MOORE, GEORGE T., Earth System Science Center, The Pennsylvania State University, University Park, PA; ERIC J. BARRON, Earth System Science Center, The Pennsylvania State University, University Park, PA; and DARRYL N. HAYASHIDA, Chevron Petroleum Technology Company, La Habra, CA

ABSTRACT: Testing GCM Model Results by Comparing Different Greenhouse Conditions of Late Jurassic Simulations with the Geologic Record

General Circulation Models (GCM) are designed to replicate the present climatic conditions on Earth and they simulate it very well. The fact that GCMs must be "tuned" to accomplish this indicates that any GCM results for times and conditions, other than those specified for the present-day, should be treated with some degree of caution. On the other hand, using the geologic record as a framework to test paleoclimate model simulations for discrete time intervals has merit. Such experiments and sensitivity tests have been completed successfully in the Eocene, mid-Cretaceous, Late Jurassic, and mid-Silurian. Our purpose here is to present the Late Jurassic example.

The Late Jurassic (Kimmeridgian; 154.7-152.1 my) was a time of expanded continental rifting, increased seafloor spreading, and rising eustatic sea level. From the Early Jurassic onward, these processes collectively caused the fragmentation and flooding of the megacontinent Pangea as well as a gradual amelioration of the global paleoclimate. The Kimmeridgian likewise contains extensive source rocks that contributed significantly to the world's petroleum reserves. For both geologic and economic reasons, the Late Jurassic is an important epoch in which to model paleoclimate and test the validity of the model results.

Using Chevron's GCM, we report two Late Jurassic paleoclimate seasonal simulations with different concentrations of carbon dioxide, 280 ppm (pre-Industrial Revolution) and 1120 ppm, 4X that value, respectively. Simulation of a warmer planet with an elevated greenhouse effect (1120 ppm carbon dioxide) fits the known distribution of paleoclimatically sensitive faunas, floras, and sedimentary rocks for this stage.

While these numerical GCMs are still primitive, given to certain simplistic parameterizations, and limited spacial resolution, they provide a method of evaluating greenhouse conditions in the past by using the preserved geologic record. Studies to date indicate that the greenhouse conditions have varied widely throughout the Phanerozoic Eon. The biosphere has survived these changes, albeit not without certain disruptions and extinction events. Futuristic studies into global warming need to consider these documented sensitivity tests.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.