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The thorough breakdown of primary minerals within inter-basaltic paleosols of the Ordovician Dunn Point Formation of eastern Canada suggests that they experienced penetrative chemical weathering under a warm and humid climate. Nowadays, a lush vegetation cover would thrive in such a climate, and the system would be necessarily acidic due to large inputs of organic acids.
However, the accumulation of K, the preservation of some calcite, and the replacement of Al by Si in these paleosols suggest that paleo-weathering conditions were strongly alkaline. This apparent dichotomy may be linked to the absence of a substantial land plant cover in Ordovician times. As atmospheric CO2 gradually decreased in Precambrian times due to its storage in carbonaceous rocks, and as reactions between minerals and rainwater naturally tend to promote alkalinity, groundwater pH must have been gradually on the rise, which may explain why Proterozoic to early Palaeozoic paleosols tend to be abnormally rich in K.
In conclusion, because groundwaters were apparently able to maintain their alkalinity even under a warm and humid climate, the thoroughly weathered Ordovician paleosols of the Dunn Point Formation developed a K-rich, Si-rich and Al-poor illitic facies that has no modern equivalent. In the modern world, alkaline soils are only found in settings that are too arid to support significant plant life. In this view, early land plants may have evolved in a predominantly alkaline world during the Ordovician, although their radiation subsequently led to a rapid lowering of global groundwater pH.
This may explain why primitive land plants have an affinity for alkalis, a trait that should be taken into consideration in models on early land plant evolution.
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