REVISITING THE NEODYMIUM PARADOX IN THE OCEAN

全球气候变化是当今社会面临的重大挑战之一。气候系统的变暖是明确的，从全球平均气温上升的观测中可以明显看出。在海洋中也观察到变暖，并伴随着盐度的变化，高纬度地区变得“更新鲜”（即，盐份减少），亚热带和热带地区的盐份增加-这种性质的重新分配有可能影响海洋环流。气候变化对海洋化学也有明显影响。虽然增加吸收更丰富的大气二氧化碳导致海洋酸化，威胁海洋生态系统，但人们对某些微量金属浓度较高的影响知之甚少，这是人为污染和土地侵蚀模式变化的结果。然而，这种变化非常重要，因为海洋从大气中吸收二氧化碳的能力与所谓的营养物的供应密切相关，这些营养物是海洋生命所必需的元素。作为该项目的一部分，我们将更好地了解这种“地球化学循环”。我们选出了三种微量金属，钕（Nd），镉（Cd）和铅（Pb），它们共同代表了海洋中许多不同元素的行为。例如，镉和铅都是由人类活动提供给环境的，这可能会改变它们的自然循环。由于镉是海洋中重要的微量营养元素，这种变化也可能影响全球碳循环。作为我们项目的一部分，一名博士生将专注于了解从沙漠地区到海洋的自然尘埃通量和大气中的人为尘埃清除剂颗粒是否对海洋Cd和Pb循环产生重要影响。学生将进一步研究，如何通过改变氧气浓度来改变海洋中这些元素的循环。氧气是（仅次于营养物）地球化学循环中的另一个重要角色，其在海水中的溶解度取决于温度。气候模式预测，未来海洋中将形成广阔的低氧区。海洋系统的另一个重要方面是，洋流是分布热量的关键机制，因此它们对区域和局部气候有重大影响。此外，水团运动（垂直和横向）对碳循环非常重要，因为深海的碳含量是大气的50-60倍。今天，我们可以通过测量海水的物理性质来监测海洋环流。然而，过去50年的观测并没有给我们任何明确的迹象表明海洋环流的模式是否正在发生变化。然而，从过去的研究中我们知道，海洋水团在冰河时代和过去的极端温暖时期有不同的配置。海水中的钕同位素经常被用于这种重建，结果显示了过去温度、二氧化碳水平和海洋环流之间惊人的关系。然而，对现代Nd循环的不完整理解限制了我们对这种重建的信心，从而限制了我们将推断的机制转移到未来模型中的能力。特别是，一般认为，远离海洋边缘，钕同位素是一个理想的海洋环流示踪剂，因为它们只通过水团之间的混合修改。然而，有许多潜在的海洋过程，可能不符合这种简单化的观点雅阁。目前的项目将根据从大西洋战略地点收集的一整套新的观测数据，处理这种不确定性。结合建模工作，我们的新数据将揭示海洋Nd循环的过程和Nd同位素作为循环示踪剂的适用性。

Global climate change is one of the big challenges society faces today. Warming of the climate system is unequivocal, and evident from observations of increasing global average temperatures. Warming is also observed in the oceans, and is accompanied by a change in salinity, with the high latitudes becoming 'fresher' (i.e., less saline) and the subtropics and tropics becoming more saline - a redistribution of properties that has the potential to affect ocean circulation. There are also clear effects of climate change on the chemistry of the oceans. Whilst increased uptake of more abundant atmospheric carbon dioxide leads to an acidification of the oceans that threatens marine ecosystems, only little is known about the effects of higher concentrations of certain trace metals, as a result of anthropogenic pollution and changing erosion patterns on land. Such changes are very important, however, as the ability of the ocean to take up carbon dioxide from the atmosphere is strongly coupled to the supply of so-called nutrients, elements that are essential for life in the ocean. As part of this project, we will develop a better understanding of such 'biogeochemical cycles'. We picked out three trace metals, neodymium (Nd), cadmium (Cd), and lead (Pb), which together represent the behaviour of many different elements in the ocean. For example, both Cd and Pb are today supplied to the environment by human activity and this may alter their natural cycles. As Cd is an important micronutrient in the ocean, such changes could also affect the global carbon cycle. As part of our project, a PhD student will focus on understanding whether the natural flux of dust from desert areas to the ocean and the anthropogenic particles the dust scavenges in the atmosphere have an important impact on the marine Cd and Pb cycles. The student will furthermore study, how the cycling of these elements in the ocean is altered by changing oxygen concentrations. Oxygen is (next to the nutrients) another important player in biogeochemical cycles, and its solubility in seawater is temperature dependent. Climate models predict that extended zones with low oxygen concentrations will develop in the future oceans.Another important aspect of the ocean system is that ocean currents are the key mechanism for distributing heat, and thus they have a significant impact on regional and local climate. Furthermore, water mass movements (both vertical and lateral) are very important for the carbon cycle, as the deep ocean contains 50-60 times more carbon than the atmosphere. Today we can monitor ocean circulation by measuring the physical properties of seawater. Observations over the past 50 years, however, do not give us any clear indication whether the pattern of ocean circulation is changing. From studies of the past we know, however, that ocean water masses had a different configuration during the ice ages and past periods of extreme warmth. Neodymium isotopes in seawater are often used for such reconstructions, and the results show stunning relationships between past temperatures, carbon dioxide levels, and ocean circulation.A patchy understanding the modern Nd cycle however limits our confidence in such reconstructions, and thus our ability to transfer the inferred mechanisms to future models. In particular, it is generally assumed that away from ocean margins, Nd isotopes are an ideal ocean circulation tracer as they are only modified by mixing between water masses. However, there are many potential marine processes, which may not be in accord with this simplistic view. Such uncertainties will be addressed by the current project, based on a comprehensive suite of new observational data that will be collected for samples from strategic locations in the Atlantic Ocean. In conjunction with modelling efforts, our new data will shed light on the processes governing the marine Nd cycle and the suitability of Nd isotopes as circulation tracer.