How are stable isotopes used to study environmental history?

Reconstruction of past air temperatures using hydrogen isotopes of water in ice cores.

The hydrogen and oxygen isotope ratios of water have been used to reconstruct temperature patterns. The basis for this observation is that the isotopic fractionation associated with cloud vapor to precipitation formation is temperature dependent. Therefore deep ice profiles, such as the ice in Antarctica, provide a long-term record of temperature changes. Samples from the Vostok ice core show that isotopes have oscillated during glacial-interglacial cycles. The bottom of the Vostok ice core extends back to 420,000 years before the present. When the isotope ratios in the ice core samples are converted to temperature, we see that the temperature at this location varied by 10o C between glacial and interglacial periods. By analyzing the ice records from different locations scientists are able to reconstruct the spatial patterns of temperature changes and the extent to which climate oscillations were of regional versus global impact.

Carbon isotopes in fossils and fossil soils are used to reconstruct past terrestrial ecosystems.

The differences in the carbon isotope composition of C3 and C4 plants can also be used to answer questions about past climate.  Many grasses are C4 plants, and in some ecosystems all grass species are C4.  Animals that feed on grasses are called grazers; some animals feed exclusively on grass and are called hypergrazers.  Animals that eat broad-leaved plants are browsers. Researchers have studied the isotopic composition of teeth in modern animals with a known diet, such as animals in zoos, and found that carbon isotope ratios in teeth are predictable based on the known values of C3 and C4 plants.  By examining the teeth of grazers, hypergrazers, and browsers in the fossil record, it's possible to detect changes in the diet of herbivores that are related to the influence of climate on vegetation. Animals that will feed on both C3 or C4 plants will show changes in their diet over time as changes in climate alter the proportion of grasses and other vegetation.  Other animals that cannot change their diet may go extinct and disappear from the fossil record when their food of choice becomes unavailable.


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