The impact of Mid-Ocean Ridges on the Ocean's Iron cycle

海洋浮游植物的光合作用为较高营养水平(如鱼类和海洋哺乳动物)提供能量，并有助于二氧化碳在大气和海洋之间的分配。铁是浮游植物生长所必需的，因为它是参与光合作用和呼吸作用的许多重要酶所必需的。与陆地系统相比，铁在公海中的浓度非常低(每10亿个水分子中不到1个铁原子)。因此，海洋大部分地区的浮游植物光合作用受到铁的限制。这种铁缺乏对地球系统有重要影响，因为浮游植物光合作用是海洋调节全球气候的重要手段。大洋中脊是铁的重要来源，据估计，来自大洋中脊的铁占全球大洋铁储量的25%-75%。这些大洋中脊是深海山脉，它们构成了贯穿世界大洋的单一全球大洋中脊系统，使其成为世界上最长的山脉。在这些山脊，新的岩浆与海水混合，以高温流体的形式喷出。虽然这种海脊流体已经被认为是深海铁的一大来源，但这种铁的远场影响取决于它在海洋化学中以溶解形式保留下来。我们最近的研究表明，来自大洋中脊的铁的寿命似乎比之前认为的要长得多，并被出口到距离大洋中脊4000公里的地方。尽管海脊衍生铁的作用正在显现，但我们不了解它对深海铁储量的影响，也不了解铁是如何混合到表层水域以推动生物活动的。我们强调，要了解海脊来源的铁的去向及其对海洋的最终影响，需要更多关于海脊提供的铁的数量和化学形态(例如，溶解或颗粒)的信息，以及这些信息如何随着距离源头的距离而变化。要做到这一点，我们需要评估被称为配体和所谓的铁纳米颗粒的有机小分子的作用，它们被用来控制脊形铁的寿命。考虑到铁在热液系统中的特殊性，是限制其更广泛影响的关键。此外，我们的同事最近的工作表明，深海潮汐和海脊本身之间的相互作用可以提高物理混合的速度。如果大洋中脊的垂直混合增加是典型的，这意味着这些区域可能还会显示出铁向表层水域的有效转移。鉴于大洋中脊的普遍存在，这些现象的协同结合可能是向地表水大规模供应铁的关键。对海水中极低浓度的铁进行采样和测量具有挑战性，申请者是世界上少数几个能够可靠地做到这一点的研究小组之一。我们小组处于代表铁的作用的前沿是全球海洋模型，这些模型是评估对生物生产力的更大规模影响的关键工具。该项目将参加由NERC资助的一次研究巡航，在那里，拥有测量混合和其他常量营养素的专业知识的科学家将研究大洋中脊的营养和碳泵。因此，这项建议将从这些测量中受益，并将通过确定铁的相关作用而增加这次巡航的价值。总体而言，该项目将在海洋铁循环的两个重要方面提供最先进的观测和建模约束：1)大洋脊如何影响铁到表面的物理混合；2)什么化学过程控制由大洋中脊直接供应的铁的大范围影响。最终，我们将能够解决更广泛的问题，即来自大洋中脊的铁的数量和化学形态如何影响公海浮游植物的生长。

Photosynthesis by marine phytoplankton provides energy to higher trophic levels (such as fish and marine mammals), as well as contributing to the partitioning of carbon dioxide between the atmosphere and the ocean. Iron is essential for phytoplankton growth as it is required for a number of important enzymes that participate in both photosynthesis and respiration. In contrast with terrestrial systems, iron is present at very low concentrations (less than 1 iron atom to every billion water molecules) in the open ocean. Thus phytoplankton photosynthesis is limited by iron over large parts of the ocean. This iron deficiency has important ramifications for the earth system since phytoplankton photosynthesis is an important means by which the ocean regulates global climate. Mid-ocean ridges are an important source of iron with estimates suggesting that ridge-derived iron makes up 25-75% of global ocean iron stocks. These mid-ocean ridges are the deep-sea mountain ranges that form a single global mid-ocean ridge system throughout the world's ocean, making it the longest mountain range in the world. At these ridges, new magma mixes with seawater and is exhaled as a high temperature fluid. While this ridge fluid has been noted to be a large source of iron to the deep-sea, the far field influence of this iron depends on its retention in dissolved forms by ocean chemistry. Our recent work shows that iron from mid-ocean ridges appears to have a much longer lifetime than previously thought and be exported up to 4000km away from the ridge. Despite the emerging role for ridge-derived iron, we do not understand its impact on deep ocean iron stocks, as well as how iron is mixed into surface waters to drive biological activity. We have highlighted that understanding the fate of ridge-derived iron and its ultimate influence on the ocean requires more information on the quantity and chemical form of iron supplied by ridges (e.g. dissolved or particles) and how these change with distance from source. To do this we need to appraise the role of small organic molecules called ligands and so-called iron nanoparticles, which have been invoked to control the lifetime of ridge-derived iron. Accounting for the specificity of iron within hydrothermal systems is key to constraining its wider impact. In addition, recent work by our colleagues has shown that interactions between the deep ocean tide and the ridge itself can elevate rates of physical mixing. If increased vertical mixing typifies mid-ocean ridges it implies that these regions may also exhibit efficient transfer of iron to surface waters. Given the ubiquity of mid-ocean ridges, the synergistic combination of these phenomena may be key to the large-scale supply of iron to surface waters.Sampling and measurement of iron at very low concentrations in seawater is challenging and the applicants are among the few research groups in the world who are able to do this reliably. Our group is at the forefront of representing the role of iron is global ocean models, which are crucial tools for assessing larger scale impacts on biological productivity. This project will participate in a NERC funded research cruise where scientists with expertise in measuring mixing and other macronutrients will be studying the nutrient and carbon pump over mid-ocean ridges. This proposal will therefore benefit from these measurements and will add value to this cruise by determining the associated role for iron.Overall, this project will provide state of the art observational and modelling constraints on two important aspects of the ocean iron cycle: 1) How does the ocean ridge impact physical mixing of iron to the surface and 2) what chemical processes control the large scale influence of the iron directly supplied by mid-ocean ridges. Ultimately we will be able to address the broader question of how the amount and chemical form of iron from mid-ocean ridges can influence phytoplankton growth in the open ocean.