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Geological climate proxies can be any geological phenomena that provides information about the contemporary climate. They include fossil faunas and floras, sediments such as soils and evaporites, geochemistry (e.g. oxygen isotopes), stratigraphic architecture (e.g. cyclicity). For any climate proxy to be of use it's position in climate space (the multidimensional space delimited by climate variables) must be determined. With the advent of more robust climate models (GCMs) the need for quantitative descriptions of climate proxies has become essential, since descriptions such as "arid" or "wet" provide only the most basic qualitative information.
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LEFT: Climate space represents the multi-dimensional space within which physical and biological climate proxies exist. In this example only two axes are shown.
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Like petrological phase space, the boundaries in climate space for climate proxies can be determined by experiment. For faunal and floral proxies such experiments are rarely pursued since any move outside of their climate space results, by definition, in death. In my thesis and the papers that followed (Markwick, 1996, 1998) I relied on a different approach, which used the modern distribution of each taxonomic group or assemblage to define at least part of the climate space. The assumption was that the distribution of these taxa was more or less in equilibrium with their environment and that the occurrence of a taxon did represent the presence of a viable, re-producing population. By intersecting these distributions with modern climate information (the last 30 years at least), I could define a basic climate space for each group.
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| RIGHT: The modern distribution of climate proxies (crocodilians in this example) used to help define climate space.
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However, while this technique was able to define where a taxon could exist (it's presence), the opposite was not necessarily true, since an absence today could be due to lack of other environmental factors (for crocodilians this was often standing water), or a historical biogeographical artifact (e.g. the absence of alligatorids from Africa and Australia). Thus absences had to be investigated, but given the environmental datasets available this was possible (today it is even easier using GIS). Through this process, together with reference to what direct biological observations had been made, a climate space could be constructed (with caveats).
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LEFT: The climate space (temperature only) occupied by modern crocodilians. Properly constrained this can then be used to help intepret the climatic significance of fossil crocodilian occurrences (Markwick, 1998)
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There are a wealth of different geological climate proxies available, of lesser or greater significance.
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| RIGHT: The climate space (temperature only) occupied by the giant tortoise Geochelone and palms (Markwick, 1996: PhD thesis).
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Other proxies include phosphorites, evaporites, 'red-beds', peats and coals, dune beded sandstones. Climate interpretations from all of these have to be treated with some care since no proxy is totally without caveats! The important fact is the use of multiple climate proxies.
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LEFT: Dune bedding in the Navajo Sandstone, western USA.
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The goal is to seek convergence between geological data and model results--since these models are also instrumental in predicting the potential direction and nature of future climates, whether naturally or anthropogenically induced, studies of the climate of the geological past have a direct application to present concerns.
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