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

浮游有孔虫中硼同位素代理的实验细化 - 温度、非共生敏感性和海水元素组成

• 批准号: 2103461
• 负责人: Baerbel Hoenisch
• 金额: $ 65.84万
• 依托单位: Columbia University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-08-15 至 2025-07-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2103461&HistoricalAwards=false
• 关键词: Experimental; refinement; boron; isotope; proxy

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.

将利用培养实验来研究浮游有孔虫的壳成分。浮游有孔虫是生活在海洋表面的单细胞生物。科学家们利用有孔虫外壳化石的化学成分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，硼同位素可能受到其他因素的影响，包括温度和海水钙浓度。拟议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支持一名研究生和两名本科生助手。PI将试图从地球科学领域代表性不足的群体中招募本科生。 南极有孔虫壳中的硼同位素代用指标是重建表层海水酸度的最成功的工具之一，并通过推断大气CO2。一个强有力的经验框架已经建立了这个代理，但是，仍然存在一些未知因素，影响的确定性与古pH值和古CO2可以重建从这个代理。具体而言，温度对溶解的硼酸根离子和硼酸之间的水硼同位素分馏的影响已被建议的热力学原理和建模和几条线的证据表明，这种影响表现在南极有孔虫的硼同位素组成，但缺乏坚实的实验证据。如果这种效应存在，它可以解释硼替代敏感性之间的差异表面和深居有孔虫物种。解释物种差异的另一种假设是基于这样的观察，即实验室培养实验在比天然海洋更宽的pH值和方解石饱和状态范围内进行，这可能会影响有孔虫的生长速率。建议进行以下实验来解决这些不确定性：（1）建立了在宽范围的实验pH下共生体贫瘠的有孔虫物种的第一个实验室校准，（2）在宽范围的温度但恒定的pH下生长浮游有孔虫，（3）在恒定pH下，在宽范围的钙浓度和饱和状态下生长浮游有孔虫。在模拟古新世海水条件下进行校准，当时海水中钙浓度高于现代海洋，但镁和硼浓度显著降低。这项研究将提供硼同位素代理应用的基本限制，并改善海洋表面pH值和大气CO2的重建，从更新世到新生代早期。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。