Enhanced weathering and its potential connection to ocean oxygenation and eukaryotic evolution at 1.57 Ga

Xi Chen, Ying Zhou, Simon W Poulton, Fred Toby Bowyer, Colin Mettam, Shihong Zhang, Maoyan Zhu, Da Li, Philip A.E. Pogge von Strandmann, Benjamin Mills, Graham Anthony Shields 

Chemical weathering is a critical Earth system process that regulates climate, ocean chemistry and the long-term carbon cycle. During the mid-Proterozoic (~1.8‒0.8 Ga), chemical weathering is generally considered to have been relatively muted, but this perception remains largely untested, limiting our understanding of the drivers of purported oxygenation events and coeval biological evolution. Here, we report the first coupled positive seawater 87Sr/86Sr (~0.0007) and δ7Li (~5‰) isotope excursion of the Mesoproterozoic Era (1.6‒1.0 Ga). Geochemical box modelling suggests the concurrent Sr - Li isotope excursions signify a substantial weathering event at ~1.57 Ga, characterised by increased silicate weathering rates and decreased weathering congruency. Drawing on independent geological evidence, we posit that enhanced volcanic CO2 degassing, possibly alongside accretional orogenesis, increased denudation rates and invigorated the hydrological cycle, amplifying silicate weathering and secondary clay formation. This weathering pulse coincided with the ocean oxygenation and carbon cycle disruption, implicating it in the coeval preservation of the earliest known decimetre-scale, multicellular eukaryotic fossils.