『Abstract
Paleosols (fossil soils) are preserved throughout the geologic
record in depositional settings ranging from alluvial systems
to between basalt flows. Until recently, paleosols were studied
using primarily qualitative methods. In recent years, paleopedology
has shifted from a largely qualitative field based on comparisons
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 new techniques to new materials, including thermodynamic
modeling of soil formation, isotope geochemistry, and applications
of empirical relationships derived from modern soils. A variety
of semi-quantitative and quantitative tools has been developed
to examine past weathering and pedogenesis, and to reconstruct
both paleoenvironmental and paleoclimatic conditions at the time
that the paleosols formed. Though it is often not possible to
achieve the same temporal resolution as with marine records for
paleoclimatic reconstructions, proxies based on paleosols are
potentially a much more direct means of making paleoclimatic reconstructions
because soils from 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.
Keywords: paleosols; paleoclimate; paleoenvironments; isotopes;
geochemistry; pedogenesis』
Contents
1. Introduction
2. Qualitative methods
2.1. Taxonomic and stratigraphic approaches
2.2. Semi-quantitative methods
2.2.1. Compaction
2.2.2. Ichnology
3. Quantitative methods overview
4. Clay mineralogy of soils and paleosols
4.1. Occurrence of clay minerals
5. Whole rock geochemistry
5.1. Analytical methods
5.2. Provenance and pedogenesis
5.2.1. Major element ratios and pedogenic processes
5.2.2. Major element weathering indices
5.2.3. Trace element ratios
5.2.4. Rare earth elements
5.3. Mass-balance calculations
5.3.1. Pedogenesis and diagenesis
5.3.2. Precambrian atmospheric CO2 from
mass balance
5.4. Paleotemperature
5.5. Paleoprecipitation
5.5.1. Content of Fe-Mn nodules in vertisols
5.5.2. Depth to Bk horizon
5.5.3. Bw/Bt horizon geochemistry
5.6. Long-term chemical weathering
6. Thermodynamic approaches
6.1. Simple versus complex systems
6.2. Single-equation approaches
6.2.1. Precambrian atmospheric CO2
6.2.2. Earliest Triassic soil formation
6.3. Multiple-equation approaches
7. Stable isotope approaches
7.1. Stable isotopic composition of pedogenic minerals as
paleoenvironmental proxies
7.1.1. Mineral-water isotope fractionation and the jargon of
stable isotope geochemistry
7.1.2. Stable isotope fractionation factors of common pedogenic
minerals
7.1.3. Relationship between hydrogen and oxygen isotopes in
continental waters
7.2. Carbon in soils
7.2.1. One-component soil CO2
7.2.2. Two-component soil CO2
7.2.3. Three-component soil CO2
7.3. Soil and paleosol carbonate
7.3.1. Pedogenic calcite δ18O values
7.3.1.1. Pedogenic calcite as a proxy for soil moisture δ18O
values
7.3.1.2. Pedogenic calcite δ18O values as a proxy
of paleotemperature
7.3.2. Pedogenic siderite as a proxy for soil moisture δ18O
values
7.4. δ13C values of carbonate
7.4.1. Calcite from one-component of soil CO2
7.4.2. δ13C of pedogenic siderite
7.4.3. Calcite derived from 2-component soil CO2
mixing
7.4.3.1. Estimates of paleoatmospheric pCO2
7.4.3.2. Soil calcite δ13C as a means of assessing
soil pCO2 and productivity
7.4.3.3. Pedogenic goethite
7.4.4. Soil carbonates formed by mixing of three-components
of soil CO2
7.4.4.1. Calcite
7.4.4.2. Goethite
7.5. δ18O and δD of hydroxylated minerals
7.5.1. Origin of residual deposits
7.5.2. Variations in soil moisture δ18O and δD values
7.5.3. Single-mineral paleotemperature estimates
7.5.3.1. Goethite
7.5.3.2. Smectite and mixed phyllosilicates
7.5.3.3. Kaolinite
7.5.4. Mineral-pair δ18O values
7.6. Paleo-vegetation/paleo-photosynthesis
8. Future approaches and challenges
8.1. Boron isotopes
8.2. Energy balance models
8.3. “Clumped isotope” paleothermometry
9. Summary
Acknowledgements
Appendix A. Supplementary data
References