『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