Paleosols in clastic sedimentary rocks: their geologic applications

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64( )M.J. KrausrEarth-Science ReÕiews 47 1999 41–70Although Caudill et al. Ž 1996.concluded thatcalcareous Vertisols might provide an excellent toolfor estimating ancient MAP, Aslan and Autin Ž 1998.have suggested that caution should be used whenestimating paleo-precipitation from the depth of calcichorizons, even in Vertisols. They observed calciczones Ž Bk and Ck horizons.in Mississippi Riverfloodplain soils, many of which are Vertisols. Applyingthe Retallack equation to both natural levee soilsand backswamp soils, they calculated MAP between545 and 908 mm, figures that seriously underestimatethe actual MAP of 1500 mm. Aslan and Autinconcluded that the position of the calcite was notcontrolled by the climate. Rather the depth to calcitein these soils is controlled by water-table levels andthat calcite precipitates from ascending, not descending,groundwaters during the wet season. Consequently,they suggested that paleosols that formed inaggradational floodplain settings are more likely topreserve information on floodplain sedimentation andpaleohydrology than on paleoclimates.A similar concern about carbonates formed fromascending vs. descending waters was sounded bySlate et al. Ž 1996 .. Those authors compared andcontrasted the morphology and isotopic compositionsof carbonates that formed in well-drained paleosolsand hydromorphic paleosols. They emphasized thatthe Cerling model should be restricted to soil carbonatethat formed in the vadose zone because only hereare all oxidized carbon species in isotopic equilibrium.Carbonates from hydromorphic paleosols areunsuitable for isotopic analysis because ascendingground waters probably contaminated their isotopiccomposition. This paper is also important because itprovides criteria for distinguishing between soil carbonatesthat formed in well-drained paleosols fromthose that formed in hydromorphic paleosols. Similarly,Pimentel et al. Ž 1996.concluded that carbonatesin Paleogene alluvial rocks were the product ofgroundwater diagenesis, although the carbonates resembledthose resulting from pedogenesis. The authorswarned that such carbonates have no particularclimatic interpretation, nor should they be used toestimate sediment accumulation rates on the basis oftheir degree of development.Finally, depending on the nature of the exposureand amount of data available, it may be difficult todistinguish climatic controls from other mechanismsthat can produce vertical changes in paleosol properties.The uncertainty that may exist is demonstratedby a study of Lower Carboniferous paleosols byWright et al. Ž 1991 .. An upward change from calcrete-bearingVertisols to paleosols interpreted asferrolytic Vertisols indicates a change in soil moisturefrom better drained to more poorly drainedconditions. The ferrolytic paleosols also contain anunusual clay assemblage that was attributed to intenseleaching. The authors suggested that either achange in local drainage conditions Žan intrinsicmechanism.or a climate change to more prolongedprecipitation could have been responsible. Althoughthey favored the climatic interpretation, they admittedthat this interpretation was tenuous because itwas based on only two paleosol profiles from onlytwo localities.6. Interpreting ancient atmospheres6.1. Isotopic analysisCerling Ž 1991, 1992.provided a model for estimatingancient atmospheric rŽ CO .2 by showing thatthe isotopic composition of soil carbonate is directlyrelated to soil CO 2, which depends on the concentrationof CO2in the atmosphere. As noted above, thecarbon isotopic composition of soil carbonates isalso sensitive to the kind of vegetation ŽC 3 vs. C 4plants .. The model assumes that the pre-Miocenevegetation consisted entirely of C 3 plants, becausevarious isotopic studies ŽCerling, 1992; Cerling etal., 1993; Quade et al., 1994, 1995; Latorre et al.,1997.have shown a major change from C 3 to C 4vegetation between 8 and 7 Ma. Also of importancefor paleosols studies is the depth of the pedogeniccarbonate because d 13 CO2values become more negativedownward in a soil profile to a depth of ;20cm.Using paleosol carbonates and the Cerling model,atmospheric CO2values have been estimated forvarious times in the Phanerozoic Ž Fig. 12 .. In a studyof pedogenic carbonates of Late Silurian throughPennsylvanian age, Mora et al. Ž 1996.concluded thatatmospheric CO2levels were high for the Late SilurianŽ 3200–5200 ppm V ., then steadily declined
