PROTEROZOIC SOILS EXPERIENCED LESS CHEMICAL WEATHERING THAN DID PHANEROZOIC SOILS

Mass fluxes of weathering in six Proterozoic, two Cambrian, and two Cretaceous paleosols, which formed on igneous protoliths in the Lake Superior region of midcontinental Laurentia, are compared to those in five modern soils worldwide; weathering of both the paleosols and modern soils occurred in a diversity of protolith compositions and over a range of (paleo)latitudes. The bulk of mass flux in both suites is accounted for by the removal of SiO₂, CaO, Na₂O, and K₂O, the magnitude of which has been calculated relative to Al₂O₃ as an immobile constituent.

The Proterozoic and Cambrian paleosols all experienced post-weathering potassium metasomatism, as documented by application of the Al₂O₃-CaO*Na₂O-K₂O plot and evidenced by the presence of neoblastic muscovite, illite, or microcline. In such paleosols, an estimate of pre-metasomatic K₂O removal by weathering may be obtained by normalizing to the removal of Na₂O, as calibrated by comparison with the average depth variations of K₂O and Na₂O in modern soils, for which:

(% change K₂O) / (% change Na₂O) = – 1.40z³ + 0.95z² – 0.31z + 0.75

where z is normalized depth. Following this approach, the removal of K₂O ranges from 41.5% to 51.7% in the six Proterozoic paleosols, and 33.4% to 54.9% in the two Cambrian paleosols. Such quantification of potassium removal is essential for obtaining MAP and MAT estimates using paleoclimate proxies that include K₂O.

The percentage removal by weathering for the sum of SiO₂, CaO, Na₂O, and K₂O progressively increases from Proterozoic (17.6 ± 1.9%) to Cambrian (28.5 ± 4.2%) to Cretaceous (36.5 ± 8.6%) paleosols. In comparison, the percentage of mass removed from the five modern soils is 30.4 ± 7.6%, which overlaps with values for the Cambrian and Cretaceous paleosols. We suggest that the greater magnitude of weathering in Phanerozoic soils compared to Proterozoic ones is due to the effects of higher concentrations of organic acids during Phanerozoic soil formation, notably with the emergence of sparse cryptophytes in biological soil crusts in Cambrian time and subsequent greening of the continents with vascular plants from the Devonian to present. Deeper rooting by more advanced vascular plants in the Cretaceous contributed to greater weathering and percentage mass removal in these paleosols compared with those of Cambrian age.