TUTORIAL

 

Catchment scale: the exchange of isotopes in water

Groundwater and rainwater generally have different isotopic signatures.  In addition, summer and winter rain may be distinguished isotopically in many areas. To understand why, remember that heavier molecules, such as molecules containing heavy isotopes, will condense into liquid faster than lighter molecules. This is called Rayleigh distillation.  Picture a cloud that forms by evaporation over the ocean - as the cloud moves across land, heavy water molecules containing D and 18O rainout first in coastal areas. The water vapor remaining in the cloud is depleted in these isotopes. As the cloud continues to move inland and more water condenses into rain, the isotope ratio of the liquid water becomes more and more negative, or depleted in the heavy isotopes. Temperature also influences the isotopic composition of rainwater because fractionation decreases with temperature. At northern latitudes where temperatures are cooler, even less 18O and D fractionates into rainwater, so that isotope ratios of precipitation become even more negative.  Finally, the isotope composition of precipitation is also more negative at higher elevations. As air masses move up mountainsides, changes in air pressure cause cooling and precipitation -the heavier isotopes rainout first at lower elevations.

Because of all these effects, clouds that originate in different areas produce rain with different isotope ratios.  This can cause predictable seasonal variation.  For instance, here in Utah, winter storm fronts generally come from the Pacific, while summer monsoons originate from convective thunderstorms at much warmer temperatures.  Precipitation that falls in the Utah desert from the winter Pacific fronts has a dD of about -90‰.  Most desert plants are inactive during the winter, so there is little transpiration to remove winter rainwater from the soil. Soil moisture in deeper soil layers is recharged during the winter.  In the summer, monsoonal rains are isotopically "heavier", with dD of about -25‰.  During this time, greater evapotranspiration and runoff prevent the soil from being deeply recharged - only the upper soil layers contain water from the summer rain. Ecologists have used this isotopic difference to sample water in the stems of desert plants and determine which species were deeply rooted in order to use winter recharge, and which relied only on the shallow soil layers. They found that while many species used both deep and shallow water, several desert shrubs did not use summer precipitation at all.  Conversely, annual species, which live for only one year, and succulents, fleshy plants such as cacti, relied completely on summer precipitation. If climate patterns shifted such that summer rain became much less frequent, these species could disappear from the Utah desert ecosystem.

Atmospheric gradients of 18O/16O also contain important information about the role of the terrestrial biosphere in the carbon cycle. We have discussed that CO2 leaving ecosystems carries the 18O signature of water in leaves and soil, as oxygen in CO2 exchanges with oxygen in water. This signature is very different from 18O/16O of ocean water, so that measurements of oxygen isotopes in atmospheric CO2 provide an additional, independent method for partitioning oceanic and terrestrial sinks of carbon. To utilize oxygen isotopes on a global scale, modelers must estimate both photosynthetic discrimination of 18O and the oxygen isotopic composition of plant and soil water globally.

 

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