Изотоптық геохимия: табиғи ортада изотоптардың құрамы мен мағынасы

Aspect of geology studying variations in isotope abundances in the natural environment

Isotope geochemistry is an aspect of geology based upon the study of natural variations in the relative abundances of isotopes of various elements. Variations in isotopic abundance are measured by isotope ratio mass spectrometry, and can reveal information about the ages and origins of rock, air or water bodies, or processes of mixing between them. Stable isotope geochemistry is largely concerned with isotopic variations arising from mass dependent isotope fractionation, whereas radiogenic isotope geochemistry is concerned with the products of natural radioactivity.

Carbon has two stable isotopes, 12C and 13C, and one radioactive isotope, 14C. The stable carbon isotope ratio, δ13C, is measured against Vienna Pee Dee Belemnite (VPDB). The stable carbon isotopes are fractionated primarily by photosynthesis (Faure, 2004). The 13C/12C ratio is also an indicator of paleoclimate: a change in the ratio in the remains of plants indicates a change in the amount of photosynthetic activity, and thus in how favorable the environment was for the plants. During photosynthesis, organisms using the C3 pathway show different enrichments compared to those using the C4 pathway, allowing scientists not only to distinguish organic matter from abiotic carbon, but also what type of photosynthetic pathway the organic matter was using. The 14C ratio has been used to track ocean circulation, among other things.

Nitrogen has two stable isotopes, 14N and 15N. The ratio between these is measured relative to nitrogen in ambient air.

Oxygen has three stable isotopes, 16O, 17O, and 18O. Oxygen ratios are measured relative to Vienna Standard Mean Ocean Water (VSMOW) or Vienna Pee Dee Belemnite (VPDB). Typically, the VPDB oxygen reference is used for paleoclimate, while VSMOW is used for most other applications. Isotope ratios in fossilized foraminifera have been used to deduce the temperature of ancient seas.

Sulfur has four stable isotopes, with the following abundances: 32S (0.9502), 33S (0.0075), 34S (0.0421) and 36S (0.0002). These abundances are compared to those found in Cañon Diablo troilite.

Radiogenic isotopes provide powerful tracers for studying the ages and origins of Earth systems. They are particularly useful to understand mixing processes between different components, because (heavy) radiogenic isotope ratios are not usually fractionated by chemical processes. Radiogenic isotope tracers are most powerful when used together with other tracers: The more tracers used, the more control on mixing processes. An example of this application is to the evolution of the Earth's crust and Earth's mantle through geological time.

Lead has four stable isotopes: 204Pb, 206Pb, 207Pb, and 208Pb. Lead is created in the Earth via decay of actinide elements, primarily uranium and thorium. Lead isotope geochemistry is useful for providing isotopic dates on a variety of materials. Because the lead isotopes are created by decay of different transuranic elements, the ratios of the four lead isotopes to one another can be very useful in tracking the source of melts in igneous rocks, the source of sediments and even the origin of people via isotopic fingerprinting of their teeth, skin and bones. It has been used to date ice cores from the Arctic shelf, and provides information on the source of atmospheric lead pollution. Lead–lead isotopes has been successfully used in forensic science to fingerprint bullets, because each batch of ammunition has its own peculiar 204Pb/206Pb vs 207Pb/208Pb ratio.

Samarium–neodymium is an isotope system which can be utilised to provide a date as well as isotopic fingerprints of geological materials, and various other materials including archaeological finds (pots, ceramics). 147Sm decays to produce 143Nd with a half life of 1.06x1011 years. Dating is achieved usually by trying to produce an isochron of several minerals within a rock specimen. The initial 143Nd/144Nd ratio is determined. This initial ratio is modelled relative to CHUR (the Chondritic Uniform Reservoir), which is an approximation of the chondritic material which formed the solar system. CHUR was determined by analysing chondrite and achondrite meteorites. The difference in the ratio of the sample relative to CHUR can give information on a model age of extraction from the mantle (for which an assumed evolution has been calculated relative to CHUR) and to whether this was extracted from a granitic source (depleted in radiogenic Nd), the mantle, or an enriched source.

Rhenium and osmium are siderophile elements which are present at very low abundances in the crust. Rhenium undergoes radioactive decay to produce osmium. The ratio of non radiogenic osmium to radiogenic osmium throughout time varies. Rhenium prefers to enter sulfides more readily than osmium. Hence, during melting of the mantle, rhenium is stripped out, and prevents the osmium–osmium ratio from changing appreciably. This locks in an initial osmium ratio of the sample at the time of the melting event. Osmium–osmium initial ratios are used to determine the source characteristic and age of mantle melting events.

Natural isotopic variations amongst the noble gases result from both radiogenic and nucleogenic production processes. Because of their unique properties, it is useful to distinguish them from the conventional radiogenic isotope systems described above.

Isotopes in the decay chains of actinides are unique amongst radiogenic isotopes because they are both radiogenic and radioactive. Because their abundances are normally quoted as activity ratios rather than atomic ratios, they are best considered separately from the other radiogenic isotope systems.

Uranium is well mixed in the ocean, and its decay produces 231Pa and 230Th at a constant activity ratio (0.093). The decay products are rapidly removed by adsorption on settling particles, but not at equal rates. 231Pa has a residence equivalent to the residence time of deep water in the Atlantic basin (around 1000 yrs) but 230Th is removed more rapidly (centuries). Thermohaline circulation effectively exports 231Pa from the Atlantic into the Southern Ocean, while most of the 230Th remains in Atlantic sediments. As a result, there is a relationship between 231Pa/230Th in Atlantic sediments and the rate of overturning: faster overturning produces lower sediment 231Pa/230Th ratio, while slower overturning increases this ratio. The combination of δ13C and 231Pa/230Th can therefore provide a more complete insight into past circulation changes.

Allègre C. J., 2008. Isotope Geology (Cambridge University Press). Dickin A. P., 2005. Radiogenic Isotope Geology (Cambridge University Press). Faure G., Mensing T. M. (2004), Isotopes: Principles and Applications (John Wiley & Sons). Hoefs J., 2004. Stable Isotope Geochemistry (Springer Verlag). Sharp Z., 2006. Principles of Stable Isotope Geochemistry (Prentice Hall).