Illinois).Is it possible for drinking water in a city or town to have different dD and d18O compared tothe local precipitation? Why?

Standards used for oxygen and hydrogen isotopesOcean water is used as the standard for all H isotope analyses and for most O isotope analyses.Actually, there is slight variation in ocean water, so we have something called:- Standard Mean Ocean Water (SMOW) as a standard. V-SMOW is an equivalent standard

developed by a group in Vienna. The original SMOW supply may be completely used up.- PDB (now VPDB) is a carbonate material used for oxygen isotopes in carbonate rocks.

d18O(VSMOW) = d18O(VPDB) + 30.92 !!!+ d18O(VPDB)* 30.92/1000

Fractionation of O and H: equilibrium and kinetic- Precipitation is an equilib. process.: During precipitation of water from H2O vapor, the

intimate contact between water and vapor, and the fact that humidity is very close tosaturation (100% relative humidity)

- Precipitation is a Rayleigh process: Precip. is removed from the cloud and thus back-reaction ceases and we have a Rayleigh process.

- Evaporation is kinetic (it may get somewhat close to equilibrium if humidity is high and thusback reaction is almost as great as forward reaction, but when humidity is higher, netevaporation is slower)

Sizes of fractionations- 1000lna values given as a function of T in White’s Fig. 27.2. Actuallythat figure is very confusing, so see handout of Faure, 1986 Fig. 24.1)- At the 0˚C: 1000lna = 11‰ for oxygen isotopes, 99‰ for hydrogen isotopes- At the 100˚C: 1000lna = 3.2‰ / 29‰ for hydrogen isotopes

Application of Rayleigh fractionation and transport models to watervapor in our atmosphere and precipitation.A: The classic model (has some flaws):1) Most water vapor on earth originates in the tropics2) It is then transported, via winds and mixing, toward the poles3) The warm air in the tropics is moisture-laden4) As a packet of air cools, it drops moisture as rain or snow5) The transport of moisture poleward thus involves a distillation process

- remaining vapor becomes progressively depleted in heavier isotopes6) The fraction of the original moisture remaining should be a function of temperature

- and thus, the isotopic depletion should be a function of temperature7) Using a Rayleigh fractionation model, we can then link temperature to d18O

NOTE: 1000lna increases as T decreases, so this Rayleigh model is more complex thanthe one we have done for constant a.See Alley and Cuffey, Fig. 3 for the constant a case. Note that in White’s notes, hepresents a model for fractionation of water vapor for a simplified case as an exampleonly. Don’t use that model for the real world.

B: A more realistic picture involves the following:1) “Rain-out” effect as given in the model above2) Mixing of different air masses3) Exchange of H20 between atmosphere and oceans after the air leaves the tropics4) Exchange of H20 between atmosphere and soils, lakes, and rivers5) Orographic effects, e.g., Nevada, USA has isotopically light rain because of strong

rainout over the Sierra Nevada mountains of California.

6) Evaporative/Recycling effects: Water that evaporates is more depleted in heavy isotopesthan the rain. Also operates in Nevada, possibly.

7) For interpretation of ancient conditions: Changes in Moisture source T and humidity.8) For interpretation of ancient conditions: Changes in d18O of oceans (about 1 per mil

heavier during last glacial period).- See Hendricks et al. (2000) [Global Biogeochem. Cycle Vol. 14, pp. 851-861] and

Jouzel et al., [JGR-D Vol. 92, 14739-14760] if you want to see, in more detail, morerealistic models that help us understand some of these issues.

Actual dD and d18O on earth (annual average) as a function of meanannual air temperatureLook at the Dansgaard (1964) curve- Measurements of d18O plotted versus temperature. Seehandout. Also see Fig. 4a from Alley and Cuffey. This plot is particularly useful, as he usesdifferent symbols for different latitude ranges. NOTE that most of the line is defined by datafrom Greenland and Antarctica.1) Interesting that the data are close to linear. Theory gives some curvature. Actually, the plotin Alley and Cuffey looks somewhat curved.2) However, there are some locations that fall far off the line. Especially above the line. This iscaused by local, low-altitude moisture derived from the ocean (e.g., think about some island inthe arctic, surrounded by ocean- where does the moisture come from?)

