Atmospheric fluxes of mineral dust-derived soluble trace elements to the ocean using thorium isotopes (ThorMap)

生物生产力(浮游植物的生长)受到海洋中至少30%的铁(Fe)有效性的限制。铁不溶于海水，因此从河流进入的大量铁不能被输送到远离大陆边缘的地方。落在海洋上的尘埃提供的铁成为将铁(和其他对生命重要的元素，如磷)添加到开阔海洋的主要方式。因此，铁从大气到表层海洋的模式和通量对海洋生态系统和全球碳循环都很重要(因为海洋生物消耗碳)。尽管这很重要，但人们对尘埃及其结合的金属流入海洋的流量知之甚少。直接测量这种通量是具有挑战性的，而其他观测方法需要非常基本的假设，这限制了精度。因此，目前对沙尘通量的大多数估计都依赖于大气模型，通常被认为是十倍的不确定，特别是在偏远地区。在拟议的工作中，我们将评估和使用一种新的方法来量化沙尘及其相关的微量营养素到海洋的输入。这种方法依赖于对尘埃中两种生物活性的、部分可溶的成分的测量：钍(Th)和铝(Al)。在这项评估中使用了Th的两个同位素。232Th，存在于陆相岩石中。如果被发现溶解在开阔的海洋中，232Th一定是最近从大陆运来的尘埃溶解而添加的。另一种同位素230在海水中是通过铀同位素的衰变而形成的。它在海水中的浓度反映了已知的形成速度和由于其不溶性质而被去除之间的竞争。因此，我们可以使用230Th来评估海水中Th的去除速度，包括232Th。去除的232Th必须被来自尘埃的输入取代，以保持观测到的232Th浓度，这样我们就可以计算出尘埃的输入。从Th同位素重建尘埃通量有两个主要挑战。其一是Th在尘埃中的溶解度，这是通量计算中的一个关键项，并不是很清楚。我们的新结果表明，Th是一小部分元素，其溶解度受大气中尘埃输送的影响很小，而Fe、Al和其他几种生物活性元素的溶解度在输送过程中都发生了很大的变化。利用在一系列研究巡航和百慕大取样塔收集的气溶胶样本，我们将评估Th的溶解度，控制其在大气输送过程中的变化，以及它与Al和Fe溶解度变化的关系。我们还将对沙漠沙尘母质土壤进行实验室研究，以更好地了解Th在沙尘气溶胶中的异常溶解度。粉尘通量也可以根据溶解的铝浓度计算，但这些估计受到大气输送过程中铝的溶解度变化的影响。第二个挑战是，我们不知道在输入到海岸后，232Th可能会从大陆转移到多远的地方。我们将通过将232Th纳入海洋模型来解决这个问题。这样的模型已经被证明有能力重建230度，我们将把它们发展成也对232Th进行建模，并指出232Th主要由沿海输入而不是尘埃主导的地方。这些模型还将用于评估使用Th同位素重建尘埃输入的不确定性。最近在一个名为GeoTRACE的国际方案中测量了大量海水中Th同位素的观测结果。我们将增加另外两个航次的数据，以完成对大西洋Th和Al测量的详细覆盖。因此，结合使用Th和Al示踪剂将使我们能够制作大西洋(通过使用Al运输过程中的溶解度变化)的尘埃输入(来自Th)和可溶Fe输入(以及相关的不确定性地图)的可靠地图。

Biological productivity (the growth of phytoplankton) is limited by the availability of iron (Fe) in at least 30% of the ocean. Fe is so insoluble in seawater that the large amounts entering from rivers cannot be transported far from the continental margins. The supply of Fe from dust falling on the ocean becomes the primary way to add Fe (and other elements important to life such as phosphorus) to the open ocean. The pattern and flux of Fe from the atmosphere to the surface ocean is therefore important for ocean ecosystems, and for the global carbon cycle (because ocean life consumes carbon). Despite this importance, the flux of dust and of its incorporated metals to the ocean is poorly known. It is challenging to measure this flux directly, and other observational approaches require quite fundamental assumptions, which limit accuracy. At present, therefore, most estimates of dust flux rely on atmospheric models, and are generally considered to be uncertain by a factor of ten, particularly in remote regions.In the proposed work, we will assess and use a new approach to quantify the inputs of dust and its associated micronutrients to the ocean. This approach relies on measurements of two biologically inactive, partially soluble components of dust: thorium (Th) and aluminium (Al). Two isotopes of Th are used in this assessment. 232Th, is present in continental rocks. If found dissolved in the open ocean, 232Th must have been recently added by dissolution of dust transported from the continents. Another isotope, 230Th, is formed within seawater by the decay of a uranium isotope. Its concentration in seawater reflects a competition between this known rate of formation, and removal due to its insoluble nature. We can therefore use 230Th to assess the removal rate of Th, including 232Th, from seawater. The 232Th removed must be replaced by input from dust to maintain the observed 232Th concentrations, so we can calculate the input of dust. There are two main challenges to the reconstruction of dust fluxes from Th isotopes. One is that the solubility of Th in dust, a critical term in the flux calculation, is not well known. Our new results indicate that Th is amongst a small group of elements whose solubility is very little impacted by transport of dust through the atmosphere, while the solubilities of Fe, Al and several other biologically active elements are all altered greatly during transport.Using aerosol samples collected on a series of research cruises, and at a sampling tower on Bermuda, we will assess the solubility of Th, the controls on how that varies during atmospheric transport, and its relationship to changes in Al and Fe solubility. We will also conduct laboratory studies on desert dust parent soils aimed at better understanding the unusual Th solubility in dust aerosols. Dust fluxes can also be calculated from dissolved Al concentrations, but these estimates are affected by changes in Al solubility during atmospheric transport. The second challenge is that we do not know how far 232Th from the continents might travel after input at the coast. We will address this by incorporating 232Th into an ocean model. Such models have a proven ability to reconstruct 230Th, and we will develop them to also model 232Th, and to indicate where 232Th is dominated by coastal inputs rather than by dust. These models will also be used to assess the uncertainty in using Th isotopes to reconstruct dust inputs.A large number of observations of Th isotopes in seawater has recently been measured during an international programme: GEOTRACES. We will add data from two further cruises, to complete a detailed coverage of Th and Al measurements for the Atlantic Ocean.Combined use of the Th and Al tracers will therefore allow us to produce robust maps of dust inputs (from Th) and soluble Fe inputs (by taking account of the changes in solubility during transport using Al) for the Atlantic (with associated maps of uncertainty).