Quantitative paleoenvironmental and paleoclimatic reconstruction ...

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ARTICLE IN PRESS 2 N.D. Sheldon, N.J. Tabor / Earth-Science Reviews xxx (2009) xxx–xxx 5.5. Paleoprecipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 5.5.1. Content of Fe–Mn nodules in vertisols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 5.5.2. Depth to Bk horizon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 5.5.3. Bw/Bt horizon geochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 5.6. Long-term chemical weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 6. Thermodynamic approaches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 6.1. Simple versus complex systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 6.2. Single-equation approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 6.2.1. Precambrian atmospheric CO 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 6.2.2. Earliest Triassic soil formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 6.3. Multiple-equation approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7. Stable isotope approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.1. Stableisotopiccompositionofpedogenicmineralsaspaleoenvironmentalproxies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.1.1. Mineral-water isotope fractionation and the jargon of stable isotope geochemistry . . . . . . . . . . . . . . . . . . . . . . . 0 7.1.2. Stable isotope fractionation factors of common pedogenic minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.1.3. Relationship between hydrogen and oxygen isotopes in continental waters . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.2. Carbon in soils. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.2.1. One-component soil CO 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.2.2. Two-component soil CO 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.2.3. Three-component soil CO 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.3. Soil and paleosol carbonate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.3.1. Pedogenic calcite δ 18 O values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.3.2. Pedogenic siderite as a proxy for soil moisture δ 18 O values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.4. δ 13 C values of soil carbonate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.4.1. Calcite from one-component of soil CO 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.4.2. δ 13 C of pedogenic siderite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.4.3. Calcite derived from 2-component soil CO 2 mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.4.4. Soil carbonates formed by mixing of three-components of soil CO 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.5. δ 18 O and δD of hydroxylated minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.5.1. Origin of residual deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.5.2. Variations in soil moisture δ 18 O and δD values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.5.3. Single-mineral paleotemperature estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.5.4. Mineral-pair δ 18 O values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 7.6. Paleo-vegetation/paleo-photosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 8. Future approaches and challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 8.1. Boron isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 8.2. Energy balance models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 8.3. “Clumped isotope” paleothermometry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 9. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 1. Introduction Increasing recognition of paleosols (fossil soils; Fig. 1) in nonmarine strata has opened up new types of paleoenvironmental and paleoclimatic reconstructions. While good quantitative paleoclimatic data may be obtained from plant fossil assemblages using either nearest living relative (e.g., Leopold and Clay-Poole, 2001; Utescher and Mosbrugger, 2007) or leaf morphometric approaches (e.g., Wolfe, 1994; Uhl et al., 2007), those approaches yield snap shots of past environments and are relatively rare in the fossil record. Paleosols preserved in continental basins on the other hand, raise the possibility of long-term, fairly continuous paleoclimatic records, potentially with a temporal resolution that equals that of marine proxy records (e.g., Retallack et al., 2004b; Sheldon and Retallack, 2004; Retallack, 2007). The last point is crucial because unlike most marine proxies, which are fundamentally indirect climatic records, paleosols formed at the Earth's surface, in direct contact with climatic and environmental conditions that prevailed at the time of their formation. Potentially then, paleosol-based proxies could be among the most powerful tools for reconstructing past environments. Until recently, paleopedology (the study of paleosols) was largely a qualitative science that relied on the recognition of features similar to modern soils that allowed for a “nearest living relative” comparison. That is, a paleosol was identified as indicating an ancient grassland if it had features similar to a modern grassland (Fig. 2). This approach still underlies most paleopedology, but has some fundamental limitations, namely the need for taxonomic uniformitarianism. Continuing on with the grassland example, the origin of grasses is most likely in the Cenozoic (e.g., Strömberg, 2002). However, paleosol-based estimates for the origin of grasslands as evidenced by Mollisol-like paleosols range from Eocene to Miocene (Retallack, 1997a,b, 2001a,b) depending on the strictness of one's taxonomy. Furthermore, others (Terry, 2001) have described Oligocene-age paleosols as “Mollisols” without meaning to connote that a grassland ecosystem was present. On the other hand, there are some types of paleosols found in the Earth's past for which there are imperfect analogues (e.g., Retallack, 1997c; Sheldon, 2005), or for which there is no appropriate modern analogue owing to different environmental and ecological conditions in Earth's past. For example, weathering at the Earth's surface was occurring in the Precambrian (e.g., Driese, 2004; Sheldon, 2006b), but under the influence of microbial enhancement only in the absence of higher plants. The emergence of new quantitative techniques for reconstructing various paleoenvironmental and paleoclimatic conditions using whole rock and isotopic geochemistry has fundamentally changed the field of paleopedology. Proxies for variables ranging from levissage to mean annual precipitation to the composition of the paleoatmosphere at the time of the paleosols' formation have now been developed. Some of the approaches that we describe in this review have been widely applied, others have not but could have wider applicability. We also attempt to “gaze into the crystal ball” to evaluate Please cite this article as: Sheldon, N.D., Tabor, N.J., Quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols, Earth- Science Reviews (2009), doi:10.1016/j.earscirev.2009.03.004
ARTICLE IN PRESS N.D. Sheldon, N.J. Tabor / Earth-Science Reviews xxx (2009) xxx–xxx 3 Fig. 1. Photos of representative paleosols. A) Painted Hills Unit, Eocene, Oregon. Dominated by Alfisol-like paleosols. B) Mascall Formation overlook, Miocene, Oregon. Near-mollic Inceptisol-like paleosols. C) Classic “red beds” from the Permian Cala del Vino Formation, Sardinia. D) Close-up of root mat carbonate rhizoliths, Renova Group, Montana. some future directions and some ways in which existing proxies may be extended. In general, no single proxy for weathering processes or past environmental conditions, or single type of quantitative data is without limitations. Therefore, a multi-proxy approach is key both to giving the investigator the widest possible range of information, but also to ensure reproducibility of her/his results. Thus, the approach Fig. 2. Comparison between modern soil and paleosol equivalent. A) Modern xeric Mollisol, southwestern Montana; B) late Miocene xeric Mollisol, central Oregon. Please cite this article as: Sheldon, N.D., Tabor, N.J., Quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols, Earth- Science Reviews (2009), doi:10.1016/j.earscirev.2009.03.004
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