Boron and calcium isotope composition in Neoproterozoic carbonate rocks from Namibia: evidence for extreme environmental change

The level and evolution of atmospheric carbon dioxide throughout Earth's history are key issues for palaeoclimate reconstructions, especially during times of extreme climate change such as those that marked the Neoproterozoic. The carbon isotope ratios of marine carbonates are crucial in the correlation and identification of Neoproterozoic glacial deposits, and they are also used as a record for biogeochemical cycling and potential proxy for atmospheric pCO(2). Likewise, the boron and calcium isotope compositions of marine carbonates are potential proxies for palaco-seawater pH and the ratio of calcium fluxes into and out of seawater, respectively, and together they may be used to estimate atmospheric carbon dioxide. Here we use B and Ca isotopes to estimate palaeoenvironmental conditions in the aftermath of a major Neoproterozoic glaciation in Namibia. The validity of the B and Ca isotope variation in the ancient marine carbonates is evaluated using the oxygen isotope composition of the carbonates and its correlation to the carbon isotope variation. A negative (2.7 to -6.2parts per thousand) delta(11)B excursion occurs in the postglacial carbonates and is interpreted to reflect a temporary decrease in seawater pH. Associated variations in delta(44) Ca values (ranging between 0.35 and 1.14parts per thousand) are linearly coupled with the carbon isotope ratios and imply enhanced postglacial weathering rates. The reconstructed seawater pH and weathering profiles indicates that high atmospheric CO, concentrations were likely during the melt back of Neoproterozoic glaciations and precipitation of cap carbonates. However, the B isotope trend suggests that these concentrations rapidly ameliorated and they do not co-vary with delta(13)C. Thus models attempting to link long-lived negative delta(13)C excursions to elevated pCO(2) need to be reconsidered. (C) 2004 Elsevier B.V. All tights reserved.