Other questions: Shouldn’t d18O depend on the temp. at the time and place of precip, but NOTnecessarily on the mean annual temp? Clearly, these two are closely related, so this is a minorerror, but in some areas most of the precipitation is in one season only and thus the meanannual temp. is not appropriate for the plot.

Relationship between dD and d18O on earth: The “meteoric waterline” and how to detect evaporation.

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1) The solid line gives the approximate compositions of rain/snow on earth. This is knownas the Global Meteoric Water Line (GMWL). dD =8(d18O) +10Rain at low latitude plots along the line to the left of the seawater point. Rain-out drivesthe remaining water vapor along the line to the lower left. Higher latitudes plot to the

Plot of dD vs. d18O: Close linear relationshipbetween d18O and dD, as expected, because therelative sizes of the fractionations should beconstant (for precipitation; see below forevaporation). The diamond gives thecomposition of seawater. Tropical rains plot justto the left of the seawater value. Precipitationsamples from cooler places plot along theGMWL; colder places are farther to the lowerleft. The horizontal arrow gives the effect ofextensive water-rock interaction (e.g., hot springwaters).

lower left. The GMWL is the effect of Rayleigh-type fractionation driven byequilibrium fractionation during precipitation.

2) The dotted line gives the trajectory followed (approximately) by a mass of water (e.g., alake) as a large fraction of it is evaporated away. The kinetic fractionation occurringduring evaporation is different from equilibrium fractionation occurring duringprecipitation, so the slope (relative effects on hydrogen versus oxygen isotopes) isdifferent.- The exact slope depends on the humidity (see plot below). Low humidity leads to a

slope very different from that of the GMWL. High humidity- slope more similar toGMWL.

- - Evaporation from soils tend to be more like the high humidity case- Transpiration: Water uptake by plants does not involve much of an isotopic

fractionation and therfore the overall transpiration flux must NOT be highlyfractionated isotopically

So... compositions falling to the right side of the line suggest evaporation. Hydrologists can usethis as a means of detecting lake water that has moved into the subsurface.

3) Departure from the GMWL can be quantified by a parameter know as the “deuteriumexcess” = DE. This expresses howis defined as the y-intercept of the line with a slope of 8.0,drawn through a sample’s composition. The GMWL has a DE value of +10‰. The deuteriumexcess can be used to:- Infer humidity at the moisture source. If most of the moisture in an air mass was transferred

to the atmosphere under low-humidity conditions, the air mass will have a larger DE thanother air masses.

- Distinguish different moisture sources that produced recharge for old groundwaterJoel Gat (Wiezmann Institute, Israel) has done much research on the meteoric water lines and the effects ofevaporation. See also: Craig, H. and Gordon, L.I., 1965. Deuterium and oxygen-18 variations in the oceanand marine atmosphere. In: E. Tongiorgi (Editor), Stable Isotopes in Oceanographic Studies and Paleo-Temperatures. Pisa, Lab. Geol. Nucl.: 9-130.

4) Some places have precipitation that generally does not fall on the GMWL. For example, inNevada and Israel, the climate is very dry and evaporation effects drive the water vapor tounusual compositions. So we defined a local meteoric water line (LMWL) and use that for localstudies.

5) The horizontal arrow shows the effects of extensive water-rock interaction at hightemperatures. This happens because the water exchanges oxygens with the host rock, which has

relatively heavy oxygen isotope ratios. Hydrogen is affected little by this process, becausethere’s so much more hydrogen in water than in rock.

d18O and dD data in Ice cores: A record of climate change.Ice cores from Antarctica: Snow d18O was 8 to 10 ‰ lighter during last glacial period- why?1) Temp. effect on the dprecip. - dvapor during precip.: This site was likely a few degrees colder,so, if the vapor were the same then as now, the snow would be isotopically heavier duringglacial time- REVERSE of observed2) Rainout effect: This is the dominant effect. The vapor must have been, say, 10‰ lighterthen- must be greater rainout during glacial times.

