Ocean micronutrient cycles: UK GEOTRACES

20世纪发展起来的一种模式，认为海洋中生命的数量和类型在很大程度上取决于向表层海洋提供的三种营养物质--磷酸盐、硝酸盐和二氧化硅(常量营养素)。国际研究工作绘制了这些常量营养素的详细分布图，并充分了解了这些常量营养素是如何以化学循环进入、离开和进入海洋的。海洋生物学和全球碳循环的模型现在纳入了这一理解。然而，在20世纪90年代初，很明显，这种对海洋营养物质的看法是不完整的。在不污染海水的情况下对海水进行采样并进行敏感测量的新能力表明，海水中存在的一系列低浓度金属是生命所必需的。在这些微量营养素中，最突出的是铁，它现在被认为是海洋大片地区生命的主要限制因素。锌和钴等其他微量营养素对关键的生物过程也是必不可少的。尽管它们很重要，但我们对这些微量营养素的化学循环的了解是初级的，特别是与大量营养素相比。我们知道微量营养素以尘埃形式进入海洋，但其他输入(来自河流、沉积物变化或海底火山)的大小尚不清楚。即使是这些微量营养素在海洋中的分布也鲜为人知，测量数据也很少，特别是在深海中。为了了解对海洋中生命和碳循环的控制，迫切需要大幅提高对微量营养素分布和循环的了解。这是一项新的重大国际研究方案--地球轨道系统的目标。该方案力求通过一系列横跨所有大洋盆地的章节，使人们对微量营养素循环的了解与对大量营养素循环的理解一样全面。这项提议代表了英国对该计划的贡献。我们将绘制七种最重要的微量营养素的浓度图，沿着大西洋东西40 oS的全水柱。对这片海洋的研究很少，但它是海洋生物地球化学循环的重要区域。在这个纬度的海面上，海洋生产力很高，需要添加微量营养素，但这些微量营养素的来源尚不清楚。在深处发现了三个不同的水团。最上层向北流向赤道表面并向上流动，为这一非常高产的地区提供微量营养素，而中间层向南流动，然后上升到南大洋，那里的铁供应不足被认为是对生命的主要限制。因此，了解这些深水团中的微量营养素输入对更广泛区域的生命来说是重要的，并将使我们总体上了解控制微量营养素循环进入全球地表和深水的过程。我们将研究来自四个海洋边界的微量营养素的输入--来自南美洲吹来的灰尘来自大气；来自河流(大普拉塔河)；来自沉积物；来自大西洋中部的活火山。我们将使用各种工具--包括用作微量营养素示踪剂的其他化学品和计算机模型--来评估微量营养素是如何从其来源进入公海的。我们将研究这些微量营养素与出现在南纬40度多产海域的生态系统的性质之间的关系。这项工作将依赖于包括大学和研究中心在内的10个英国领先机构之间的合作，也将有来自其他国家的顶尖科学家参与(部分通过GEOTRACES计划)。这一国家和国际努力将大大改善我们对控制海洋生物和碳系统的金属循环的理解。

A paradigm developed during the 20th Century that the amount and type of life in the oceans depended to a large degree on the supply to the surface ocean of three nutrients - phosphate, nitrate, and silica (the macronutrients). International research efforts mapped the distribution of these macronutrients in detail and developed a full understanding of how these macronutrients are chemically cycled into, out-of, and within the oceans. Models of ocean biology and the global carbon cycle now incorporate this understanding. In the early 1990s, however, it became clear that this view of ocean nutrients was incomplete. New ability to sample seawater without contaminating it, and to make sensitive measurements, demonstrated that a range of metals, present at low concentrations in seawater, were required by life. Of these 'micronutrients', the most prominent is iron which is now known to be the major limitation on life in large areas of the ocean. Other micronutrients, such as zinc and cobalt, are also essential for critical biological processes. Despite their importance, our knowledge of the chemical cycle of these micronutrients is rudimentary, particularly compared to that of the macronutrients. We know micronutrients enter the ocean in dust, but the size of other inputs (from rivers, alteration of sediments, or from undersea volcanoes) is not known. Even the distribution of these micronutrients in the ocean is poorly known and measurements are sparse, particularly in the deep ocean. To understand controls on life and the carbon cycle in the ocean, there is an urgent need to dramatically improve knowledge of the distribution and cycling of micronutrients. This is the goal of a major new international research programme - GEOTRACES. The programme seeks to develop an understanding of micronutrient cycles as comprehensive as that of the macronutrients, through a series of sections spanning all the ocean basins. This proposal represent the UK contribution to that programme. We will map the concentration of the seven most important micronutrients through the full water column along an east-west section at 40oS in the Atlantic. This ocean is little studied but is an important region for ocean biogeochemical cycles. In the surface at this latitude the ocean is very productive, requiring addition of micronutrients, but the source of these micronutrients is not known. At depth are found three different water masses. The uppermost flows northwards and upwells to the surface at the equator to provide micronutrients to this very productive region, while the middle layer flows southward before upwelling in the Southern Ocean where low iron supply is known to be the primary limitation on life. Understanding micronutrient inputs to these deep water masses is therefore important for life in a much broader region, and will teach us generally about the processes that control cycling of micronutrients into surface and deep waters around the globe. We will study the inputs of micronutrients from four ocean boundaries - from the atmosphere as dust blown from South America; from rivers (the large Plata River); from sediments; and from the active volcanoes found in the mid Atlantic. We will use a variety of tools - including other chemicals that act as tracers of the micronutrients, and computer models - to assess how micronutrients get from their sources into the open ocean. And we will study the relationship between these micronutrients and the nature of the ecosystems that occur in the productive seas of 40oS. This work will rely on co-operation between 10 leading UK institutes, including universities and research centres, and also involves leading scientists from other countries (partially through the GEOTRACES programme). This national and international effort will lead to a significant improvement in our understanding of the cycles of the metals that control the biology and carbon system in the ocean.