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

[9726009 . Hemming]地球科学家面临的一个主要挑战是记录过去地球大气和化学的变化，并查明这些变化的原因。我们建议利用中中新世底栖有孔虫的硼同位素组成来验证Woodruff和Savin（1991）的假设，即底栖有孔虫中d13C含量异常高的“蒙特雷漂移”时期是碳固定降低大气CO2的结果。有孔虫的硼同位素组成是检验这一假设的理想方法，因为较低的二氧化碳分压应导致该区间较高的海洋pH值（从而较高的d11B值）。确定古ph值的能力将使我们能够更好地了解过去海洋和大气之间的CO2-H2O平衡，这将有助于我们了解全球气候变化和导致冰川作用的过程。这反过来又可以帮助我们更好地理解地球化学的最新变化，比如化石燃料燃烧导致大气中二氧化碳含量增加了30%。最近的证据表明，碳酸盐的硼同位素组成受其母体流体的pH值控制（例如，Vengosh等人，1991；Hemming和Hanson, 1992； Spivack等人，1993；Hemming等人，1995）。硼同位素作为更新世冰川海洋pH代用物的成功应用（Sanyal et al., 1995; Sanyal et al., in press）为研究更早地质时期的海洋化学奠定了基础。该工具的可行性取决于有孔虫中原始海洋信号的保存。Sanyal等人（1995）研究的样品中似乎保存了海洋成分，但需要对更老的样品进行保存评估。我们研究感兴趣的时间间隔是一个保存特别高的时期，所以我们有理由相信，在中新世中期，有可能确定主要的海洋硼同位素特征。

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.