Abstract
Ca. 548 Ma old paleosols developed on Ediacaran basalts and almost unaltered by diagenesis were discovered and sampled from several core profiles in Volyn (NW Ukraine). Their mineral composition, elemental geochemistry (major and trace, including REE, plus Fe2+/Fe3+ ratio by Mössbauer spectroscopy), stable isotope geochemistry (O and C in carbonates, B in kaolinite and hematite), age (U-Pb in carbonates), and age of diagenetic alteration (K-Ar of illite and aluminoceladonite) were studied. The technique of mass balance calculation independent from bulk density measurements was applied.The profiles are mineralogically and geochemically very mature: primary basaltic and hydrothermal minerals disappear completely and neoformed minerals appear: first dioctahedral smectite and hematite, then smectite alters into kaolinite-smectite, and finally kaolinite and hematite remain as dominant phases. Geochemical indices classify Volyn paleosols as paleo-Ultisols on the border of lateritization zone with pH close to neutral. 50–60% of the rock mass was dissolved from the top layer. Weathering involved biological activity as documented directly by microbial mats (Kremer et al., 2018) and supported by several geochemical indicators, including the presence of immature organic matter and stable isotope ratios.Volyn paleosols are close analogues of recent weathering of basalts on Hawaii and the coast of Costa Rica, including the presence of mixed-layer kaolinite-smectite. They document hot and humid Ediacaran climate on the East European Craton at ca. 550 Ma. The lack of aeolian contamination implies the lack of dry regions on the craton in this period. Mass-balance calculations suggest >90% of P loss from the top saprolitic layer. Given the area of the basalt trap, this reflects a significant release of P into the Ediacaran ocean, which possibly contributed to the Ediacaran life explosion and atmospheric O2 rise.
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