Li, Mg, and Sr isotope trends track changes in continental weathering in carbonate rocks of the Salitre Formation, Irecê Basin

During the Ediacaran-Cambrian transition, carbonates of the Salitre Formation were deposited in the Irecê Basin, eastern Brazil. These carbonates record marine transgressions that occurred in West Gondwana and potentially reflect the paleoenvironmental changes and marine dynamics of this period. This interval is also significant for recording the appearance of the first biomineralized organisms, with one theory suggesting a shift from dolomitic-aragonitic seas to aragonitic seas. Given that continental weathering transfers chemical elements from continents to oceans, this could have been a decisive factor in changing ocean chemistry. To track weathering regimes and test this hypothesis, we applied coupled lithium, magnesium, and strontium isotope compositions of carbonate rocks to investigate weathering fluxes in this basin. From the base to the top, there is a notable increase in Sr/Ca ratios and concomitant decreasing trends in δ⁷Li, δ²⁶Mg values, and ⁸⁷Sr/⁸⁶Sr ratios, suggesting an environmental shift from incongruent to congruent weathering in the source areas. This trend also implies a greater discharge of alkalinity into seawater, which could have altered the chemical composition of seawater to conditions conducive to biomineralization. However, no significant fossil record of biomineralized macroorganisms has been reported in the Salitre Formation. We suggest that the lack of fossil fauna in this formation results from the predominance of restricted-sea depositional environments in the Irecê Basin, at the core of the Gondwana paleocontinent. In this restricted environment, the high nutrient supply related to congruent weathering could lead to eutrophication and a consequent decline in habitability. The Salitre Formation illustrates how regional and local conditions could have prevailed over global ones. In this context, the assembly of the supercontinent and the creation of epicontinental seas in West Gondwana may have hindered animal dispersion.