Collaborative Research: Taking the reliability of Cenozoic boron isotope pH and pCO2 reconstructions to the next level

如果人为的二氧化碳（CO2）排放继续有增无减，这种气体在大气中的持续积累将通过温室效应使地球气候变暖，而二氧化碳在海洋表面的溶解使海水变得更酸。虽然历史上的二氧化碳水平和全球变暖在过去几十年里有相当充分的记录，但科学家对气候，特别是生态系统对高得多的二氧化碳水平的敏感性的理解是有限的，这极大地阻碍了对未来变化的准确预测。地球历史可以帮助科学家提高他们的认识，因为它提供了过去海水化学变化的更长的地质记录，以及与之相关的生态反应（如果有的话）。这项研究旨在提供过去6500万年中海洋表面酸度和大气二氧化碳水平的基本约束条件，当时发生了许多气候变化，其中包含有关海洋和大气化学变化的重要信息。这项研究将结合浮游生物化石壳中储存的地球化学信息重建海洋表面酸度，以及海洋碳化学的计算机模型模拟。除了让科学家和政策制定者了解大气中二氧化碳浓度上升对全球气候的影响外，这项研究的结果对想要了解海洋酸化对海洋生物影响的古生态学家也很有价值。这项研究将支持哥伦比亚大学和加州大学河滨分校的两名研究生的研究，并将为感兴趣的研究人员提供碳循环建模方面的培训。具体来说，该项目将测量有孔虫化石壳中记录的硼同位素组成（delta11B），以重建过去的海水ph值，并由此推断大气二氧化碳水平。使用这种方法的早期研究只能重建海水ph值和大气二氧化碳的相对变化，因为单个有孔虫物种记录的三角洲11b的微小差异使这种替代方法在遥远的过去的应用复杂化，因为大多数古老的有孔虫物种现在已经灭绝。此外，需要海洋碳酸盐体系的第二个参数（如碱度）来从表面海洋化学记录中准确计算大气CO2水平，但研究人员尚未发现海洋沉积物中碱度的可靠地球化学指标。本研究旨在克服这些局限性，通过(1)交叉校准现代和已灭绝的浮游有孔虫物种，(2)改进地球系统模型对海洋表面碱度的估计，(3)从硼同位素和模拟碱度中得到精细且自一致的新生代pCO2重建。除了本研究预测的低分辨率6500万年前的古构造外，交叉校准对有孔虫三角洲11b的重要影响和改进的地球化学模拟碱度估计将有助于未来对绝对pH和二氧化碳分压的研究，而不是过去的相对pH和二氧化碳分压。

If anthropogenic carbon dioxide (CO2) emissions continue to increase unabated, the continued accumulation of this gas in the atmosphere will warm Earth's climate via the greenhouse effect, while the dissolution of CO2 in the surface ocean makes seawater more acidic. Although historical CO2 levels and global warming are reasonably well documented for the past few decades, scientists' understanding of climate and particularly ecosystem sensitivity to much higher CO2 levels is limited and greatly impedes accurate projections of future changes. Earth history can aide scientists in improving their understanding, by providing much longer geological records of variations in past seawater chemistry and their associated ecological responses, if any. This research aims to provide fundamental constraints on surface ocean acidity and atmospheric CO2 levels throughout the past 65 million years, when a number of climate shifts containing important information about oceanic and atmospheric chemical changes occurred. The research will combine surface ocean acidity reconstructions from geochemical information stored in the fossil shells of plankton organisms, as well as computer model simulations of ocean carbon chemistry. In addition to informing scientists and policy makers about the impact of rising atmospheric CO2 levels on global climate, the outcomes of this study are also of great value to paleoecologists, who want to understand the effect of ocean acidification on marine life. The study will support the research of two graduate students at Columbia University and University of California at Riverside, and will provide training in carbon cycle modeling to interested researchers .Specifically, the project will measure the boron isotopic composition (delta11B) recorded in fossil foraminifera shells to reconstruct past seawater-pH and, by inference, atmospheric CO2 levels. Earlier studies using this approach have only been able to reconstruct relative shifts in seawater-pH and atmospheric CO2, because slight differences in the delta11B recorded by individual foraminifera species complicate the application of this proxy to the distant past, as most ancient foraminifera species are now extinct. In addition, a second parameter (e.g., alkalinity) of the marine carbonate system is needed to accurately calculate atmospheric CO2 levels from surface ocean chemistry records, but researchers have not yet found a reliable geochemical indicator for alkalinity in ocean sediments. This research aims to overcome these limitations, by (1) cross-calibrating modern and extinct planktic foraminifera species, (2) refining Earth system model estimates of surface ocean alkalinity, and (3) deriving a refined and self-consistent reconstruction of Cenozoic pCO2 from boron isotopes and modeled alkalinity. In addition to the low resolution 65-million year paleoreconstruction anticipated by this study, the cross-calibration of vital effects on foraminiferal delta11B and improved alkalinity estimates from geochemical modeling will facilitate future studies of absolute rather than relative pH and pCO2 in the past.