PRECAMBRIAN WEATHERING PROFILES IN THE LAKE SUPERIOR REGION: COMPARISONS AND CONSTRAINTS ON MASS FLUX AND ATMOSPHERIC CO2 ESTIMATES FOR METASOMATIZED PALEOSOLS

Mass fluxes have been calculated for Precambrian weathering profiles in granitic protoliths at six localities in the Lake Superior region (Steeprock, 2700 Ma; Denison, 2450 Ma; Pronto, 2450 Ma; Ville Marie, 2450 Ma; McGrath, 2200 Ma; and Baraboo, 1700 Ma). Profiles at all localities have experienced significant K‒metasomatism. Despite such metasomatism, estimates of mass fluxes can be made by appropriate projections in an A‒C*N‒K diagram, although the resulting estimates are minimum values due to uncertainty in the actual amount of K removed by weathering. An extreme example of such underestimation is provided by the Baraboo profile, where all K‒feldspar was removed by weathering. In this instance, the calculated removal of K₂O is 40% less than the actual amount removed (0.40 vs. 0.67 moles/cm²), although the calculated total mass flux is only 5% less than the actual total (4.86 vs. 5.13 moles/cm²).

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.