reference is used for paleoclimate, while VSMOW is used for most other applications. Oxygen isotopes appear in anomalous ratios in atmospheric ozone, resulting from mass-independent fractionation. Isotope ratios in fossilized foraminifera have been used to deduce the temperature of ancient seas. Sulfur 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. Variations in sulfur isotope ratios are used to study the origin of sulfur in an orebody and the temperature of formation of sulfur–bearing minerals as well as a biosignature that can reveal presence of sulfate reducing microbes. Radiogenic isotope geochemistry 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–lead isotope geochemistry 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 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