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

海洋是大气中二氧化碳的主要汇。然而，全球大部分海洋的碳吸收能力受到地表水中营养物质铁和/或磷的数量的限制。因此，了解表层海洋中铁和磷的起源和命运对于模拟气候系统并因此预测气候变化非常重要。在开放海洋的表层水中发现的铁和磷的主要外部来源之一是通过大气尘埃。这些来自尘埃的铁和磷可以增加海洋的碳吸收率，改变海洋生物地球化学，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总沙尘沉积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，沙漠尘埃中的大多数铁和磷是不活跃的，因此不是生物可利用的，但它们在进入大气后会变得更加活跃。例如，在更遥远的海洋上的尘埃中，溶解（<200nm）的铁和磷占总铁和磷的比例（定义为溶解度）要比在沙漠地区高几个数量级。现在人们了解到，铁，可能还有磷，在粉尘中的溶解度在很大程度上是由污染源和大气中的过程控制的。对某些过程的机理理解已经发展起来，并/或被参数化为全球模型。然而，在沉积到海洋的粉尘，特别是湿沉积的粉尘中铁和磷的溶解度、不稳定性和生物利用度方面，仍存在主要差距。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尘埃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。目的：(1)阐明控制雨水粉尘中不稳定、可溶（<1nm）、溶解和总铁磷浓度和分配的基本参数和过程；(2)阐明雨水中铁与磷及微量金属相互作用及影响其溶解度的机理；(3)量化沙尘源区下风5个站点的不稳定和/或可溶、溶解和总铁磷沉降通量，用于模型约束。这些目标将通过采用最先进的分离技术（例如，流场流动分馏，FFF）和原位不稳定痕量金属形态形成技术（即薄膜中的扩散梯度，DGT）以及高分辨率ICP-MS和电子和原子显微镜的现场测量和实验室模拟来实现。这些科学结果将被输入到全球模型中，以改进对大气养分沉积通量的估计，并且可以很容易地输入到Met Office和NERC地球系统模型中，以预测大气养分输入在现在和未来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.