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

9726009地球科学家面临的一个主要挑战是记录地球大气和化学过去的变化，并限制这些变化的原因。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)的假说，即“蒙特利漂移”是底栖有孔虫中d13C组分异常高的时期，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含量应该会导致在这个区间内较高的海洋pH值(因此也就是较高的d11B值)。确定古pH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的过程。这反过来可能有助于我们更好地理解最近地球化学的变化，例如化石燃料燃烧导致大气中二氧化碳增加30%。最近的证据表明，碳酸盐的硼同位素组成受其母流体的pH控制(例如，Vengosh等人，1991；Hemming和Hanson，1992；Spivack等人，1993；Hemming等人，1995)。将硼同位素作为更新世冰川海洋pH替代物的成功应用(Sanyal等人，1995；Sanyal等人，正在出版中)为进一步研究地质时代的海洋化学奠定了基础。这一工具的生存能力取决于有孔虫中原始海洋信号的保存。Sanyal等人研究的样品中似乎保存了海洋成分。(1995)，但保存需要在较老的样本中进行评估。我们的研究感兴趣的时间间隔是一个保存特别高的时期，因此我们有理由相信，有可能在中中新世确定原始海洋硼同位素特征。

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.