Novel approaches to the evaluation of iron and phosphorus availability in dust deposited to the oceans

海洋是大气CO2的主要汇。然而，全球大部分海洋的碳吸收能力受到表层沃茨中营养物铁和/或磷数量的限制。因此，了解表层海洋中铁和磷的来源和归宿对于模拟气候系统并预测气候变化非常重要。公海表层沃茨水中铁和磷的主要外部来源之一是大气尘埃。沙尘中的铁和磷可以增加海洋碳的吸收，改变海洋的地球化学，从而影响气候。沙尘输入对海洋碳吸收和气候的影响程度取决于沙尘沉降通量以及沙尘中铁和磷的生物利用度。全球模型似乎能够相当好地模拟前者，但不能模拟后者。一个重要的原因是，沙漠灰尘中的大多数铁和磷是不活泼的，因此不能被生物利用，但它们在大气中被运输后会变得更加活泼。例如，在更偏远的海洋上，尘埃中溶解的（<200 nm）铁和磷的比例（定义为溶解度）比沙漠地区高出几个数量级。现在已经知道，粉尘中铁的溶解度，很可能还有磷的溶解度，在很大程度上是由源区和大气中的过程控制的。对某些过程的机械理解已经发展和/或参数化为全球模型。然而，在沉积到海洋中的灰尘，特别是湿沉积物中的铁和磷的溶解度、不稳定性和生物利用度方面，仍然存在重大差距。这些差距极大地影响了全球模式代表当前气候系统的能力，从而影响了预测气候变化的能力。这项工作的目的是评估铁和磷在灰尘中的溶解度和不稳定性（可用性），特别是在沉积到海洋的雨水中。目标是：（1）阐明了控制雨水降尘中活性态、可溶态（<1 nm）、溶解态和总态铁磷浓度和分配的基本参数和过程：（2）阐明了雨水中铁与磷和微量金属的相互作用及其对溶解度的影响;（3）对沙尘源区下风向5个站点的不稳定和/或可溶性、溶解性和总铁磷沉降通量进行定量分析，作为模式约束条件。这些目标将通过采用最先进的分离技术（例如，流场流分级，FFF）和原位不稳定痕量金属形态技术（即，扩散梯度薄膜，DGT）结合高分辨率ICP-MS和电子和原子显微镜。科学结果将被输入全球模型，以改善对大气营养物沉积通量的估计，并可以随时输入气象局和NERC地球系统模型，以预测大气营养物输入对海洋生产力和气候的影响。

Ocean is a major sink for atmospheric CO2. The carbon uptake capacity of a large part of the global ocean is however limited by the amount of nutrients iron and/or phosphorus in surface waters. Therefore, understanding the origins and fate of iron and phosphorus in surface oceans is important in modeling the climate system and therefore predicting climate change. One of the primary external sources of iron and phosphorus found in the surface waters in the open ocean is through atmospheric dust. Such iron and phosphorus from the dust can increase ocean carbon uptake and alter ocean biogeochemistry, thus affecting climate. The magnitude of the impact of dust input on oceanic carbon uptake and climate is dependent on total dust deposition fluxes as well as the bioavailability of iron and phosphorus in the dust. Global models seem able to simulate the former reasonably well but not the latter. One important reason is that most iron and phosphorus in desert dust are unreactive and thus not bioavailable but they can become much more reactive after being transported in the atmosphere. For example, the fraction of dissolved (<200nm) to total iron and phosphorus (defined as solubility) are orders of magnitude higher in dust over more remote oceans than over desert regions. It is now understood that solubility of iron, and probably phosphorus, in dust is controlled to a large extent by processes in the source area and in the atmosphere. Mechanistic understanding of some of the processes have been developed and/or parameterized into global models. However, major gaps remain on the solubility, lability and bioavailability of iron and phosphorus in dust deposited to the ocean, particularly from the wet deposition. These gaps significantly affects the ability of global models to represent the current climate system and therefore to predict climate change. The aim of this work is to evaluate the solubility and lability (availability) of iron and phosphorus in dust, particularly in rainwater deposited to the oceans. The objectives are: (1) To elucidate the fundamental parameters and processes controlling the concentration and partition of labile, soluble (<1nm), dissolved, and total iron and phosphorus in dust from rainwater; (2) To clarify how iron interact with phosphorus and trace metals in rainwater and affect their solubilities; and (3) To quantify the labile and/or soluble, dissolved and total iron and phosphorus deposition fluxes at five sites downwind of dust source regions for model constraining. These objectives will be met by field measurements and laboratory simulations employing state-of-the-art separation techniques (e.g., flow field flow fractionation, FFF) and in-situ labile trace metal speciation techniques (i.e., diffusive gradients in thin Films, DGT) coupled with high resolution ICP-MS and Electron and Atomic Microscopy. The scientific results will be fed to global models to improve the estimation of atmospheric nutrient deposition fluxes, and can be readily fed to Met Office and NERC Earth system models to predict the impact of atmospheric nutrient input on ocean productivity and climate in the present and the future.