PRECAMBRIAN WEATHERING PROFILES IN THE LAKE SUPERIOR REGION: COMPARISONS AND CONSTRAINTS ON MASS FLUX AND ATMOSPHERIC CO2 ESTIMATES FOR METASOMATIZED PALEOSOLS
Additional uncertainty arises from possible erosion of the weathering profiles. 10% erosion of a profile reduces calculated mass flux by 5% to 20%, depending on individual profiles, and calculated pCO2 (following Sheldon, 2006) by 11% to 35%. However, erosion has a much smaller influence on the Feldspar Index of Weathering [FIW = 100 × (amount of K2O+Na2O+CaO removed)/(total removal of K2O+Na2O+CaO)], which is reduced by only ~2%. Thus, the FIW is an effective parameter for comparing the intensity of weathering among different paleosols.
Plagioclase was removed from all the weathering profiles, but K‒feldspar was removed only from the Baraboo profile, which is the most intensely weathered among the six, as reflected by its higher FIW value of 84, compared to 47 to 72 for the others. Observed depths of weathering in the profiles range from 4.2 to 13.4 m, which are poorly correlated with calculated mass removals of SiO2 + CaO + Na2O + K2O (2.7 to 8.5 moles/cm2), well correlated with addition of K2O (0.32 to 1.42 moles/cm2), and uncorrelated with FIW. The calculated level of atmospheric pCO2 for the Baraboo profile is 15.8 × PAL, which is consistent with that predicted by solar luminosity models for a Precambrian atmosphere at 1700 Ma. The remaining profiles yield pCO2 values ranging from 2.5 to 11 × PAL, which are all less than predicted, probably due to the effects of erosion and incomplete preservation of weathering profiles.