The C/B Excursion During Marinoan Glaciation (Snowball Earth) may not be Meaningful

Boron during meteoric diagenesis and its potential implications for Marinoan snowball Earth δ11B-pH excursions

Authors:

Stewart et al

Abstract:

Large negative carbon (δ13C) and boron (δ11B) isotope excursions (both >6‰) within the widely distributed Neoproterozoic carbonates associated with the Marinoan "snowball Earth" event are interpreted to represent considerable perturbations of the carbon cycle and the accompanying reduction in global ocean pH. Yet this interpretation is predicated on these isotopic signals being primary in origin. Recent studies of Pleistocene carbonate platform sediments from the Great Bahama Bank (western Atlantic Ocean; Clino core, drilled by the Bahamas Drilling Project) and elsewhere demonstrate that δ13C excursions of similar magnitude and global distribution to the snowball Earth excursions are formed following eustatic sea-level fall and exposure of shelf carbonates to meteoric diagenesis. Here we present δ11B and trace element data (B/Ca, Na/Ca, Mg/Ca, and Sr/Ca) from the same Clino core carbonate sediments in order to explore the influence of this diagenetic process on the boron system. We find that within the interval of meteoric diagenesis the δ11B of bulk carbonate is reduced by ∼6‰ in conjunction with a drop in the B/Ca ratio of 90%. Our results clearly demonstrate that the boron system is impacted by meteoric diagenesis, implying that a rigorous assessment of the diagenetic history of all ancient carbonates is required to ensure any paleoceanographic interpretation based on δ11B and/or B/Ca are robust.