The Nitrogen Isotopic Composition of Diatom Resting Spores in Southern Ocean Sediments: A Source of Bias and/or Paleoenvironmental Information?

南大洋硅藻化石的化学成分提供了有关南极洲环境史的信息，包括海冰范围、生物生产和海洋营养分布。海冰区是一群硅藻的重要栖息地，这些硅藻主要来自毛藻属，它们在环境压力下有一个独特的生命周期阶段，当它们产生一种叫做静止孢子的结构时。休眠的孢子是为了在条件更有利的时候在海洋表面重新播种。这些沉重的休眠孢子的产生往往会从海洋表面带走大量的碳和硅，这是必需的营养物质。因此，这个群体有可能从海洋表面去除碳，并可能影响科学家用来重建环境变化的沉积记录。该项目利用这些化石的氮同位素特征来探索静止孢子在沉积记录中的作用，以及如何使用这些测量来估计碳循环变化。这项工作将包括这些有机体的实验室孵化，以回答休眠孢子的化学成分是否以及如何与典型的硅藻细胞不同。孵育结果将用于评估在地球历史上的两个不同时期，即最后一个冰川期和温暖的上新世，海冰环境中的营养物质减少。这项工作将对科学界如何考虑南大洋季节性海冰覆盖对全球气候的响应和调节的影响产生重大影响。该项目为本科生和研究生提供培训和研究机会。正在进行的南极地球科学研究工作将通过本国机构的互动式展览进行传播。本文提出的工作将解决以下方面的不确定性：毛囊动物静止孢子如何在其氮稳定同位素组成中记录表面营养条件，它们的特定信号对整个沉积组合的相对影响，以及它们对海冰带环境重建的偏见或增强的潜力。对实验室中生长的单个物种的硝酸盐、生物量、硅藻结合氮和硅氮比的氮稳定同位素的测量将用于量化静止孢子如何记录水柱中的营养减少，以及它们的特征在多大程度上偏向于低营养条件。这些关系将用于硅藻结合氮同位素重建上新世沿海多聚藻和跨越末次盛冰期的开放海洋核心的营养流失。该提案利用了2017年为拟议的培养工作收集的南大洋毛羽类分离物的可用性，以及存档的沉积物岩心和/或现有数据。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。

The chemical composition of diatom fossils in the Southern Ocean provides information about the environmental history of Antarctica, including sea ice extent, biological production, and ocean nutrient distribution. The sea ice zone is an important habitat for a group of diatoms, largely from the genus Chaetoceros, that have a unique life cycle stage under environmental stress, when they produce a structure called a resting spore. Resting spores are meant to reseed the surface ocean when conditions are more favorable. The production of these heavy resting spores tends to remove significant amounts of carbon and silicon, essential nutrients, out of the surface ocean. As a result, this group has the potential to remove carbon from the surface ocean and can impact the sedimentary record scientists use to reconstruct environmental change. This project explores the role of resting spores in the sedimentary record using the nitrogen isotopic signature of these fossils and how those measurements are used to estimate carbon cycle changes. The work will include laboratory incubations of these organisms to answer if and how the chemistry of the resting spores differs from that of a typical diatom cell. The incubation results will be used to evaluate nutrient drawdown in sea ice environments during two contrasting intervals in earth history, the last ice age and the warm Pliocene. This work should have significant impact on how the scientific community considers the impact of seasonal sea ice cover in the Southern Ocean in terms of how it responds to and regulates global climate. The project provides training and research opportunities for undergraduate and graduate students. Ongoing research efforts in Antarctic earth sciences will be disseminated through an interactive display at the home institution.The work proposed here will address uncertainties in how Chaetoceros resting spores record surface nutrient conditions in their nitrogen stable isotopic composition, the relative impact of their specific signal with respect to the full sedimentary assemblage, and their potential to bias or enhance environmental reconstructions in the sea ice zone. Measurements of nitrogen stable isotopes of nitrate, biomass, and diatom-bound nitrogen and silicon-to-nitrogen ratios of individual species grown in the laboratory will be used to quantify how resting spores record nutrient drawdown in the water column and to what degree their signature is biased toward low nutrient conditions. These relationships will be used to inform diatom-bound nitrogen isotope reconstructions of nutrient drawdown from a Pliocene coastal polyna and an open ocean core that spans the last glacial maximum. This proposal capitalizes on the availability of Southern Ocean isolates of Chaetoceros spp. collected in 2017 for the proposed culture work and archived sediment cores and/or existing data.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.