Boron and diagenesis: Questioning the fidelity of Snowball δ11B excursions

Abstract:

Large negative carbon (δ¹³C) and boron (δ¹¹B) isotope excursions (both >6‰) within the widely distributed Neoproterozoic “Snowball Earth” cap carbonates are interpreted as evidence for considerable perturbation of the carbon cycle and the associated reduction, then recovery of global ocean pH. Yet, before conclusive interpretations may be drawn, isotopic data must first be shown to be primary in origin and non-diagenetic. Recent studies of Quaternary carbonate platform sediments from a number of locations worldwide reveal δ¹³C excursions of similar magnitude and distribution to the “Snowball Earth” excursions. However, these recent analogues were formed following eustatic sea level fall and exposure of recent carbonates to meteoric diagenesis (Swart and Kennedy, 2012). Here we present δ¹¹B and B/Ca data from Pleistocene-aged carbonate platform sediments recovered by the Clino Core from the Bahamas to examine the effects of diagenesis on the boron system. We find that within the interval of meteoric diagenesis the δ¹¹B of bulk carbonate is substantially reduced by approximately 6‰ in conjunction with a drop in B/Ca of 90%. These isotopic and elemental down-core patterns are strikingly similar to those reported for δ¹¹B and B/Ca in the cap carbonates of the Neoproterozoic snowball Earth events (Kasemann et al., 2005; Kasemann et al., 2010). Our results may therefore question the primary nature of “Snowball Earth” isotope excursions. We recommend more rigorous assessment of the diagenetic history of these ancient carbonates to ensure palaeoclimatological interpretations are robust.