x' may be heavier or lighter than x, one can obtain the isotope-ratio Sx' in %% as follows:
Thus, in fact, one is operating with isotope-ratios, which provides the high precision in comparisons of different samples, expressed as a differential against the particular standard.
Isotope effects are a measure of the behaviour of individual molecules containing different isotopes of an element and occur during diffusion or transport in various media as well as in enzymatic reactions. The differences in the kinetic properties and in the behaviour of molecules with different isotopic composition in thermody-namic equilibria form the basis for many applications of the stable-isotope technique, including:
• biochemistry and physiology of metabolism;
• ecology and environmental research.
The technique originated from geochemistry. Its great importance in meteorology may be illustrated by considering water. The vapour pressure of water is proportional to the mass. Thus, heavy H2O-molecules with the isotopes 2H and 18O instead of 1H and 16O need higher temperatures to evaporate and are discriminated against. However, at higher temperatures the absolute H2O content in the gas-phase is higher. During rain the heavier molecules then precipitate more readily than the lighter ones, and hence, evaporation and precipitation provide a climate effect on isotope composition of rain (Ziegler 1989):
• according to latitude;
the content of heavy isotopes 2H and 18O declines with increasing latitude following the temperature gradient;
• according to altitude;
the content of 2H and 18O declines with increasing altitude;
• according to seasons;
at higher latitudes (> 30°) the rain contains more 2H and 18O in summer and less in winter;
• according to continentality;
the 2H and 18 O contents decrease with increasing distance from the coast;
• according to the amount of rain;
the 2H and 18O contents decrease with increasing amount of rain falling.
This explains hydrological isotope effects. The isotope composition of groundwater, flowing surface-water, and recent precipitation is different. Thus, although no
fractionation occurs during uptake by plants, organisms which take up such water are also distinguished by their own isotope content. Although there are additional discrimination processes when water evaporates from leaves (enriching leaf 2H and 18O), a further fractionation occurs in favour of the heavy isotopes during incorporation into organic material. By analysis of the most recently produced biomass, allowing for certain exchange reactions one can conclude from which direction the last rain falls or one can determine the geographical origin of plants and food items (Smith 1975).
These climate effects have led to applications in palaeoclimatology. From the isotope composition of fossil water in subterranean water reservoirs, which have formed in geological periods, one can draw conclusions on the climate at that time. Analysis of small gas inclusions in layers of ice in glaciers, or of water from the ice itself, allow similar conclusions about the past. For example, in the ice of a glacier at 5,670 m a.s.l. in the Cordillera de Carabaya of the tropical Andes in Peru, relatively low levels of 18O [more negative values of S18O, see (2.5)] indicate a warmer period between 1000 and 1500 A.D. This was correlated with lower accumulation of ice, although higher 18O levels (less negative S 18O), then represent a colder period from 1500 to about 1875 A.D. when ice-deposition was initially high (Fig. 2.12). Further opportunities are provided by the incorporation of water into organic constituents of plants, and the analysis of gross remnants of plants in peat accumulations or from the study of isotope composition of annual rings in old trees and wood.
When ô13 C signatures of several ecosystem components such as the air, the plant biomass, fallen litter and the soil are studied systematically with a spatiotemporal resolution insights into the carbon dynamics, e.g. of tropical primary rainforests can be obtained (Buchmann et al. 2004). In extant current physiology of water relations of plants the isotope effects of evaporation and diffusion of water are useful for the study of transpiration and water use efficiency (WUE) dur o o
o 1980 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000
Was this article helpful?
Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.