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Department of Meteorology, University of Reading, 2 Earley Gate, Whiteknights, Reading RG6 2AU, UK
1 Postgraduate Research Institute for Sedimentology, The University, Whiteknights, Reading RG6 2AB, UK
Although largely circumstantial in character, evidence for orbitally-forced (Milankovitch) climate changes in Jurassic microrhythmic successions, such as those of the Lias and Kimmeridgian, is becoming more persuasive. We present here the results of experiments, using a general circulation model, testing ways in which orbitally-induced variations in solar energy might be translated into a Jurassic climate response. In particular, we address the problem of the 100 ka (eccentricity-forced) cycle. This is generally considered to have only a small direct effect on solar input. It would be expected to have little impact on an ice-free Earth (commonly assumed for the Jurassic). Nonetheless, this weak signal is claimed to have been recognized in many Jurassic successions. Our results simulate, for the Late Jurassic, the possible effects at the ‘minimum’ and ‘maximum’ extremes of seasonal forcing (i.e. comparable with those affecting the Earth at 115 and 9 ka BP, respectively). Model predictions are critically evaluated against the geological database. At times of ‘minimum seasonal forcing’ there is a significant expansion in the area of the Northern hemisphere monsoon. In the tropics, changes in precipitation predominate over changes in temperature, whereas at high southern latitudes there are very large seasonal variations in temperature, and heavy winter snows. During these times the model is close to predicting a modest, but significant, Jurassic ice-cap in the Antarctic. Ice build-up is particular larly likely over uplands. Such an ice-cap disappears at times of ‘maximum seasonal forcing’. Waxing and waning of ice may thus provide the elusive mechanism for metre-scale sea-level changes. It is argued that apparently similar microrhythms (e.g. limestone-shale) might be the sedimentary response to different climatic signals in different climate zones, cause and effect being exactly the opposite in some circumstances.
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