Hydrologic patterns are imprinted in the geologic record and play a prominent role in the investigations of Pangean climate. However, the hydrologic aspects of climate are complex (1) many variables are required to analyze hydrology, (2) hydrologic processes act on spatial scales that are smaller than GCM (General Circulation Model) grid spacing, and (3) observational data bases for calibrating modern climate simulations are few. This study, which uses a GCM to depict arid climates of the past and present, is designed to evaluate a variety of simulated hydrologic variables that are becoming increasingly available for paleoclimate model/data comparison studies. Simulations of the current climate show that there are quantifiable levels of precipitation (P), soil moisture, and surface runoff that delineate the locations of modern deserts. However, in a Pangean simulation, using the same model, the threshold values of these variables reveal disparate views of desert extent. Altered boundary conditions and atmospheric circulation can invalidate thresholds based on modern climatology. The credibility of these variables thus rests heavily on the accuracy of reconstructed boundary conditions. A more fundamental meteorological approach defines deserts as regions where the atmospheric demand for moisture (potential evapotranspiration, E p ) exceeds the supply (precipitation, P). GCM values of E p are overestimated; thus simulated P–E p fields are unusable. However, simulated Ep values can be derived independently using GCM-produced temperature values in empirically derived equations (E T p ). Similar methods, using observed temperatures, are employed by atmospheric scientists to construct global and regional E p data sets. The results of this study indicate that P–E T p is more accurate than other individual variables for calculating moisture balance and is more useful for paleoclimate model/data comparisons because of its fundamental meteorological basis and lack of dependence on prescribed vegetation and soil conditions. Ultimately, hydrological misinterpretations can be avoided by understanding the limitations of GCM parameterizations and by strictly cross-checking variables with each other and with available paleoclimate data.

Abstract In this chapter the principles developed in Chapter 9 are applied to a unique geometric set of lakes, more specifically a regional Early Cretaceous system of meridionally-oriented rift lakes. Northern Gondwana existed as a separate but disintegrating, compound continent for no more than about 50 my, from around the Jurassic-Cretaceous boundary to the mid-Cretaceous. By the Cenomanian (early Late Cretaceous), the continents of South American and Africa were completely separated from one another and the Atlantic became a continuous ocean. This case history is devoted to examining a meridionally-oriented string of lakes that formed in the interior of Northern Gondwana during the rifting phase. The focus will be on the African margin from the Walvis Ridge northward to the present-day Niger Delta, a distance which spans a latitude of about 22° (2400 km). An isopach map showing the distribution of syn-and post-breakup sediments on the Atlantic margins of both continents is given in Figure 14.1. This Early Cretaceous rifting event created a series of parallel, elongate depressions in which paleoclimatic conditions favored the development of lakes (Gerrard and Smith, 1982; Ojeda, 1982; Asmus and Baisch, 1983; Reyre, 1984; McHargue, 1990). The lakes occupied what became the present-data Atlantic margin of South America and Africa. They range in age from Neocomian (Berriasian, Valanginian, Hauterivian) to Barremian (Early Cretaceous) 145.5 to 124.5 my (Harland and others, 1990). Most contain organic-rich lacustrine source rocks which vary regionally in richness and kerogen content. The basal rocks in each basin are composed of siliciclastics with

Abstract This chapter deals with interpreting stratigraphy from a suite of paleoclimate maps generated from a global paleoclimate simulation and comparing the results with the geologic record. The CCM results can be viewed globally or, by focussing on a particular set of latitude/longitude coordinates, regionally. In this chapter we utilized the principles developed thus far in this short course to focus on a region occupied by a seaway, the western part of the Tethys Sea in the Kimmeridgian stage (154.7-152.1 Ma; Harland and others, 1990). The paleolatitudes range from 5°S to 35°N and the paleolongitudes from 50°W to 30ºE. It is important to remember that while we are examining only a small area of the simulation, the detail and accuracy are not enhanced from the global results. The surface grid still is approximately 4.5° latitude by 7.5° longitude.