Ocean micronutrient cycles: UK GEOTRACES

20世纪形成了一种范式，认为海洋中生命的数量和类型在很大程度上取决于海洋表面三种营养物质的供应——磷酸盐、硝酸盐和二氧化硅（宏量营养物质）。国际上的研究工作详细地绘制了这些大量营养素的分布图，并对这些大量营养素如何在海洋中化学循环进出海洋有了充分的了解。海洋生物学和全球碳循环的模型现在包含了这一认识。然而，在20世纪90年代初，人们清楚地认识到，这种关于海洋营养物质的观点是不完整的。在不污染海水的情况下对海水进行取样并进行灵敏测量的新技术表明，存在于海水中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，现在已知它是海洋大片地区生命的主要限制因素。其他微量营养素，如锌和钴，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还很初级，特别是与对大量营养素的了解相比。我们知道微量营养物质以灰尘的形式进入海洋，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知之甚少，测量也很少，特别是在深海中。为了了解对海洋中生命和碳循环的控制，迫切需要大大提高对微量营养素分布和循环的认识。这是一个重要的新的国际研究项目——GEOTRACES的目标。该方案力求通过跨越所有海洋盆地的一系列部分，发展对微量营养素循环的理解，就像对大量营养素循环的理解一样全面。这项建议代表联合王国对该方案的贡献。我们将绘制7种最重要的微量营养素的浓度图，沿着大西洋东西向40度的剖面，绘制整个水柱的浓度图。这片海洋的研究很少，但却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，海洋的生产力非常高，需要添加微量营养素，但这些微量营养素的来源尚不清楚。在深处发现了三种不同的水团。最上层向北流动，向上流到赤道表面，为这个非常富有生产力的地区提供微量营养素，而中间层在南大洋上涌之前向南流动，那里的铁供应不足是已知的生命的主要限制。因此，了解这些深水群的微量营养素输入对更广泛地区的生命非常重要，并将使我们大致了解控制微量营养素进入全球表层和深水的循环过程。我们将研究来自四个海洋边界的微量营养素的输入——来自南美洲吹来的大气尘埃；来自河流（大的普拉塔河）；从沉积物;以及大西洋中部的活火山。我们将使用各种工具——包括作为微量营养素示踪剂的其他化学物质，以及计算机模型——来评估微量营养素是如何从它们的来源进入公海的。我们将研究这些微量营养素与40年代多产海洋中生态系统性质之间的关系。这项工作将依靠英国10个主要研究所的合作，包括大学和研究中心，也包括来自其他国家的主要科学家（部分通过GEOTRACES项目）。这项国家和国际的努力将大大提高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.