Paleoclimatic Reconstructions and Qualitative Models
Paleoclimatic indicators, including the economic deposits of interest to this course, range from the very specific to the very general. Latentes, for example, seem to indicate very specific conditions of temperature and precipitation. In contrast, biogeographic patterns are very general indicators, which usually cannot reveal specific climatic parameters but which are excellent for understanding gradients, particularly in temperature. Only a few quantitative methods exist for empirically determining paleoclimates. They are stable isotopes of oxygen (e.g., Savin, 1977), angiosperm leaf morphology (e.g., Wolfe, 1979), and biogeographical patterns in very young rocks (e.g., CLIMAP, 1976). Although certain other indicators, such as latentes, are thought to have very specific climatic requirements, they are so sparsely distributed in the rock record as to be of limited use in paleoclimatic recontructions.
The biogeographic patterns that can be used to quantitatively determine ocean temperature are those on which transfer functions have been performed. The use of transfer functions is a numerical method that infers the temperature requirements of ancient biotic assemblages from the degree of taxonomic similarity to modern assemblages, whose temperature requirements are known (Sachs et al., 1977). The problem is that, as fewer modern forms are found in the successively older sediments, the less reliable is the paleotemperature determination. Therefore, although the technique may be useful for the late
Pleistocene (CLIMAP, 1976), it is virtually useless for older rocks. An informal version of this approach is to use individual taxa, for example, crocodilians, as paleoclimatic indicators (Colbert, 1964). However, this technique presupposes similar environmental
Figures & Tables
Burgeoning interest in paleoclimatology has been spurred by growing awareness of the control of paleoclimates on the formation of economic deposits. In past studies, paleoclimatic patterns were derived empirically from biogeographic patterns, and to a lesser extent, from the distributions of sedimentary paleoclimatic indicators, such as coals. The problems with this approach are numerous. In early studies, the paleoclimatic patterns appeared to make very little sense because they were reconstructed on modern continental positions. Even after the acceptance of continental drift, problems arose when the paleoclimatic indicators were poorly dated or when geologists chose paleoclimatic indicators from too long a time period, during which major paleoclimatic changes could have occurred. More recently, qualitative and quantitative models of paleoclimate have proved useful for understanding the distributions of climatically significant geologic data. These models are founded on basic principles of atmospheric and oceanic circulation as applied to global paleogeography, including reconstructed plate positions. With climate models, geologists can formulate hypotheses about the paleoclimatic patterns that might be expected during the various intervals in Earth history and test those hypotheses with the paleoclimatic indicators.