REVISITING THE NEODYMIUM PARADOX IN THE OCEAN

全球气候变化是当今社会面临的一大挑战。气候系统变暖是明确的，从全球平均气温上升的观察中可以明显看出这一点。在海洋中也观察到气候变暖，并伴随着盐度的变化，高纬度地区变得更新鲜(即盐分更少)，亚热带和热带地区变得更盐分--这是一种可能影响海洋循环的特性的重新分配。气候变化对海洋化学也有明显的影响。虽然对更丰富的大气二氧化碳的吸收增加导致海洋酸化，威胁到海洋生态系统，但由于人为污染和陆地侵蚀模式的变化，人们对某些痕量金属浓度较高的影响知之甚少。然而，这些变化非常重要，因为海洋从大气中吸收二氧化碳的能力与所谓营养物质的供应密切相关，营养物质是海洋中生命所必需的元素。作为这个项目的一部分，我们将更好地理解这种“生物地球化学循环”。我们挑选了三种痕量金属，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，今天，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Cd是海洋中一种重要的微量营养素，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沙尘的自然通量以及沙尘在大气中清除的人为颗粒是否对海洋Cd和Pd循环产生重要影响。学生还将进一步学习，这些元素在海洋中的循环是如何通过改变氧气浓度来改变的。氧(仅次于营养物)是生物地球化学循环中的另一个重要角色，它在海水中的溶解度与温度有关。气候模型预测，未来海洋将形成氧气浓度较低的延伸区。海洋系统的另一个重要方面是洋流是分配热量的关键机制，因此它们对区域和当地气候有重大影响。此外，水团运动(垂直和横向)对碳循环非常重要，因为深海中的碳含量是大气的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年的观察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改变。然而，从过去的研究中我们知道，在冰河时代和过去的极端温暖时期，海水团有不同的形态。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令人震惊的关系。然而，对现代ND循环的片面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的能力。特别是，人们普遍认为，在远离海洋边缘的地方，钕同位素是一种理想的海洋环流示踪剂，因为它们只能通过水团之间的混合进行修改。然而，有许多潜在的海洋过程，这可能不符合这种简单化的观点。目前的项目将根据从大西洋战略位置收集的样本收集的一整套新的观测数据来解决这些不确定性。结合建模工作，我们的新数据将阐明管理海洋ND循环的过程以及作为循环示踪剂的ND同位素的适宜性。

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.