Collaborative Proposal: A Field and Laboratory Examination of the Diatom N and Si Isotope Proxies: Implications for Assessing the Southern Ocean Biological Pump

合作建议：硅藻N和Si同位素代用物的实地和实验室研究：对评估南大洋生物泵的影响大气中二氧化碳浓度的上升和相关的气候变化使得了解海洋在大规模碳循环中的作用成为当务之急。地质样本可以通过使用地球化学代用物来探索冰期二氧化碳减少的潜在机制。来自化石硅藻（微观植物）的氮和硅同位素特征被用于研究南大洋中营养物（如氮、硅和碳）的物理供应和生物需求的变化。该项目将通过一系列实验室实验和南大洋实地取样来评估氮和硅同位素替代指标的使用。研究结果将量化南大洋中氮和硅同位素与营养物质使用之间的真实关系，并允许探索其他因素的作用，包括生物多样性、冰盖和混合，在改变硅藻记录的化学特征方面。海底沉积物样本将被用来评估埋在海底的硅藻化石记录水柱中产生的信号的效果。改进营养物同位素代用指标将使我们能够更定量地了解极地生物学在调节大气二氧化碳自然变化中的作用。该项目还将培训一名研究生，并编制面向广大大众的外联材料。该项目旨在测试硅藻氮和硅同位素代用物的保真度，这是两种常用的古海洋学工具，用于研究南大洋生物泵在冰期-间冰期时间尺度上调节大气CO2浓度的作用。现有的地面真实数据，包括培养实验、表面沉积物数据和岩心重建，都表明养分利用是南大洋同位素变化的主要驱动因素。然而，最近的培养结果暗示了种间变异的强烈贡献。此外，野外和实验室研究在种间变化的相对重要性和推断的营养同位素信号的沉积后变化方面提出了一些相互矛盾的结果。本文提出了氮和硅硅藻营养同位素古代用物的一级测试方法，包括水柱溶解和颗粒取样以及野外分离物的实验室培养。对南大洋水体、生物量、活硅藻和化石硅藻进行采样，研究硅藻氮、硅同位素的物种和组合变异及其与表层营养场和早期成岩作用的关系。获取在与主要感兴趣的沉积物相似的环境背景下产生的新鲜物质对于进行可靠的古海洋学重建至关重要。野外采样将沿着175°W进行，横贯南极环极流从亚热带到边缘冰缘。将使用从13个站点收集的水、下沉/悬浮颗粒和多核样品以及3个船上孵育实验来检验提出的四种假设，这些假设共同评估现有培养结果的重要性，并寻求在评估生物泵时最佳地利用硅藻营养同位素代用物。

Collaborative Proposal: A field and laboratory examination of the diatom N and Si isotope proxies: Implications for assessing the Southern Ocean biological pumpThe rise in atmospheric carbon dioxide concentrations and associated climate changes make understanding the role of the ocean in large scale carbon cycle a priority. Geologic samples allow exploration of potential mechanisms for carbon dioxide drawdown during glacial periods through the use of geochemical proxies. Nitrogen and silicon isotope signatures from fossil diatoms (microscopic plants) are used to investigate changes in the physical supply and biological demand for nutrients (like nitrogen and silicon and carbon) in the Southern Ocean. The project will evaluate the use the nitrogen and silicon isotope proxies through a series of laboratory experiments and Southern Ocean field sampling. The results will provide quantification of real relationships between nitrogen and silicon isotopes and nutrient usage in the Southern Ocean and allow exploration of the role of other factors, including biological diversity, ice cover, and mixing, in altering the chemical signatures recorded by diatoms. Seafloor sediment samples will be used to evaluate how well the signal created in the water column is recorded by fossil diatoms buried in the seafloor. Improving the nutrient isotope proxies will allow for a more quantitative understanding of the role of polar biology in regulating natural variation in atmospheric carbon dioxide. The project will also result in the training of a graduate student and development of outreach materials targeting a broad popular audience.This project seeks to test the fidelity of the diatom nitrogen and silicon isotope proxies, two commonly used paleoceanographic tools for investigating the role of the Southern Ocean biological pump in regulating atmospheric CO2 concentrations on glacial-interglacial timescales. Existing ground-truthing data, including culture experiments, surface sediment data and downcore reconstructions, all suggest that nutrient utilization is the primary driver of isotopic variation in the Southern Ocean. However, strong contribution of interspecific variation is implied by recent culture results. Moreover, field and laboratory studies present some contradictory results in terms of the relative importance of interspecific variation and of inferred post-depositional alteration of the nutrient isotope signals. Here, a first order test of the N and Si diatom nutrient isotope paleo-proxies, involving water column dissolved and particulate sampling and laboratory culturing of field-isolates, is proposed. Southern Ocean water, biomass, live diatoms and fossil diatom sampling will be conducted to investigate species and assemblage related variability in diatom nitrogen and silicon isotopes and their relationship to surface nutrient fields and early diagenesis. Access to fresh materials produced in an analogous environmental context to the sediments of primary interest is critical for making robust paleoceanographic reconstructions. Field sampling will occur along 175°W, transecting the Antarctic Circumpolar Current from the subtropics to the marginal ice edge. Collection of water, sinking/suspended particles and multi-core samples from 13 stations and 3 shipboard incubation experiments will be used to test four proposed hypotheses that together evaluate the significance of existing culture results and seek to allow the best use of diatom nutrient isotope proxies in evaluating the biological pump.