Paleosols in clastic sedimentary rocks: their geologic applications

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62( )M.J. KrausrEarth-Science ReÕiews 47 1999 41–70Ž Cerling, 1991, 1992 .. Cerling Ž 1991.and othersŽ e.g., Driese and Mora, 1993.have shown that levelsof CO2in the atmosphere have varied over time andhave differed significantly from the modern levelsunder which the empirical curve was constructedŽ see Section 6 .. A second potential problem is thatMAP can be underestimated if the upper part of apaleosol has been truncated. Retallack Ž 1994.hassuggested ways to avoid these two problems, and thereader is referred to his discussion.A more contentious problem is the effects ofsediment compaction, especially if the rocks haveundergone significant burial. Retallack Ž 1994.hastended to calculate depth to the carbonate horizonafter decompacting the strata using the compactioncurves of Baldwin and Butler Ž 1985 .. Yet, severalstudies suggest that these curves are not appropriatefor paleosols and, in fact, that many paleosols arelittle affected by compaction. In an innovative study,Caudill et al. Ž 1996.used vertical micro-cracks inPaleozoic Vertisols to show that the paleosols underwentno more than 10% compaction despite thedepth of burial. Caudill et al. concluded that, becauseVertisols have initial bulk densities that are quitehigh, they undergo little compaction with burial. Intheir case, they calculated 650 mm of precipitationon the basis of depth to the carbonate horizon in theVertisols, which is comparable to the precipitationunder which modern Vertisols form. Evidence fromboth modern and ancient floodplain deposits alsoindicates that, because paleosols require surface exposure,they underwent significant dewatering andcompaction prior to burial Ž Nadon and Issler, 1997 ..These studies suggest that depth to the calcic horizoncan be used directly to estimate MAP as long asproblems with truncation and paleoatmospheric CO 2can be reconciled.5.3. Isotopic analysisFor a thorough discussion of the factors controllingthe isotopic composition of soil minerals and theproblems encountered in interpreting isotopic compositions,the reader is referred to Cerling and Quade18Ž 1993 .. In brief, the d O value of soil carbonatedepends on the isotopic composition of the meteoricwaters from which it precipitated. The isotopic compositionof the meteoric waters is, in turn, influencedby the mean annual temperature Ž MAT. ŽCerling,1984; Cerling and Quade, 1993 .. Furthermore, thecarbon isotopic composition of modern soil carbonatestends to be higher in warmer areas because thekind of vegetation Ž C vs. C plants.3 4 is temperaturedependent. C 3 plants, which include most trees,shrubs, and cool-season grasses, produce soil carbonated 13 C values around y27% whereas C 4 plants,which include most of the summer grasses andsedges, produce a value around y12%. On the basisof studies that show a major change from C 3 to C 4vegetation between 8 and 7 Ma ŽCerling, 1992;Cerling et al., 1993; Quade et al., 1994, 1995; Latorreet al., 1997 ., most paleoclimatic interpretationsof pre-Miocene paleosols assume that the vegetationconsisted entirely of C 3 plants. Because of theserelationships between isotopic composition and temperature,paleosol carbonates have been analyzed toreconstruct paleoclimates Že.g., Mack et al., 1991;Koch et al., 1995 .. Other soil minerals, particularlyclay minerals, are also potentially useful Že.g., Sternet al., 1997 ..Reconstructing paleoclimates using the isotopiccomposition of paleosol minerals suffers from severalproblems. First is climatic overprinting, whichcan occur if sediment accumulates very slowly andthe pedogenic carbonate precipitates under more thanone climatic regime Ž Cerling, 1984 .. In those circumstances,the carbonate will contain an isotopicmix. Second, the oxygen isotopic composition of soilminerals is more prone to diagenetic modificationthan is the carbon isotope composition. Studies ofLate Paleozoic pedogenic carbonates by Driese andMora Ž 1993.and Eocene carbonates by CerlingŽ 1991.showed little if any diagenetic alteration ofthe carbon isotopes; however, the d 18 O in both exampleswas depleted as a result of recrystallization.To understand more thoroughly the effects of diagenesison oxygen isotopes, Mora et al. Ž 1998.analyzedthe isotopic compositions of pedogenic calciteand illite in three vertic paleosols that underwentdifferent burial depths and burial temperatures. Theirresults clearly demonstrate that the oxygen isotopesin this example were affected by burial diagenesisand are not appropriate for paleoenvironmental andpaleoclimatic interpretations.Cognizant of the limitations to oxygen isotopes,Mack et al. Ž 1991.used the isotopic composition of
