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）受到至少30%海洋中铁（Fe）可用性的限制。铁不溶于海水，从河流进入的大量铁不能从大陆边缘运到很远的地方。落在海洋上的尘埃提供的铁成为向开阔的海洋添加铁（以及其他对生命重要的元素，如磷）的主要方式。因此，铁从大气到海洋表面的模式和通量对海洋生态系统和全球碳循环（因为海洋生物消耗碳）很重要。尽管具有这种重要性，但人们对灰尘及其含金属进入海洋的情况知之甚少。直接测量这种通量具有挑战性，而其他观测方法需要相当基本的假设，这限制了准确性。因此，目前对尘埃通量的大多数估计依赖于大气模式，而且通常被认为是不确定的，尤其是在偏远地区。在拟议的工作中，我们将评估并使用一种新的方法来量化灰尘及其相关微量营养素对海洋的输入。这种方法依赖于对灰尘中两种生物无活性、部分可溶成分的测量：钍（Th）和铝（Al）。在这个评估中使用了Th的两种同位素。钍存在于大陆岩石中。如果在开阔的海洋中发现了溶解的元素，那么232Th一定是最近从大陆运来的尘埃溶解而加入的。另一种同位素，230，是在海水中由铀同位素衰变形成的。它在海水中的浓度反映了已知的形成速率和由于其不溶性而导致的去除速率之间的竞争。因此，我们可以使用230Th来评估海水中Th（包括232Th）的去除率。为了维持观测到的232Th浓度，必须用粉尘的输入来取代去除的232Th，因此我们可以计算粉尘的输入。从Th同位素重建尘埃通量有两个主要挑战。一是尘埃中Th的溶解度，这是通量计算中的一个关键项，并不为人所熟知。我们的新结果表明，Th是少数溶解度受大气中尘埃运输影响很小的元素之一，而Fe， Al和其他几种生物活性元素的溶解度在运输过程中都发生了很大的变化。利用在一系列研究巡航中收集的气溶胶样本，以及在百慕大的一个采样塔，我们将评估Th的溶解度，控制其在大气运输过程中的变化，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土进行实验室研究，以更好地了解沙尘气溶胶中不寻常的Th溶解度。尘埃通量也可以从溶解的铝浓度计算，但这些估计受到大气输送过程中铝溶解度变化的影响。第二个挑战是，我们不知道在海岸输入后，232Th从大陆可能走多远。我们将把232Th并入一个海洋模型来解决这个问题。这些模型已经被证明有能力重建230Th，我们将开发它们来模拟232Th，并指出232Th是由海岸输入而不是尘埃主导的。这些模型还将用于评估使用Th同位素重建粉尘输入的不确定性。最近在一个名为GEOTRACES的国际项目中对海水中的大量Th同位素进行了观测。我们将添加另外两次巡航的数据，以完成大西洋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为大西洋生成可靠的粉尘输入图（来自Th）和可溶性铁输入图（通过考虑使用Al运输过程中溶解度的变化）（以及相关的不确定性图）。

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).