- Dansgaard model predicts that isotope ratio should be a strict function of temp(because the amount of rainout is). So one interpretation is that the d18O shift tells usit was 13˚C colder and no other changes occurred.

- Perhaps there was more rainout without a T change? Is this plausible? Duringcolder periods, we expect less vigorous circulation of the atmosphere, and possiblyless efficient transport of moisture toward poles. But this requires either:- 1) Colder temp needed to get more rainout as above- 2) Different ”starting point” for the rainout- e.g., maybe rainout started at lower

latitude? Maybe conditions at the moisture source were different?3) Preservation effects? Maybe summer snow is ablated away more during warmer times- thiscauses a loss of the heaviest snow of the yearBottom line: The temperature was almost certainly colder during glacial periods, but theisotopes are probably not a perfect paleothermometer. Maybe the conditions were different atthe moisture sources and maybe preservation of the snow changed.

Note that dD in ice cores is very strongly correlated with the CO2 concentration in theatmosphere at the time of precip. (this is determined by analyzing tiny gas bubbles in the ice).Clearly, the isotopes are recording climate changes.

The timing of these changes (note the rapid deglaciations) can be precisely determine viacosmogenic isotope dating of the ice.

Some ice cores go back very far at low temporal resolution. Other cores, because the snowaccumulation rate is greater, have greater temporal resolution but don’t go as far back in time.

Ice cores from lower latitudes have been measured as well. However, these ice cores are takenfrom mountain glaciers- much more difficult! Lonnie Thompson at Ohio State is famous fortaking drill rigs via backpack to 6,000 m altitude and above to get drill cores of mountainglaciers. These may tells us more than the high-latitude ice cores because they are morerelevant to climate where we live.

Water Isotopes in Hydrology

1) Seasonal Effects

Chicago d18O (monthly averages)

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Above is a plot of d18O in precipitation, averaged monthly, in Chicago for a few years in the60’s and 70’s. Note the seasonal variation AND the year-to-year variability for any givenmonth. For other global data, go to: http://isohis.iaea.org/GNIP.asp (you may have to register toaccess the site).

The ISOHIS site has animations showing the evolution of isotope values during the annualcycle.

2) Orographic effects- “The altitude effect” and water tracingMountain ranges usually have isotopically lighter water than surrounding areas. Myunderstanding of this is that the mountains get rain from many air masses because of theorographic effect, whereaslower areas get rain less frequently, and only from air masses that have more moisture and thushave been subjected to less rainout previously.

3) Orographic effects- Paleoaltitude studiese.g., Chamberlain and Poage, 2000, Geology 28, 115-118.1) Rain-out over all mountain ranges- even small ones2) Rain-out removes heavier isotopes preferentially, vapor becomes lighter3) How big is this effect? How much lighter per km elevation?

- Chamberlain and Poage get 2.1‰/km- Temperature decrease for adiabatic cooling is about 10˚C per km- Rain out effect is about 7‰ per 10˚C- too large but…- Temperature decrease for adiabatic cooling plus condensation < 10˚C per km

4) Detecting old glacial meltwaterIsotopically light water found in deep aquifers of upstate New York and the Midwest.Upstate NY data were collected by Don Siegel, group.

5) Detecting water that is piped in from remote areas

Many cities get their water from far away areas. For example, San Francisco gets its water fromthe Sierra Nevada, where the water is isotopically lighter than the rain in San Francisco. It isthus possible to detected groundwater leaking from water supply pipes.

6) PaleohydrologyIsotope ratios of minerals left behind by ancient waters (typically, calcite, quartz, or clayminerals) reflect the isotopic composition of the parent waters. In many cases, the isotope ratiosare easier to interpret than major or trace element chemical data, and may even give informationabout the temperature of the water.