Paleosols in clastic sedimentary rocks: their geologic applications

60( )M.J. KrausrEarth-Science ReÕiews 47 1999 41–70temperature Ž MAT .. Ancient climatic conditions canbe interpreted by classifying the paleosols and usingmodern analogs to infer the paleoclimatic regime orby identifying particular pedogenic properties thatmodern studies show to have climatic significance.Stable carbon and oxygen isotopes are also used tointerpret ancient climate and some effort has beenmade to use isotopic composition to estimate MATŽ e.g., Mack et al., 1991; Koch et al., 1995 .. Finally,the depth at which calcic horizons originally formedhas been used to estimate paleo-precipitation. Thefollowing sections provide overviews of these approachesas well as their limitations.5.1. Modern soil analogsThe paleosol literature provides numerous examplesof the classification approach to paleoclimaticinterpretation. For example, Mack Ž 1992.used alluvialpaleosols to interpret a climate change across theLower Cretaceous–Upper Cretaceous boundary inNew Mexico. Older red, calcic paleosols, interpretedas Aridisols, were attributed to semi-arid or aridconditions. These are overlain by Inceptisols andAlfisols that have somber colors and lack carbonate,suggesting a subhumid or humid climate. Similarly,Bestland Ž 1997.used paleosols to interpret a climatechange across the Eocene–Oligocene boundary. Theolder paleosols were interpreted as ‘Ultisol-like’ paleosolsthat formed under humid, subtropical conditions,whereas the younger smectitic paleosols werejudged to be ‘Alfisol-like’ and indicative of humid,temperate conditions.In a more unifying approach to paleoclimaticinterpretation, Mack and James Ž 1994.generated apaleosol analog to the Soil Map of the World Že.g.,FAO, 1974 ., which links modern soils to particularclimatic zones. Using the Mack et al. Ž 1993.paleosolclassification Ž Table 1 ., they suggested that highlyweathered paleosols, such as Oxisols and Argillisols,should be characteristic of wet equatorial paleoclimatesin which MAP and MAT are high and havelittle seasonal variation. Argillisols, Spodosols, andGleysols are more apt to form in humid ŽMAP)1000mm.midlatitude climates, whereas Calcisols are indicativeof a dry Ž MAP-1000 mm.subtropicalpaleoclimatic zone. One caveat to this global approachis that it is not appropriate for paleosols thatformed prior to the advent of vascular plants.The second approach to climate reconstruction—focusing on a particular soil property or group ofproperties—is based on Quaternary soil studies,which have shown that particular features, includingclay and carbonate accumulations and the depth ofsoil oxidation can be quantitatively related to soilformingfactors, including climate Že.g., Bockheim,1980; Harden and Taylor, 1983; Birkeland, 1984 ..Molecular weathering ratios Že.g., silicarsesquioxidesor basesralumina.provide a good illustration.In some cases, the ratios indicate intense leaching ofbase cations and the loss of silica, which are characteristicsof modern soils that formed in humid, tropicalclimates Že.g., Arndorff, 1993; Retallack andGerman-Heins, 1994; Gill and Yemane, 1996 .. Claymineralogy has also been used to detect and interpreta climatic change. Robert and Kennett Ž 1994 ., forexample, found that clay minerals on the Antarcticcontinent showed a dramatic increase in smectiteŽ and corresponding decrease in illite.during thelatest Paleocene. The clay change corresponds to awell-documented isotopic shift that marks the latestPaleocene thermal maximum Že.g., Zachos et al.,1993 .. Robert and Kennett concluded that the increasein smectite was the result of increased chemicalweathering as a result of warmer temperaturesand greater rainfall at that time.In some cases, the paleosols are parts of cyclothemsin which alternations of particular kinds ofpaleosols are linked to climatic cycles. Good examplesare provided by the wetrdry cycles linked topedogenically-unmodified sediment and paleosols inloess and loessite. In some, the cyclic climaticchanges are linked to either sea-level changes ortectonic activity. For example, working in LowerPermian cyclothems of the U.S. mid-continent, Milleret al. Ž 1996.described cyclic changes from verticpaleosols to Alfisols with calcic horizons and attributedthese to fluctuations between semi-aridrsubhumidconditions and humid monsoonal conditions.In a subsequent study Ž McCahon and Miller, 1997 .,these paleosolrclimate cycles were linked to glaciallycontrolled sea-level changes. The change to increasedprecipitation followed by a rapid changeback to arid conditions was attributed to gradualregression caused by glacial activity followed by a