Paleoclimate of the Kimmeridgian/Tithonian (Late Jurassic) world: II. Sensitivity tests comparing three different paleotopographic settings

Moore, G.T., Sloan, L.C., Hayashida, D.N. and Umrigar, N.P., 1992. Paleoclimate of the Kimmeridgian/Tithonian (Late Jurassic) world: II. Sensitivity tests comparing three different paleotopographic settings. Palaeogeogr,, Palaeoclimatol., Palaeoecol., 95: 229-252. Topography and location of continents largely determine present-day climate. We conclude that in the geologic past paleotopographic expression was equally important. However, in the geologic record paleotopography is difficult to assess and compile because it is largely a self-destructive environment without record and is rarely addressed in the literature. An objective of this study is to test the sensitivity of paleoclimate to paleotopography by comparing three different Late Jurassic scenarios. Paleotopography influences many paleoclimate parameters to varying degrees. To test the sensitivity of paleoclimate to modeled paleotopography for simulations incorporating a Late Jurassic reconstruction, we ran three simulations using the same boundary conditions of paleogeography (land and ocean) and atmospheric CO2 concentration, 4x the pre-lndustrial level (1120 ppm). One simulation contained mountain ranges of variable height to 3 kin. In another, all the mountain ranges were reduced to 1 km highlands. The third simulation used a constant 500 m height for all land grid cells. The sensitivity tests indicate that paleotopography is an important boundary condition. Without realistic paleotopography, flat or idealized continents surrounded by bodies of water do not produce realistic pateoclimate results. We conclude that in paleogeographic reconstructions the location and extent of mountain ranges and highlands should be recognized and given suitable elevations consistent with their plate tectonic origins, settings, and geologic ages. Introduction The Late Jurassic, part icularly the Kimmeridgian and Ti thonian (154.7-145.6 Ma) (Har land et al., 1990), were significant stages o f the Mesozoic Era for several reasons. First, the time represented the impor tan t interval o f Pangea ' s tectonic disintegration by the format ion o f new and the reestablishment o f ancient seaway connect ions (Fig. 1). N o r t h America completed its separat ion f rom G o n d w a n a , and G o n d w a n a was split into a nor thern and southern cont inent by the rift system opening the p ro to Ind ian Ocean. By the late Correspondence to: G.T. Moore, Chevron Oil Field Research Company, La Habra, California. Ti thonian, two connections were established between the Tethys Sea and the Panthalassa Ocean (Ross et al., 1992). Second, the Triassic Jurassic high sea level stand occurs within this interval. Third, Upper Jurassic rocks contain immense, economically impor tan t deposits including petroleum source rocks, reserves o f oil and gas, coal, and evaporites. Finally, paleoclimate modeling o f this epoch has not been at tempted previously with a general circulation model (GCM). Modeling o f this interval and correlation with paleoclimatically-sensitive sedimentary rocks confirms that the Late Jurassic was a time of high CO2 (Budyko et al., 1985; Berner, 1990) and an elevated greenhouse effect (Hallam, 1985; Moore et al., 1992). In this study, we used l l 2 0 p p m CO2, 4 × 0031-0182/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved. 230 G . T . M O O R E E T A L .