Precambrian Si isotope mass balance, weathering, and the significance of the authigenic clay silica sink

The geological cycles of carbon and silicon are linked via the silicate weathering feedback, but growing understanding of reverse weathering—the formation of authigenic clay minerals via back reaction of silica with cations—indicates that the nature of the connection between these two elemental cycles may be more complex than previously assumed. Recent carbon cycle modeling has suggested that enhanced reverse weathering during Precambrian time could have played an important role in regulating climate. The topology of the Precambrian silica cycle, including the size of the authigenic clay sink and its influence on the carbon cycle, is challenging to reconstruct due to inherent preservation biases in the rock record. Here we used an alternative approach using Si isotope values to invert for the magnitudes of Precambrian silica fluxes, by assuming that the silica cycle must be in mass balance with the bulk silicate Earth over geological time scales with respect to Si isotopes. To estimate uncertainties associated with these flux reconstructions, we used Monte Carlo simulations based on the natural variation exhibited in Si isotope ratio data from the Precambrian sedimentary record (n = 2118, spanning ~3.8 Ga to ~525 Ma) and modern authigenic clays (n = 123). The perspective of Si isotope mass balance shows that the previously observed increase in chert δ^(30)Si values across Precambrian time is a signal of an evolving silica cycle rather than a secular change in seawater temperature. Results yielded a secular increase in the relative contribution of the authigenic clay sink across most of Precambrian time—this view is consistent with previous interpretations of a concomitant decrease in the concentration of dissolved silica in seawater. This result is consistent among a variety of model versions incorporating different assumptions about the abundance of iron formations and possible relationships between authigenic clay formation and concepts that invoke slow growth in the absolute amount of the continental crust. This approach also highlights key areas where improving Si isotope records could further enhance our constraints on the Precambrian silica cycle, and its relationship with the carbon cycle and climate stability