Regions of persistent high organic productivity (e.g., coastal upwelling regions) and environmental conditions conducive for organic matter preservation provide a setting for petroleum source bed formation. Both productivity and preservation of organic matter are strongly related to the atmospheric and oceanic circulation. Accurate predictions of ancient circulation patterns will therefore be useful as source rock predictors.

In the past, predictions of ancient circulation patterns and upwelling regions have been based on an analogy with the modern circulation. The approach is essentially one of moving the continents "beneath" the present-day atmospheric circulation. Because topography, continental positions, and sea level almost certainly modify the nature of the circulation, the utility of such a simple research approach is suspect. The most promising approach is through the use of numerical climate models based on the dynamic and thermodynamic equations thought to govern the circulation, given specific geologic boundary conditions.

A mathematical general circulation model of the atmosphere, capable of using realistic geographic surface boundary conditions, has been employed to examine the sensitivity of the circulation to changes in continental position, sea level, topography and land-sea thermal contrast based on mid-Cretaceous geography. Each experiment consisted of a single change in a boundary condition in the following order: (1) a present-day control; (2) the present-day with no topography; (3) rotation of present-day "flat" continents to mid-Cretaceous positions; (4) reduction of Cretaceous land area associated with higher sea level; (5) addition of Cretaceous topography; and (6) addition of a warm Cretaceous ocean. The results are of particular significance because they illustrate (a) the importance of g ography as a control on the nature of the circulation and (b) the potential usefulness of numerical climate modeling in petroleum source rock prediction.

(1) Experiments with no topography (flat, sea level continents) are nearly identical, exhibiting only minor differences from a "classical" pattern of atmospheric circulation. This pattern consists of an equatorial low, high pressure centered just equatorward of 30° lat., a low pressure belt centered near 60° and a polar high.

(2) Topography and land-sea thermal contrasts substantially alter the atmospheric circulation patterns in two important aspects. First, these variables reduce the zonality of the surface pressure and wind patterns, resulting in regional convergences and divergences which would not be predicted by a simple qualitative analogy with present-day patterns. Second, the mean latitudinal position of low and high pressure regions (hence easterly and westerly wind patterns) and the intensity of these features were altered by including topography and land-sea thermal contrasts. For example, the Tethyan ocean and the topography of the bordering continents result in a 10° equatorward shift of the Northern Hemisphere subtropical high. Such a shift has considerable implications for the climate f the regions bordering the Tethys ocean. Cretaceous upwelling areas may have been substantially displaced in latitude with respect to present-day patterns.

The model results illustrate the importance of geographic variables as controls on circulation patterns. These results can be compared with the geologic record and evaluated as a petroleum source rock prediction tool.

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