Collaborative Research: Are Low-Temperature Hydrothermal Vents an Important but Overlooked Source of Stabilized Dissolved Iron to the Ocean?

热液喷口是海底的温泉。它们通常位于海洋表面以下数千米，沿着被称为海洋中脊的大型火山山脉。人们对这些热液喷口的了解相对较少，因为它们离海洋表面很远。这些喷口不是熔岩，而是向海洋排放温暖到热的液体（从几摄氏度到350摄氏度），并含有高浓度的各种溶解化学物质。其中一种化学物质，铁，已经被海洋科学家广泛研究，因为它作为一种重要的营养物质，对生长在海洋表面水域的微小光合藻类（称为浮游植物）至关重要。科学家们曾经认为，由于热喷口流体和冷海水之间的化学反应，从热喷口流体中流出的大部分溶解的铁会在喷口附近迅速沉淀（形成固体并下沉），因此只有少量溶解的铁可以从深海中洋脊运送到浮游植物生长的表层水域。然而，在最近一次对最活跃的大洋中脊之一——南东太平洋隆起（SEPR）进行的科学考察中，科学家们发现，从南东太平洋隆起的热液喷口排出的大部分溶解的铁实际上被输送到数千公里外的深海。此外，计算机模拟表明，大量的这种？水热铁吗?被运送到南极洲周围的南大洋表层，在那里它可以支持浮游植物的生长。来自华盛顿大学、伍兹霍尔海洋研究所和老道明大学的科学家们将乘坐一艘研究船返回SEPR，进行额外的测量，以更好地了解热液喷口如何将溶解的铁添加到深海中。他们将使用专门的采样设备，包括无人驾驶的微型潜艇，称为自主水下航行器（auv），来绘制该地区热液喷口的分布，并从高温（250-350ºC）和低温（150ºC）喷口收集水样。这些样品将被分析，以寻找热液喷口流体中富含的化学物质，如铁、锰和氦-3，这将提供热液喷口流体与周围海水混合并从SEPR带走时发生的化学变化的信息。科学家们将利用他们的观察和化学分析来解决以下主要问题：1)海洋中脊的热液喷口是否比之前认为的要多？(2)低温热液喷口是否特别有利于稳定溶解的铁，并最终将其输送到支持浮游植物生长的海洋表面？该项目将为一名研究生和两名大四本科生的教育和培训做出贡献，其成果将被纳入高中STEM课程。研究活动将通过公开演讲和媒体发布来传播，项目结果将通过会议报告、同行评议的科学期刊出版物和免费提供的在线存档数据向科学界传播。沿着数千公里的海底火山链，即大洋中脊，热液喷口将热的、酸性的、富含金属的液体排放到深海中。这些海底温泉是固体地球和深海之间热和化学物质交换的主要通道，在过去的40年里得到了深入的科学研究。这项研究绝大多数集中在高温（~200-350°C）热液喷口。低温（150°C）热液喷口受到的关注相对较少，尽管最近的观测和模拟研究结果表明，沿洋中脊有更丰富的低温排放物，其对元素和化合物输入深海的潜在重要性。在热液喷口流体中富集的元素中，铁作为表层海洋初级生产的必需营养物而受到关注。长期以来，人们一直认为，热液喷口排出的大部分溶解铁是从靠近洋中脊源的溶液中流失的，因此对海洋化学的重要性有限。但这一观点受到了最近研究的挑战，这些研究表明，热液溶解铁的化学稳定性可能有助于其在深海中的远程运输。特别是，来自美国GEOTRACES项目的结果揭示了热液溶解铁的羽流从东太平洋隆起（SEPR）南部的源区向西移动了数千公里。在这个项目中，来自华盛顿大学、伍兹霍尔海洋研究所和老道明大学的研究人员将对SEPR进行实地研究计划，以验证弥漫性低温热液喷口是向深海提供化学稳定的溶解铁的主要来源，因此在海洋铁循环中起着重要但以前被忽视的作用。基于船舶的现场项目将使用配备原位光学和化学传感器的自主水下航行器来识别沿SEPR扩散（低温）和离散（高温）热液排放的羽流。来自多个低温和高温热液喷口的排放物将被取样，用于船上和巡航后对铁和其他微量金属以及惰性热液“示踪剂”氦-3的分析。所得数据将用于验证两个具体假设：(1)沿洋中脊的活跃热液排放点的数量比先前估计的要多；(2)低温热液排放有利于溶解铁的化学稳定，从而促进其出口到海洋内部并最终出口到海洋表面，在那里它支持浮游植物的初级生产。这项研究获得的信息将有助于将铁和其他微量元素纳入海洋生物学和生物地球化学的数值模型，这将提高预测海洋如何响应和调节未来气候和环境变化的能力。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。

Hydrothermal vents are hot springs on the seafloor. They are often thousands of meters below the surface of the ocean along the large volcanic mountain ranges called the mid-ocean ridges. Relatively little is known about these hydrothermal vents because they exist so far from the ocean surface. Instead of lava, these vents emit warm to hot fluids (from several ºC up to 350 ºC) into the ocean, and contain high concentrations of a wide range of dissolved chemicals. One of these chemicals, iron, has been studied extensively by ocean scientists because of its importance as an essential nutrient for the microscopic photosynthetic algae called phytoplankton that grow in surface ocean waters. Scientists once thought that most of the dissolved iron coming out of hydrothermal vent fluids would quickly precipitate (form a solid and sink) near the vents, as a result of chemical reactions between the hot vent fluid and cold seawater, and therefore only small amounts of dissolved iron could be transported from the deep sea mid-ocean ridges to surface waters where phytoplankton grow. However, on a recent scientific expedition over one of the most active mid-ocean ridges called the southern East Pacific Rise (SEPR), scientists discovered that much of the dissolved iron discharged from the SEPR hydrothermal vents was in fact transported thousands of kilometers across the deep ocean. In addition, computer simulations suggest that a substantial amount of this ?hydrothermal iron? is transported as far as the surface waters of the Southern Ocean around Antarctica, where it could support the growth of phytoplankton.Scientists from the University of Washington, Woods Hole Oceanographic Institution and Old Dominion University will return to the SEPR aboard a research ship and make additional measurements to better understand how hydrothermal vents add dissolved iron to the deep ocean. They will use specialized sampling equipment, including unmanned miniature submarines called autonomous underwater vehicles (AUVs), to map the distribution of hydrothermal vents in this region, and collect water samples from both high temperature (250-350 ºC) and low temperature ( 150 ºC) vents. These samples will be analyzed for chemicals that are enriched in the hydrothermal vent fluids, such as iron, manganese and helium-3, which will provide information on the chemical