Low temperature hydrothermal vent fluxes as traced by radium isotopes

铁是大洋中脊释放出的流体中含量最丰富的元素之一，但人们一直认为，铁主要是从靠近其热液喷口源头的海水中去除的。最近出现的证据表明，这种从热液中提取的铁可以在海洋中被远距离输送，包括向阳光充足的表层输送，在那里它是浮游植物生长所必需的营养物质。这项研究将使用自然产生的镭同位素作为深海热液中铁的示踪剂，这种同位素也在热液流体中富含，但与铁相比不具反应性。镭同位素的半衰期从几天到几个世纪不等，可以用来作为“时钟”来测量洋流将热液铁带到大洋内部的精确速率。这些信息可以纳入旨在确定热液铁对海洋生产力的作用的海洋模型，由于热液铁在调节地球大气中的二氧化碳浓度方面的作用，因此具有社会意义。该项目将通过伍兹霍尔教育伙伴计划为两名学生提供研究机会，该计划旨在通过邀请大学三年级和大四学生通过夏季课堂学习和实践研究活动获得实践经验，从而增加海洋和环境科学的多样性。伍兹霍尔海洋研究所的一名科学家将量化一个主要海洋扩展中心上方铁(Fe)的运移速率，以评估热液喷口在向表层海洋供应这种基本微量营养素方面的作用。将特别注意低温热液铁通量的作用，假定这些铁通量以稳定的形式到达海底，从而减少在运输过程中的大量清除。如果是真的，低温流体可能会提供不成比例的“可运输”溶解铁，这是在大规模的深海羽流中没有观察到的。将通过同时测量Fe浓度和形态以及沿东太平洋隆起南部(EPR；南纬15-18度)探险期间的镭(Ra)“四重奏”(224Ra、223Ra、228Ra、226Ra)来完成对过程速率的评估。将使用自主水下航行器和拖曳CTD测量来确定低温和高温排放的位置。在选定的目标区域，调查人员将检验这样的假设，即低温热液输入具有明显的Re同位素比值指纹，以及低温来源中铁清除的相对速率较低。鉴于其广泛的半衰期，Ra同位素具有在生物地球化学相关的时间尺度上整合深海中铁的独特能力。同时进行的Ra同位素和总溶解铁测量的结合将使研究人员能够量化在大的水平尺度上传输的任何铁相的铁停留时间，正如先前沿着EPR观察到的那样。总而言之，Ra测量中的铁损失和迁移将导致海洋环流和生物地球化学模型预测能力的提高，这些模型评估热液铁在支持上覆表层水的初级生产力和碳吸收方面的重要性。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Iron is one of the most abundant elements in fluids released from mid-ocean ridges, yet it has been thought to be mostly removed from seawater near to its hydrothermal vent source. Recent evidence has emerged showing this hydrothermally-derived iron can be transported great distances in the ocean, including toward the sunlit surface layer where it is an essential nutrient for phytoplankton growth. This study will use naturally-occurring radium isotopes, which are also enriched in hydrothermal fluids but are non-reactive compared to iron, as tracers of hydrothermal iron in the deep ocean. With a range of half-lives from days to centuries, radium isotopes can be used as "clocks" to measure the precise rates at which hydrothermal iron is carried by ocean currents toward the ocean interior. Such information can be incorporated into ocean models designed to determine the role of hydrothermal iron on ocean productivity, which has societal relevance due to its role in regulating the carbon dioxide concentrations of Earth's atmosphere. This project will support research opportunities for two students through the Woods Hole Partnership in Education Program, which seeks to increase diversity in ocean and environmental science by inviting college juniors and seniors to gain practical experience through a summer of classroom study and hands-on research activities.A scientist from Woods Hole Oceanographic Institution will quantify rates of iron (Fe) transport above a major ocean spreading center to evaluate the role of hydrothermal venting in supplying this essential micronutrient to the surface ocean. Special attention will be paid to the role of low-temperature hydrothermal Fe fluxes, which are hypothesized to arrive at the seafloor in a stable form that mitigates significant removal during transport. If true, low-temperature fluids may supply a disproportionate amount of the "transportable" dissolved Fe that has not been observed in large-scale deep ocean plumes. Assessment of process rates will be accomplished through concurrent measurements of Fe concentration and speciation and the radium (Ra) "quartet" (224Ra, 223Ra, 228Ra, 226Ra) during an expedition along the southern East Pacific Rise (EPR; 15-18 degrees South). Autonomous underwater vehicle and towed-CTD surveys will be used to identify locations of low- and high-temperature discharge. At selected target areas, the investigators will test the hypotheses that low-temperature hydrothermal inputs have a distinct radium isotopic ratio fingerprint, and that the relative rate of Fe scavenging removal is lower in low-temperature sources. Given their wide ranging half-lives, Ra isotopes have the unique ability to integrate over biogeochemically-relevant time scales of Fe transport in the deep ocean. The combination of concurrent Ra isotope and total dissolved Fe measurements will allow the investigators to quantify Fe residence time for any Fe phase that is transported on large horizontal scales, as has been observed previously along the EPR. Together, Fe loss and transport from Ra measurements will lead to improvements in the predictive capabilities of ocean circulation and biogeochemistry models that assess the importance of hydrothermal Fe in supporting primary productivity and carbon drawdown in overlying surface waters.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.