A Test for the Interpreted Cause of the Mid-Miocene "Monterey Excursion" Using the Boron Isotopic Composition of Benthic Foraminifera

点击翻译按钮获取中文摘要

9726009 Hemming A major challenge to earth scientists is to document past changes in the earth's atmosphere and chemistry, and to constrain the causes of those changes. We propose to use the boron isotopic composition of mid-Miocene benthic foraminifera to test the hypothesis of Woodruff and Savin (1991) that the "Monterey Excursion", a time of anomalously high d13C compositions in benthic foraminifera, was the result of carbon fixation which lowered atmospheric CO2. The boron isotopic composition of foraminifera is an ideal way to test this hypothesis, as the lower pCO2 should result in higher ocean pH (and hence higher d11B values) at this interval. The ability to determine paleo-pH will allow us to better understand the CO2-H2O equilibrium between oceans and atmosphere in the past, which will contribute to our knowledge of global climate change and the processes that lead to glaciations. This in turn may help us to better understand recent changes in earth's chemistry, such as the 30% increase in atmospheric CO2 that resulted from fossil fuel burning. Recent evidence suggests that the boron isotopic composition of carbonates is controlled by the pH of their parent fluid (e.g., Vengosh et al., 1991; Hemming and Hanson, 1992; Spivack et al., 1993; Hemming et al., 1995). Success in the application of boron isotopes as a pH proxy for Pleistocene glacial oceans (Sanyal et al., 1995; Sanyal et al., in press) lays the foundation for looking at ocean chemistry further back in geologic time. The viability of this tool is dependent on preservation of a primary marine signal in the foraminifera. The marine composition appears to be preserved in the samples studied by Sanyal et al. (1995), but preservation needs to be evaluated in older samples. The time interval of interest to our study was a period of particularly high preservation, so we are reasonably confident that it will be possible to determine primary marine boron isotope signatures through the mid-Miocene.