Quantifying Iron Turnover in the Upper Ocean via Time-series Measurements at Station ALOHA

浮游植物是海洋食物网的基础，但它们在开放海洋中通过光合作用生长的能力受到关键营养物质（尤其是铁）的缺乏的限制。为了了解浮游植物对全球环境变化的反应，预测海水的营养成分将如何变化是至关重要的。铁对浮游植物的光收集机制至关重要，但它在海水中所占的比例极低（十亿分之一）。土壤中的铁含量要丰富得多，当沙尘暴把这些土壤吹到海里时，海水中的铁含量就会增加。目前尚不清楚这些铁供应事件的影响会持续多久，这取决于海洋生态系统在铁沉入海底之前恢复和再利用铁的能力。人类活动是否增加了天然铁的供应也不得而知。拟议的研究将通过在北太平洋进行为期3年的铁测量来解决这些问题。在这里，来自亚洲的灰尘主要出现在春季，因此可以记录夏秋两个月的铁流失情况。将使用独特的化学特征来区分铁供应与沙漠尘埃沉积或人类来源。海洋铁循环的这一记录对于验证用于预测未来气候变化将如何影响浮游植物生长的生态系统模型非常重要。这项研究将对夏威夷海洋时间序列做出科学贡献，帮助改进生物地球化学铁模型，在研究生和本科生水平上培训学生，并支持早期职业科学家。提出了一个3年的铁（Fe）测量时间序列，以约束外部铁输入和铁在公海的再循环的大小。近一个月的观测将在夏威夷海洋时间系列游轮上的北太平洋亚热带环流中进行，该环流在春季定期接收灰尘输入，并且受深度混合的影响最小。溶铁和颗粒铁浓度的水柱剖面——结合在痕量金属清洁沉积物捕集器中记录的铁通量——将确定铁在上层水柱中的停留时间。铁摄取率将通过使用一种新的稳定同位素技术的短期孵育来量化，并将用于推导关于生物摄取的周转时间。最后，将测量混合层中溶解铁和颗粒铁的同位素组成，以评估人为和夏威夷铁来源的潜在重要性，这些来源的约束很差。总之，这些测量将确定开放海洋铁循环的速度和可变性，并提供一种方法来验证模拟这种关键微量营养素的生物地球化学模型。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。

Phytoplankton are the base of marine food webs but their ability to grow in the open ocean by photosynthesis is limited by the scarcity of key nutrients especially iron. To understand how phytoplankton respond to global environmental changes, it is essential to predict how the nutrient content of seawater will change as well. Iron is essential to the light-harvesting machinery of phytoplankton but is an extremely small fraction of seawater (1 part per billion) . Iron is much more abundant in soils and when dust storms blow these soils out to sea, the iron content of seawater increases. It is unknown how long the effects of these iron supply events last, which depends on how well the marine ecosystem can recover and reuse iron before it sinks to the seafloor. It is also unknown if human activities have added to the natural Fe supply. The proposed research will address these questions by conducting a 3 year time-series of iron measurements in the North Pacific Ocean. Here, dust supply from Asia occurs mainly during spring, allowing the loss of iron over the summer and fall months to be documented. Unique chemical signatures will be used to distinguish iron supply from the deposition of desert dust or from human sources. This record of the marine iron cycle will be important for validating ecosystem models that are used to predict how climate change will influence the growth of phytoplankton in the future. The research would make a scientific contribution to the Hawaii Ocean Time-Series, help improve biogeochemical iron models, student training at the graduate and undergraduate level, and support an early career scientist. A 3 year time-series of iron (Fe) measurements is proposed to constrain the magnitude of external Fe input and Fe recycling in the open ocean. Near-monthly observations will be conducted in the North Pacific Subtropical Gyre onboard Hawaii Ocean Timeseries cruises, which receives regular dust input during springtime and is minimally influenced by deep mixing. Water column profiling of dissolved and particulate Fe concentrations – combined with the flux of Fe recorded in trace-metal-clean sediment traps – will define a residence time of Fe in the upper water column. Iron uptake rates will be quantified through short-term incubations using a novel stable isotope technique and will be used to derive a turnover time with respect to biological uptake. Finally, the isotopic composition of dissolved and particulate Fe in the mixed layer will be measured to evaluate the potential importance of anthropogenic and Hawaiian Fe sources, which are poorly constrained. Together, these measurements will define the tempo and variability of the open ocean Fe cycle and provide a means to validate models that simulate the biogeochemistry of this key micronutrient.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.