Experimental refinement of the boron isotope proxy in planktic foraminifera - temperature, asymbiotic sensitivity, and seawater elemental composition

培养实验将用于研究浮游有孔虫的壳组成。浮游有孔虫是生活在海洋表面的单细胞生物。科学家们利用化石有孔虫壳的化学成分来记录过去的海水状况。这些数据揭示了海洋对自然气候变化的反应。这些知识提高了科学家预测未来气候变化的能力。浮游有孔虫的硼同位素组成记录了海洋表面的酸度。然而，硼同位素可能受到温度和海水钙浓度等其他因素的影响。拟议的实验将量化这些影响。这将提高科学家了解过去海洋酸度变化的能力。该项目更广泛的影响包括支持一名研究生和两名本科生助手。PI将尝试从地球科学领域代表性不足的群体中招募本科生。浮游有孔虫壳中的硼同位素是重建表层海水酸度和推断大气CO2最成功的工具之一。然而，仍有一些未知因素影响着古ph值和古co2值重建的确定性。具体而言，温度对溶解硼酸离子和硼酸之间的硼同位素分馏的影响已经由热力学原理和模型提出，并且有几条证据表明这种影响表现在浮游有孔虫的硼同位素组成中，但缺乏坚实的实验证据。如果这种效应存在，它可以解释地表和深层有孔虫物种之间硼代剂敏感性的差异。另一种解释物种差异的假设是基于这样的观察，即实验室培养实验是在比自然海洋中存在的更宽的pH和方解石饱和状态范围内进行的，这可能会影响有孔虫的生长速度。为了解决这些不确定性，我们提出了以下实验：(1)在广泛的实验pH范围内对共生-无孔虫物种进行首次实验室校准；(2)在恒定pH范围内的广泛温度范围内培养浮游有孔虫；(3)在恒定pH条件下在广泛的钙浓度和饱和状态下培养浮游有孔虫。(4)在模拟古新世海水条件下建立pH校准，当时海水钙浓度高于现代海洋。但镁和硼的浓度明显降低。该研究将为硼同位素替代的应用提供基础约束，并改善更新世至早新生代海洋表面pH和大气CO2的重建。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Culture experiments will be used to study the shell composition of planktonic foraminifera. Planktonic foraminifera are single-celled organisms that live in the surface ocean. Scientists use the chemistry of fossil foraminifera shells to record seawater conditions in the past. Those data reveal the ocean’s response to natural climate change. That knowledge improves scientists’ ability to predict future climate change. The boron isotopic composition of planktonic foraminifera records the acidity of the surface ocean. However, boron isotopes may be influenced by other factors including temperature and seawater calcium concentration. The proposed experiments will quantify those impacts. That will improve scientists’ ability to know how ocean acidity has changed in the past. The project broader impacts include support for a graduate student and two undergraduate helpers. The PI will attempt to recruit undergraduates from groups that are underrepresented in the earth sciences. The boron isotope proxy in planktic foraminifera shells is one of the most successful tools for reconstructing surface seawater acidity, and by inference atmospheric CO2. A strong empirical framework has already been established for this proxy, however, some unknowns still remain that affect the certainty with which paleo-pH and paleo-CO2 can be reconstructed from this proxy. Specifically, the effect of temperature on the aqueous boron isotope fractionation between dissolved borate ion and boric acid has been suggested by thermodynamic principles and modeling and several lines of evidence suggest that this effect is manifested in the boron isotopic composition of planktic foraminifera, but solid experimental proof is missing. If such an effect exists, it may explain differences in boron proxy sensitivity between surface- and deep-dwelling foraminifera species. An alternative hypothesis to explain species differences is based on the observation that laboratory culture experiments are performed over a much wider range of pH and calcite saturation states than present in the natural ocean, which could affect foraminiferal growth rates. The following experiments are proposed to resolve these uncertainties: (1) establish the first laboratory calibration of a symbiont-barren foraminifera species across a wide range of experimental pH, (2) grow planktic foraminifera over a wide range of temperatures but constant pH, (3) grow planktic foraminifera across a wide range of calcium concentrations and saturation states at constant pH. (4) Establish a pH-calibration under simulated Paleocene seawater conditions, when seawater calcium concentrations were higher than in the modern ocean, but magnesium and boron concentrations significantly lower. This study will provide fundamental constraints on the application of the boron isotope proxy and improve reconstructions of surface ocean pH and atmospheric CO2, from the Pleistocene to the early Cenozoic.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.