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

将通过培养实验来研究浮游有孔虫的壳组成。浮游有孔虫是生活在表层海洋的单细胞生物。科学家使用有孔虫化石贝壳的化学物质来记录过去的海水条件。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表层海洋的酸度。然而，硼同位素可能会受到其他因素的影响，包括温度和海水钙浓度。拟议中的实验将量化这些影响。这将提高科学家了解海洋酸度在过去如何变化的能力。该项目更广泛的影响包括对一名研究生和两名本科生助手的支持。PI将尝试从在地球科学中代表性不足的群体中招收本科生。浮游有孔虫贝壳中的硼同位素替代物是重建表层海水酸度和由此推断的大气二氧化碳的最成功的工具之一。对于这一指标，已经建立了一个强有力的经验框架，然而，一些未知因素仍然存在，这些未知因素影响了从这一指标恢复古pH和古CO2的确定性。具体地说，温度对溶解的硼酸根离子与硼酸之间的水相硼同位素分馏的影响已由热力学原理和模型提出，多条证据表明这种影响体现在浮游有孔虫的硼同位素组成中，但缺乏可靠的实验证据。如果存在这样的影响，它可能解释了表层有孔虫和深海有孔虫物种对硼代理敏感性的差异。解释物种差异的另一种假设是基于观察到的实验室养殖实验比自然海洋中进行的pH和方解石饱和状态范围要大得多，这可能会影响有孔虫的生长速度。为解决这些不确定性，建议进行以下实验：(1)在实验室中首次对无共生体的有孔虫物种在广泛的实验pH范围内进行校准；(2)在广泛的温度范围内但恒定的pH条件下生长浮游有孔虫；(3)在恒定的pH条件下，在广泛的钙浓度和饱和状态下生长浮游有孔虫。(4)建立了模拟古新世海水条件下海水钙浓度高于现代海洋，而镁、硼浓度显著低于现代海洋的pH校准。这项研究将为硼同位素替代物的应用提供基本限制，并改进从更新世到早更新世的表层海洋pH和大气二氧化碳的重建。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.