EVOLUTION OF GROUNDWATER PH TRENDS THROUGH GEOLOGICAL TIME ACCORDING TO THE MINERALOGICAL AND GEOCHEMICAL COMPOSITION OF PALEOSOLS AND OTHER SEDIMENTARY ROCKS

Sedimentary rocks in general, but especially paleosols, are extensively used to derive clues about climate evolution. However, special consideration must be given to other variables than climate that have an influence on groundwater chemistry and, therefore, on the mineralogy and geochemistry of these rocks. One important variable is vegetation. Before the advent of land plants, groundwater pH must have gradually increased through time due to the gradual incorporation of atmospheric carbon in sedimentary rocks and its long-term removal from the atmosphere. Because of the substantial increase in carbon storage efficiency that took place in early Paleozoic times due to the radiation of organisms with hard parts, this trend of rising pH must have been especially rapid then, as suggested by high contents of potassic detrital minerals in sedimentary rocks of that age. These minerals were becoming increasingly stable on land due to a decreasing supply of atmospheric acids. This trend of increasing groundwater pH abruptly reversed in Late Ordovician to Early Silurian times due to the radiation of land plants, which apparently first developed under highly alkaline conditions, but which themselves promote pH reduction by producing and releasing organic acids into the soil environment. Since then, K-rich soils have been rare, whereas such soils are abundant in post-Archean to pre-Silurian successions. The high-K content of pre-Silurian paleosols is usually assumed to be related to post-burial diagenesis or metasomatism, but evidence for this is unconvincing. For example, such evidence is lacking in paleosols of the Middle Ordovician Dunn Point Formation (Nova Scotia), which were developed at the eve of land plant radiation. These Si-K enriched and Al-Fe-Ti depleted paleosols show evidence for a hot and humid equatorial climate, but strongly alkaline environmental conditions. Although water-mineral interactions promote alkalinity, modern soils cannot maintain alkalinity under humid climates because water stimulates plant growth, which in turn promotes acidity. Data from previous studies on paleosols and other sedimentary rocks of various ages are therefore combined with our data on Middle Paleozoic paleosols of Nova Scotia to develop a rough model of groundwater pH evolution through geological time.