PREDICTING THE OCCURRENCE OF PALEOSOILS USING CLIMATE MODELING (FOAM)

Because soils form under ambient climatic conditions, they are one of the best indicators of past climates. Though the factors that control soil formation are complex (Retallack, 1990), the two principal variables that determine soil type are mean annual temperature (MAT) and annual amount of precipitation. Seasonal variation in temperature and precipitation is also important. There are several modern soil classification schemes. We have chosen the scheme proposed by Zobler (1986) which is based on the FAO-UNESCO (1974) soil classification scheme. We have constructed a model that predicts the kind of soil that would be expected to form under varying temperature and precipitation conditions. For example areas with mean annual temperatures between 21°C and 24°C, and average annual rainfall of less than 1 mm/month, would most like have a Yermosol-Solonchak soil type. Whereas, Ferralsol-Nitosol type soils are predicted to develop in areas with mean annual temperatures between 22°C and 26°C, and average annual rainfall between 12 - 25 mm/month.

Using this bivariate description of soil types we have constructed “climate envelops” that characterize 10 modern soil types (Acrisol, Cambisol-Gleysol, Chernozem-Podzoluvisol-Greyzem, Ferralsol-Nitosol, Phaeozem, Kastanozem-Solonetz, Histosol-Podzol, Planosol, Vertisol and Yermosol-Solonchak). Because the climatic envelop technique (Moore, 2001) accurately predicts the distribution of modern soils, we are confident that it can be used to predict the paleogeographic distribution of ancient soil types. These predictions can be tested by comparing the predictions made by our model with the distribution of paleosols (e.g., calcisol, gypsisol, gleysol, argillisol, protosol) and other lithologic indicators of climate such as coals, evaporites, bauxites, calcretes, etc.

Predictions of the paleogeographic occurrence of paleosoils have been made for 14 time intervals during the Phanerozoic: late Miocene, Oligocene, late Eocene, K/T boundary, Cenomanian/Turonian, early Aptian, earliest Cretaceous, late Jurassic, early Jurassic, Late Triassic, Permo-Triassic, early Permian, Mississippian, and Late Devonian . We will present paleoreconstructions showing the predicted distribution of paleosoils since the Late Devonian.