Mitchell,R.L. and Sheldon,N.D.(2010): The 〜1100 Ma Sturgeon Falls paleosol revisited: Implications for Mesoproterozoic weathering environments and atmospheric CO2 levels. Precambrian Research, 183, 738-748.

『約11億年前のスタージョン・フォールズの古土壌の再考:中期原生代風化環境と大気中二酸化炭素レベルとの関係』


Abstract
 During the active rifting stage of the 〜1100 Ma Midcontinental Rift in North America, alluvial sediments were deposited intermittently between basalt flows on the north and south shores of present day Lake Superior. At times of depositional quiescence, paleosols developed in both areas on the alluvial sediments and on the antecedent basalt. New results from the Sturgeon Falls paleosol in Michigan characterizing the weathering processes at the time of its formation indicate moderate maturity, high degrees of hydrolysis and leaching, and a low degree of salinization. Geochemical provenance indices indicate a homogeneous source for the paleosols, and in contrast to earlier work, there is little evidence for K metasomatism. As a result, atmospheric CO2 levels of 46× pre-industrial atmospheric levels were calculated using a mass-balance model. This result is consistent with previous calculations from nearly contemporaneous paleosols from the other side of the Keweenawan Rift and from the 〜100 Ma younger Sheigra paleosol in Scotland. The calculated CO2 values are also consistent with the calculated weathering environment proxies that indicate weak to moderate weathering at this time frame and suggest that the higher green-house gas loads indicated by Paleoproterozoic paleosols had dissipated by the mid-late Mesoproterozoic.

Keywords: Paleosols; Sturgeon Falls; Keweenawan; Precambrian; Weathering; Midcontinental Rift

1. Introduction
2. Geological setting
 2.1. Midcontinental Rift (MCR)
 2.2. Powder Mill Group - Siemens Creek Formation
 2.3. Sturgeon Falls
3. Methods
4. Results
 4.1. Physical sedimentology
  4.1.1. Sturgeon Falls paleosol and metabasalt outcrop
  4.1.2. Jacobsville Sandstone
 4.2. Petrography
  4.2.1. Sturgeon Falls paleosol
  4.2.2. Parent metabasalt
  4.2.3. Jacobsville Sandstone
 4.3. Geochemistry
  4.3.1. Molecular weathering ratios
   4.3.1.1. Hydrolysis
   4.3.1.2. Clayeyness
   4.3.1.3. Leaching
   4.3.1.4. Salinization
   4.3.1.5. Chemical Index of Alteration (CIA)
  4.3.2. Provenance ratios
  4.3.3. Loss on Ignitions (LOI)
5. Discussion
 5.1. Formation of the Sturgeon Falls paleosol
 5.2. Comparison of geochemical data with previous work
 5.3. Mass balance calculations
 5.4. Metasomatism?
 5.5. pCO2 reconstruction
 5.6. Implications
6. Conclusion
Acknowledgements
Appendix A. Supplementary data
References


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