changes that occur as the hydrothermal vent fluids mix with surrounding seawater and are carried away from the SEPR. The scientists will use their observations and chemical analyses to address the following major questions: 1) are there more hydrothermal vents along mid ocean ridges than previously thought? and (2) are low temperature hydrothermal vents especially favorable for stabilizing dissolved iron, and ultimately transporting it to the surface ocean where it supports phytoplankton growth? The project will contribute to the education and training of a graduate student and two senior undergraduate students, and outcomes will be incorporated into high school STEM curricula. The research activities will be communicated via public lectures and media releases, and project results will be disseminated to the scientific community by conference presentations, publications in peer-reviewed scientific journals, and freely available on-line archived data.Along thousands of kilometers of the seafloor volcanic chains known as the mid-ocean ridges, hydrothermal vents discharge hot, acidic, metal-rich fluids into the deep ocean. These submarine hot springs are a major gateway for the exchange of heat and chemicals between the solid earth and the deep ocean, and have received intensive scientific study during the last 40 years. This research has overwhelmingly focused on high temperature (~200-350°C) hydrothermal vents. Lower temperature (150°C) hydrothermal venting has received relatively little attention, although results from recent observational and modeling studies point to the greater abundance of low temperature discharge along the mid-ocean ridges, and its potential importance for the input of elements and chemical compounds into the deep ocean. Among the elements that are enriched in hydrothermal vent fluids, iron has received attention because of its role as an essential nutrient for primary production in the surface ocean. It has long been thought that most of the dissolved iron discharged by hydrothermal vents is lost from solution close to mid-ocean ridge sources, and thus of limited importance for ocean chemistry. But this view has been challenged by recent studies which suggest that chemical stabilization of hydrothermal dissolved iron may facilitate its long-range transport in the deep ocean. In particular, results from the US GEOTRACES program have revealed the lateral transport of a plume of hydrothermal dissolved iron over several thousand kilometers westward from its source region on the southern East Pacific Rise (SEPR).In this project, investigators from the University of Washington, Woods Hole Oceanographic Institution and Old Dominion University will carry out a field research program on the SEPR to test the overarching hypothesis that diffuse low-temperature hydrothermal venting is a major a source of chemically-stabilized dissolved iron to the deep ocean, and hence plays an important but previously overlooked role in the ocean iron cycle. The ship-based field program will use an autonomous underwater vehicle equipped with in-situ optical and chemical sensors to identify plumes of both diffuse (low-temperature) and discrete (high-temperature) hydrothermal discharge along the SEPR. Emissions from multiple sites of both low and high temperature hydrothermal venting will be sampled for shipboard and post-cruise analyses of iron and other trace metals, and the inert hydrothermal "tracer" helium-3. The resulting data will be used to test two specific hypotheses: (1) the population of active hydrothermal discharge sites along mid ocean ridges is greater than previously estimated, (2) low temperature hydrothermal venting is conducive to the chemical stabilization of dissolved iron, thus facilitating its export to the ocean interior and ultimately to surface ocean where it supports primary production by phytoplankton. The information obtained in this research will facilitate inclusion of iron and other trace elements in numerical models of ocean biology and biogeochemistry, which will improve the ability to predict how the ocean will respond to and modulate future climatic and environmental changes.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.