( )M.J. KrausrEarth-Science ReÕiews 47 1999 41–70 65Fig. 12. Changes in atmospheric CO2based on isotopic analyses of paleosol carbonates. Paleozoic values are ranges from Mora et al.Ž 1996 .; Late Cretaceous value from Ghosh et al. Ž 1995 .; other values from Cerling et al. Ž 1992 ..through the Paleozoic Ž700–2050 ppm V for theDevonian; 450–1000 ppm V for the Mississippian;450–800 ppm V for the Pennsylvanian .. By Permiantime, atmospheric CO2levels dropped to 150–200ppm V Ž Mora et al., 1996 .. Retallack Ž 1997a.interpreteda similar drop in atmospheric CO2frompaleosols. This significant decline was linked to theexpansion of land plants and to the global climatechange that produced extensive late Paleozoic glaciationŽ Mora et al., 1996 ..Cerling Ž 1991.determined that atmosphericrŽ CO .2 then rose during the Late Triassic to EarlyJurassic Ž 2000–3000 ppm V .. Following high EarlyCretaceous atmospheric CO Ž 2500–3300 ppm V .2,values then fell through the Cenozoic. In a study ofpedogenic carbonates from central India, Ghosh etal. Ž 1995. concluded that rŽ CO .2 in the Late Cretaceousatmosphere was 800–1200 ppm V. Levels fellfurther in the Cenozoic with 600 ppm V estimatedfrom Eocene paleosols, and 400–700 ppm V determinedfrom Miocene paleosols Ž Cerling, 1991 .. Thecurrent value is approximately 300 ppm V.6.2. LimitationsTo yield reliable pŽ CO .2 estimates, the carbonatecollected from paleosols must be of indisputablepedogenic origin and cannot have undergone postpedogenicmodification. Wright and Vanstone Ž 1991.emphasized that groundwater carbonates pose a potentialproblem to this method. Groundwater carbonateshave different isotopic compositions than overlyingpedogenic carbonates but they can be difficultto distinguish from true pedogenic carbonates. Anotherproblem associated with groundwaters is that,with continued sedimentation, a soil becomes buriedand moved below the water table. Consequently,carbonate that first formed in the rooted zone can beoverprinted by groundwater precipitation. Burial diagenesisis also a potential problem, although studiesof lower Eocene paleosols Ž Cerling, 1991.and Devonianpedogenic carbonates Ž Driese and Mora, 1993.both show that the d 13 C of paleosol carbonates wasonly minimally affected by diagenesis.The particular soil environment in which the carbonateprecipitated can also affect its isotopic compositionand, thus, the pŽ CO .2 value it yields. In astudy of Devonian Vertisols, Driese and Mora Ž 1993.examined carbonate from two different sources inthe paleosols: rhizoliths and pedogenic nodules. Theyfound that the nodule carbonates were isotopicallyheavier than the rhizolith carbonates. The authorsattributed this to the depth at which the carbonatesprecipitated. The nodules precipitated in the zone ofsoil cracking, and, because Vertisols commonly developcracks of 1 m in depth, atmospheric CO2mayhave penetrated deep into the developing soil, resultingin nodules with isotopic values that overestimateatmospheric CO 2. The rhizoliths formed deeper inthe soil profile and apparently below the zone ofcracking. Consequently, the authors concluded thatthey provided a more accurate estimate of paleoatmosphericCO2than the nodules.Finally, as noted above, the proportion of C 3 orC vegetation influences the d 13 4C values of paleosolcarbonates. The Cerling model Ž 1991, 1992.assumesthat C 4 vegetation did not appear until late Miocenetime. This assumption has been questioned by Wright
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Paleosols in clastic sedimentary rocks: their geologic applications

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