Atmospheric carbon is estimated to have been ~500 times more abundant in Hadean time than at present, and its concentration has been gradually decreasing since then due to its storage in sedimentary rocks. Consequently, rain pH has been gradually increasing through geologic time, leading to the common assumption that groundwaters are less acidic today than they were in the distant past. However, this assumption overlooks the fact that root-forming land plants increase the carbonic acid concentration in soils by one or two orders of magnitude. In the absence of rooted land plants, reactions between minerals and rainwater are known to promote alkalinity. It is hypothesized that groundwater pH must have been, on average, highest shortly before the Late Ordovician to Silurian proliferation of root-forming land plants. To verify this hypothesis, we studied the mineralogy and geochemistry of the youngest known pre-Silurian paleosols to have developed on primary rocks, and provide evidence that they have evolved in predominantly alkaline groundwaters despite warm and humid paleo-environmental conditions. Today, a lush vegetation cover would thrive in such a climate, and the system would be necessarily acidic due to large inputs of organic acids. Paired with previous observations indicating that early Paleozoic sedimentary rocks are especially rich in detrital illite and K-feldspar, there is now enough evidence to believe that there was a greater tendency for alkalinity during this time period than during previous and subsequent geologic periods.

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