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Sheldon--Tabor-2009.Pdf Earth-Science Reviews 95 (2009) 1–52 Contents lists available at ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev Quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols Nathan D. Sheldon a,⁎, Neil J. Tabor b a Department of Geological Sciences, University of Michigan, 2534 CC Little, 1100 N. University Ave., Ann Arbor, MI 48109, USA b Department of Earth Sciences, Southern Methodist University, P.O. Box 750395, Dallas, TX 75275, USA article info abstract Article history: Paleosols (fossil soils) are preserved throughout the geologic record in depositional settings ranging from Received 8 April 2008 alluvial systems to between basalt flows. Until recently, paleosols were studied using primarily qualitative Accepted 15 March 2009 methods. In recent years, paleopedology has shifted from a largely qualitative field based on comparisons Available online 27 March 2009 with modern analogues to an increasingly quantitative endeavor. Some of this change has been a result of applying existing techniques to new materials, but many of the innovations have been the result of applying Keywords: new techniques to new materials, including thermodynamic modeling of soil formation, isotope paleosols paleoclimate geochemistry, and applications of empirical relationships derived from modern soils. A variety of semi- paleoenvironments quantitative and quantitative tools has been developed to examine past weathering and pedogenesis, and to isotopes reconstruct both paleoenvironmental and paleoclimatic conditions at the time that the paleosols formed. geochemistry Though it is often not possible to achieve the same temporal resolution as with marine records for pedogenesis paleoclimatic reconstructions, proxies based on paleosols are potentially a much more direct means of making paleoclimatic reconstructions because soils form at the Earth's surface, in direct contact with the atmospheric and climatic conditions at the time of their formation. Paleoclimatic and environmental properties that may be reconstructed using the new proxies include provenance, weathering intensity, mean annual precipitation and temperature during pedogenesis, nutrient fluxes into and out of the paleosols, the atmospheric composition of important gases including CO2 and O2, the moisture balance during pedogenesis, the soil gas composition, reconstructed vegetative covering, and paleo-altitude. © 2009 Elsevier B.V. All rights reserved. Contents 1. Introduction ............................................................... 2 2. Qualitative methods ........................................................... 4 2.1. Taxonomic and stratigraphic approaches .............................................. 4 2.2. Semi-quantitative methods .................................................... 5 2.2.1. Compaction........................................................ 5 2.2.2. Ichnology ........................................................ 6 3. Quantitative methods overview ...................................................... 7 4. Clay mineralogy of soils and paleosols ................................................... 7 4.1. Occurrence of clay minerals .................................................... 7 5. Whole rock geochemistry......................................................... 8 5.1. Analytical methods ........................................................ 8 5.2. Provenance and pedogenesis .................................................... 8 5.2.1. Major element ratios and pedogenic processes ....................................... 8 5.2.2. Major element weathering indices ............................................. 10 5.2.3. Trace element ratios ................................................... 11 5.2.4. Rare earth elements. ................................................... 12 5.3. Mass-balance calculations ..................................................... 13 5.3.1. Pedogenesis and diagenesis ................................................ 13 5.3.2. Precambrian atmospheric CO2 from mass balance...................................... 14 5.4. Paleotemperature ......................................................... 15 ⁎ Corresponding author. Tel.: +1 734 647 7569. E-mail address: [email protected] (N.D. Sheldon). 0012-8252/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2009.03.004 2 N.D. Sheldon, N.J. Tabor / Earth-Science Reviews 95 (2009) 1–52 5.5. Paleoprecipitation ......................................................... 16 5.5.1. Content of Fe–Mn nodules in vertisols ........................................... 16 5.5.2. Depth to Bk horizon .................................................... 16 5.5.3. Bw/Bt horizon geochemistry ................................................ 17 5.6. Long-term chemical weathering .................................................. 17 6. Thermodynamic approaches........................................................ 18 6.1. Simple versus complex systems ................................................... 18 6.2. Single-equation approaches .................................................... 18 6.2.1. Precambrian atmospheric CO2 ............................................... 18 6.2.2. Earliest Triassic soil formation ...............................................20 6.3. Multiple-equation approaches ...................................................20 7. Stable isotope approaches .........................................................22 7.1. Stableisotopiccompositionofpedogenicmineralsaspaleoenvironmentalproxies..............................22 7.1.1. Mineral-water isotope fractionation and the jargon of stable isotope geochemistry .......................22 7.1.2. Stable isotope fractionation factors of common pedogenic minerals . ............................23 7.1.3. Relationship between hydrogen and oxygen isotopes in continental waters ..........................25 7.2. Carbon in soils...........................................................25 7.2.1. One-component soil CO2 ..................................................25 7.2.2. Two-component soil CO2 ..................................................26 7.2.3. Three-component soil CO2 .................................................27 7.3. Soil and paleosol carbonate.....................................................27 7.3.1. Pedogenic calcite δ18O values ................................................27 7.3.2. Pedogenic siderite as a proxy for soil moisture δ18O values ..................................30 7.4. δ13C values of soil carbonate ....................................................30 7.4.1. Calcite from one-component of soil CO2 ...........................................30 7.4.2. δ13C of pedogenic siderite .................................................31 7.4.3. Calcite derived from 2-component soil CO2 mixing .....................................31 7.4.4. Soil carbonates formed by mixing of three-components of soil CO2 ..............................35 7.5. δ18O and δD of hydroxylated minerals ................................................36 7.5.1. Origin of residual deposits .................................................37 7.5.2. Variations in soil moisture δ18O and δD values........................................37 7.5.3. Single-mineral paleotemperature estimates .........................................38 7.5.4. Mineral-pair δ18O values ..................................................40 7.6. Paleo-vegetation/paleo-photosynthesis ...............................................40 8. Future approaches and challenges ..................................................... 41 8.1. Boron isotopes ........................................................... 41 8.2. Energy balance models.......................................................42 8.3. “Clumped isotope” paleothermometry................................................43 9. Summary ................................................................44 Acknowledgements ..............................................................44 Appendix A. Supplementary data ........................................................44 References ..................................................................44 1. Introduction but has some fundamental limitations, namely the need for taxonomic uniformitarianism. Continuing on with the grassland example, the Increasing recognition of paleosols (fossil soils; Fig. 1) in non- origin of grasses is most likely in the Cenozoic (e.g., Strömberg, 2002). marine strata has opened up new types of paleoenvironmental and However, paleosol-based estimates for the origin of grasslands as paleoclimatic reconstructions. While good quantitative paleoclimatic evidenced by Mollisol-like paleosols range from Eocene to Miocene data may be obtained from plant fossil assemblages using either (Retallack, 1997a,b, 2001a,b) depending on the strictness of one's nearest living relative (e.g., Leopold and Clay-Poole, 2001; Utescher taxonomy. Furthermore, others (Terry, 2001) have described Oligo- and Mosbrugger, 2007) or leaf morphometric approaches (e.g., Wolfe, cene-age paleosols as “Mollisols” without meaning to connote that a 1994; Uhl et al., 2007), those approaches yield snap shots of past grassland ecosystem was present. On the other hand, there are some environments and are relatively rare in the fossil record. Paleosols types of paleosols found in the Earth's past for which there are preserved in continental basins on the other hand, raise the possibility
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