Low temperature hydrothermal vent fluxes as traced by radium isotopes

铁是海洋中脊释放出的流体中最丰富的元素之一，但人们一直认为，铁主要是从热液喷口附近的海水中提取出来的。最近出现的证据表明，这种源自水热的铁可以在海洋中长距离运输，包括向阳光照射的表层运输，在那里它是浮游植物生长的必需营养物质。这项研究将使用天然存在的镭同位素作为深海热液铁的示踪剂，这种同位素也富含热液流体，但与铁相比没有反应性。镭同位素的半衰期从几天到几个世纪不等，可以作为“时钟”来测量热液中铁被洋流带到海洋内部的精确速率。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，由于其在调节地球大气二氧化碳浓度方面的作用，海洋生产力具有社会意义。该项目将通过伍兹霍尔教育合作计划为两名学生提供研究机会，该计划旨在通过邀请大学大三和大四学生通过暑期课堂学习和动手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散中心上方的铁（Fe）运输速率，以评估热液喷口在向海洋表面提供这种必需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，这些通量被假设以稳定的形式到达海底，从而减轻了运输过程中显著的去除。如果这是真的，低温流体可能会提供不成比例的“可运输的”溶解铁，这在大规模的深海羽流中是没有观察到的。在沿南东太平洋隆起（EPR；南纬15-18度）进行考察期间，将通过同时测量铁浓度和形态以及镭（Ra）“四重奏”（224Ra, 223Ra, 228Ra, 226Ra）来完成对过程速率的评估。自主水下航行器和拖曳ctd测量将用于确定低温和高温排放的位置。在选定的目标区域，研究人员将测试低温热液输入具有明显的镭同位素比值指纹的假设，以及低温源中Fe清除的相对速率较低的假设。由于其广泛的半衰期，Ra同位素具有独特的能力，可以在生物地球化学相关的时间尺度上整合铁在深海中的运输。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水平尺度上运输的任何铁相的铁停留时间，就像之前沿着EPR观察到的那样。总之，来自Ra测量的铁损失和输运将导致海洋环流和生物地球化学模型的预测能力的改进，这些模型可以评估热液铁在支持上覆地表水初级生产力和碳减少方面的重要性。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。

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.