( )M.J. KrausrEarth-Science ReÕiews 47 1999 41–70 63Permian pedogenic carbonates to interpret a climaticchange over the 18 m.y. that the stratigraphic intervalspanned. In particular, Mack et al. modified theapproach of Cerling Ž 1984 ., which was devised forenvironments dominated by C 4 plants, to examinePermian carbonates, which formed in a C 3 plantworld. They found that d 18 O values increase up-section,from which they concluded that MAT increasedfrom 158C to308C. They also concluded that precipitationdecreased as the temperature rose. d 13 C alsoincreased up-section, an isotopic change that theauthors attributed to a decrease in plant productivity,and a change that they suggested is consistent withincreased temperatures and decreased precipitation.An important aspect of this study is that Mack et al.did not analyze the isotopes in isolation; rather theyevaluated the reliability of their isotopic conclusionsin light of paleomagnetic data, which provided thepaleolatitudinal position of the study area, and macroandmicromorphological properties of the paleosols.Koch et al. Ž 1995.developed a different approachthat is based on empirical equations, developed byFriedman and O’Neil Ž 1977 ., that relate soil temperatureto the d 18 O value of pedogenic carbonate andthe d 18 O value of the soil water from which thecarbonate precipitated. The Paleogene WillwoodFormation was used to demonstrate this method. Toestimate the d 18 O values of the Paleogene meteoricwaters, Koch et al. measured the d 18 O values ofapatite from fossil teeth Žwhich are abundant in theWillwood paleosols.and aragonite in fossil freshwaterbivalves, because the oxygen in biogenic mineralscomes primarily from meteoric waters. Then, theycombined the estimated d 18 O values of meteoricwaters with measured d 18 O values of pedogeniccarbonates from the same stratigraphic levels to determineancient soil temperatures. Because BradyŽ 1990.showed that, at depths more than about 30 cmin the soil profile, air temperatures are similar to soiltemperatures, the calculated soil temperatures wereused to estimate MAT. The MAT values rangedbetween about 08C and 358C, temperatures that arereasonable when compared to MAT values estimatedfor the Willwood Formation on the basis of plantfossils. The authors, however, acknowledged that theresults were mixed and that this technique suffersfrom significant problems, which they discussed indetail. Because the Willwood Formation accumu-lated rapidly Ž e.g., Kraus and Bown, 1993 ., climaticoverprinting should not have been a problem. Moreproblematic with the Willwood paleosols is diageneticoverprinting because, as mentioned above, CerlingŽ 1991.found the Willwood carbonates to bedepleted in d 18 O due to recrystallization.Because pedogenic carbonate are not found in allsoils, Stern et al. Ž 1997.explored the possibility ofreconstructing paleoclimates using d 18 O values fromclay minerals, which, like carbonates, have beenshown to reflect the isotopic composition of meteoricwaters Ž see Refs. in Stern et al., 1997 .. They measuredthe isotopic compositions of smectite andkaolinite from strata deposited between ;12 Maand 2 Ma as part of the Siwalik succession inPakistan. When plotted against time, d 18 O values forsmectite showed similar trends to those obtainedfrom pedogenic carbonates in the same strata byQuade et al. Ž 1989 .. Both data sets show increases ind 18 Oat;7.5 Ma, and Stern et al. attributed theisotopic shift to a climatic change, probably eitherthe result of increased evaporation caused by drierclimates or the development of a rainshadow causedby uplift of the Tibetan plateau. In contrast, thekaolinite isotopes showed no significant trend upwardthrough the section, and the authors suggestedthis may reflect contamination with detrital kaolinite.Nonetheless, their results indicate that the isotopiccomposition of pedogenic clay minerals shows considerablepromise for paleoclimatic reconstructionwith the same cautions that apply to pedogenic carbonate.5.4. LimitationsSome workers have put forth caveats to the interpretationof paleoclimates from paleosols. The majorcaution is the impact of hydrologic conditions in thedepositional area. In a study of Paleogene alluvialrocks in Portugal, Pimentel et al. Ž 1996.found earlydiagenetic groundwater alteration features that mimichydromorphic soil features such as pseudogley mottlingand calcic horizons. Pimentel et al. providedcriteria for distinguishing between the diagenetic andpedogenic features; however, they cautioned that thediagenetic features could be mistaken for pedogenicfeatures and lead to incorrect paleoclimatic interpretations.The features do not have climatic significancein this case.
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Paleosols in clastic sedimentary rocks: their geologic applications

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