Neoproterozoic ice ages, boron isotopes, and ocean acidification

The Neoproterozoic Earth underwent at least two severe glaciations, each extending to low palaeomagnetic latitudes and punctuating warmer climates. In concert with the environmental changes, the rocks display large amplitude fluctuations in their stable isotopic composition. These fluctuations are stratigraphically systematic, occur in many sections worldwide and are interpreted as being globally significant1. Thus, the Neoproterozoic carbonates provide a unique geological and isotopic archive to improve our understanding of major non-anthropogenically influenced changes in Earth System behaviour. The two widespread older and younger Cryogenian glacial deposits (commonly referred to as the Sturtian and Marinoan, respectively) in Namibia are directly overlain by cap carbonates deposited under inferred periods of high atmospheric carbon dioxide concentrations. Oceanic uptake of carbon dioxide decreases ocean pH and here we present a record of Cryogenian inter-glacial ocean pH, based on boron isotopes in marine carbonates. Our data document characteristically different B isotope profiles of the two Cryogenian carbonate transects that are consistent with the presence of two panglacial climate states, but indicate that each had its own distinct environmental conditions. The Marinoan interglacial δ11B profiles are systematic and remarkably consistent, and they vary by up to 11‰. This yields a relative pH variation of up to 1.5 pH units, and implies a pH of 8.5 at the onset of cap carbonate deposition, followed by a decrease in pH to ~7 and then a return to pH ~8 for the upper part of the section. The transient ocean acidification excursion and the alkaline pH condition near the start and termination of the inferred greenhouse state suggests a rapid draw-down of CO2 initiated at the start of the deglaciation and supports inferences of a thick, global sea-ice shield with minimal air-sea gas exchange during glaciation. In contrast, largely constant B isotope values for the Sturtian-aged glacial aftermath do not indicate extreme ocean pH (~8.3) conditions and do not support a contemporaneous major ocean acidification event and associated high pCO2 at the time of the older Cryogenian glaciation and deglaciation. That leads us to speculate that the ocean during the older glaciation was not totally frozen and that the hydrological cycle was functioning2. 1Hoffman, P.F., Kaufman, A.J., Halverson, G.P. and Schrag, D.P. (1998). A Neoproterozoic Snowball Earth. Science, 281, 1342-1346. 2Kasemann, S.A., Prave, A.R., Fallick, A.E., Hawkesworth, C.J. and Hoffmann, K.H. (2010). Neoproterozoic ice ages, boron isotopes, and ocean acidification: Implications for a snowball Earth. Geology, 38, 775-778.

Kasemann, S. A., Prave, A. R., Fallick, A. E., Hawkesworth, C. J., Hoffmann, K., 2010. Neoproterozoic ice ages, boron isotopes, and ocean acidification. American Geophysical Union, Fall Meeting 2010, abstract #PP13E-03. Abstract.

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