changes in recent wet and dry periods—lets scientists draw inferences about the past, and these records come to be considered “proxies,” or indicators of the past environment.

The assumption of constancy of the relation between climate and its proxy might require little more to support it than constancy of physical law (for example, the assumption that in the past heat flowed from warm to cold rocks in the same way as today). Other assumptions might involve greater uncertainty (for example, the assumption that under different climatic conditions, marine organisms grew most vigorously during the same season and at the same water depth as in the modern environment). Testing of the underlying assumption that the present is the key to the past relies largely on the consistency of results from a wide array of proxies, particularly those depending on few assumptions. The use of multiple indicators increases the reliability of many paleoclimate reconstructions.

The following pages provide a brief synopsis of paleoclimate proxies (Table 2.1) and age indicators. The description is not exhaustive and is intended only to orient the reader to some of the current paleoclimatic tools available. For more detailed reviews of methods involved in paleoclimatic interpretation see Broecker (1995), Bradley (1999), or Cronin (1999).

Physical paleoclimatic indicators often rely on the fewest assumptions and so can be interpreted most directly. For example, old air extracted from bubbles in ice cores and old water from pore spaces in seabed sediments or continental rocks provide direct indications of past compositions of atmosphere, oceans, and groundwater (see Plate 1). Anomalously cold buried rocks or ice have not finished warming from the ice age and thus provide evidence that conditions were colder in the past. Conditions are also judged from the concentrations of noble gases found dissolved in old groundwaters. Some such records are subject to substantial loss of information through diffusion of the components being analyzed, which limits the ability to interpret older events. Physical indicators include the characteristics of sediments and land features. For example, the presence of sand dunes can indicate past arid conditions, and glacially polished bedrock is an indication of prior glacial conditions.

Isotopic indicators are widely used in paleoclimate science. The subtle differences in behavior between chemically similar atoms having different weights (isotopes) prove to be sensitive indicators of paleoenvironmental conditions. One common application is paleothermometry. The physical and chemical discrimination of atoms of differing isotopic mass increases with decreasing temperature. For example, carbonate shells grow-

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