--> Modelling Jurassic Milankovitch Climate Variations, by B. W. Sellwood, P. J. Valdes, and G. D. Price; #90986 (1994).

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Abstract: Modelling Jurassic Milankovitch Climate Variations

Bruce W. Sellwood, P. J. Valdes, G. D. Price

Although largely circumstantial in character, evidence for orbitally-forced (Milankovitch) climate changes in Jurassic microrhythmic successions (e.g. in the Lias and Kimmeridgian) is becoming accepted. Results of our experiments, using a General Circulation Model, are used to investigate ways in which orbitally-induced variations in solar energy might be translated into a Jurassic climate response. The 100k yr (eccentricity-forced) cycle 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 Jurassic Earth. Nonetheless, this weak signal is claimed to have been recognized in many Jurassic successions. Our results simulate, for the Kimmeridgian, the possible effects at the "Minimum" and "Maximum" extremes of easonal forcing (i.e. comparable with those affecting the Earth at 115k yrs BP and 9k yrs BP respectively). Control model predictions are robust when 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 predicts a modest, but significant, Jurassic ice-cap in the Antarctic. Ice buildup is particularly 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 m-s ale sea-level changes.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994