Collaborative Research: Did the SE Pacific Gyre become a Hot Spot for N2 Fixation during Dusty Glacial Conditions?

氮元素是所有生物的基本组成部分，其在环境中的循环是地球生物圈的重要组成部分。 作为一种重要的营养素，氮以生物可用形式的可用性通常“限制”陆地和海洋中植物的生长。 特别地，氮作为二氮气体（N2）在地球大气中和溶解在海水中都非常丰富。 然而，在这种化学形式中，氮不能被大多数生物利用。 只有一小部分微生物具有“固定”N2气体的能力，即将其转化为生物可用的化学形式。 因此，这些固氮生物提供了维持地球上生命的关键环境功能。 在海洋中，N2固定是生物可利用氮总量的主要控制因素，随着时间的推移，平衡损失的N2气体。 生物可利用氮的数量反过来又控制着海洋表层阳光照射区域光合生物（浮游植物）的生长（生产力），而光合生物是食物链的基础，有助于海洋控制大气中的温室气体水平。该项目关注的是了解海洋N2固定的基本，大规模控制以及它们如何随着时间的推移受到气候变化的影响。 固氮微生物本身似乎受到其他营养元素（如磷和铁）的限制。 虽然人们知道，目前在海洋的哪些部分，磷和铁的可用性更高或更低，但仍然不清楚是否有任何一个是压倒一切的重要性，或者过去的变化是否导致了N2固定的重大变化。 过去的N2固定变化可能是海洋控制大气温室气体的重要反馈。 了解这些过去的变化及其控制将提供知识基础，以改善海洋N2固定如何应对未来气候变化的预测。这是一个重大的社会意义，因为海洋N2固定的变化将最终影响海洋生态系统和生物资源，作为对温室气体驱动气候变化的反馈。为了解决这些问题，研究团队将在末次冰期循环期间对东南太平洋环流N2固定的气候敏感性及其合理的“主控制”进行研究。这个贫营养区经历很少的现代固氮，尽管接近大量供应过剩的磷酸盐从相邻的秘鲁-智利氧气最小区。这与现代铁的限制是一致的，因为在过去的多尘条件下，低风沙供应将得到缓解。研究小组将利用最后一个完整的冰川周期的自然实验，在有孔虫结合的N同位素的环流网站以及网站在其南部边缘，探索控制海洋N循环所施加的可变灰尘输入和变化的N-损失在邻近的氧气最低区，并将这些与已知的温室气候强迫的变化。通过数值模拟，研究小组还将考虑该地区过去N2固定的变化是否可能影响全球海洋N循环和预算。该项目还将资助本科生和研究生的培训，并支持代表性不足群体的高中生参与原创研究。 研究团队将通过与小学，初中和高中建立伙伴关系，继续他们的外联工作，让不同的学校人口及其家庭参与令人兴奋的相关科学。

The element nitrogen is a fundamental component of all living things and its cycling through the environment is an important component of Earth's biosphere. As a vital nutrient, the availability of nitrogen in a biologically usable form often "limits" the growth of plants both on land as well as in the ocean. Paradoxically, nitrogen is very abundant as dinitrogen gas (N2) in both the Earth's atmosphere and dissolved in seawater. However in this chemical form, nitrogen cannot be used by most living things. Only a small subset of microbes has the ability to "fix" N2 gas, that is, to convert it into a biologically usable chemical form. Thus, these N2 fixing organisms provide a critical environmental function sustaining life on this planet. In the ocean, N2 fixation is a major control on the total amount of biologically available nitrogen, balancing over the time losses back to N2 gas. The amount of biologically available nitrogen in turn controls the growth (productivity) of photosynthetic organisms (phytoplankton) in the sunlit region of the surface ocean which form the base of the food chain and contribute to oceanic control of the atmospheric levels of greenhouse gases. This project concerns itself with understanding the fundamental, large-scale controls of oceanic N2 fixation and how they are influenced by climate change over time. N2 fixing microbes themselves appeared to be limited by the availability of other nutrient elements such as phosphorous and iron. While it is known in which parts of the ocean there is at present greater or lesser availability of phosphorous and iron, it remains unclear if either is of overriding importance or if changes in the past produced significant variations in N2 fixation. Past changes in N2 fixation may have been an important feedback on oceanic control of atmospheric greenhouse gases. Understanding these past changes and their controls will provide the knowledge base for improving prediction of how ocean N2 fixation may respond to future changes in climate. This is of great societal relevance as changes in oceanic N2 fixation will ultimately impact marine ecosystems and living resources as feedback on the greenhouse gases driving climate change.To address these questions, the research team will undertake a study the climate-sensitivity of N2 fixation in the southeast Pacific gyre over the last glacial cycle as well as its plausible "master controls". This oligotrophic region experiences little modern N2 fixation despite proximity to a large supply of excess phosphate from the adjacent Peru-Chile oxygen minimum zone. This is consistent with modern iron limitation due to low aeolian supply that would have been relieved during past dusty conditions. The research team will use the natural experiment of the last full glacial cycle, captured in the foraminiferal-bound N isotopes of gyre sites as well as sites at its southern margin, to probe controls on the marine N cycle exerted by variable dust inputs and changes in N-loss in the adjacent oxygen minimum zone, and relate these to known changes in greenhouse forcing of climate. Through numerical modeling, the research team will also consider whether past variations in N2 fixation in this region may have impacted the global ocean N cycle and budget. This project will also fund the training of undergraduate and graduate students and support participation of high school students from underrepresented groups in original research. The research team will continue their outreach efforts through established partnerships with elementary, middle, and high schools, engaging a diverse school population and their families with exciting and relevant science.