A nutrient and carbon pump over mid-ocean ridges (RidgeMix)

浮游植物是水生的单细胞植物，处于大气和海洋之间的全球碳循环的中心。像其他植物一样，浮游植物需要阳光和营养来生长和繁荣。然而，在海洋中，阳光仅限于上层几十米，而营养物质浓度在海面较低，在一公里或更深的深度最大。因此，浮游植物的生长从根本上依赖于将营养物质从深层转移到阳光照射的表面的过程，在中纬度地区，获取营养物质的问题似乎特别突出。中纬度地区的风提供了一个广泛的向下输送的水，这抑制了营养丰富的深层沃茨向上转移到阳光照射的表层沃茨。因此，人们可能会认为中纬度海洋的大部分地区由于缺乏营养而成为沙漠。然而，中纬度海洋中浮游植物的增长超过了最初的预期，这在全球范围内非常重要，因为它推动了大约一半的海洋生物从大气中去除碳。海洋学家根据海洋上层惰性示踪剂的浓度，计算出支持这种增长所需的营养物质数量。然而，将已知的营养供应量加在一起，福尔斯明显低于这一总营养需求量。因此，中纬度海洋上的生物生长如何维持是一个难题。如果我们想了解碳是如何在大气和海洋之间循环的，以及它是如何影响我们的气候的，我们就需要回答这个问题。在这个提议中，我们解决了中纬度地区深层营养物质是如何被输送到表层沃茨的问题。我们建议测试一个新的观点：潮汐通过大西洋中脊产生增强的湍流和混合，这反过来又提供了营养供应的上层温跃层沃茨。然后，这些营养物质水平沿着密度表面在盆地的西侧，可能被扫沿着墨西哥湾流，并最终进入冬季混合表层。当这一表层在春季变浅变暖时，浮游植物就可以获得营养物质。工作计划包括两个主要部分。我们将开展一项实地方案，收集大西洋中脊上空和附近的湍流和营养物浓度的测量数据。这项实地工作将涉及收集数据，从一个新的长期停泊阵列的仪器在山脊上沿着与重点5周的研究巡航。我们的工作包括足够快地取样，以便能够解决海流的潮汐变化和海岭上的混合：这是以前从未做过的，我们汇集了在较浅的陆架海潮汐测量方面具有专门知识的科学家，以及在深海混合和运输方面具有专门知识的科学家。我们工作的第二部分将使用大西洋环流的计算机模型来探索实地观察的更广泛影响，使我们能够决定大西洋中脊的混合是否真的提供了足够的营养物质来解释中纬度北大西洋的浮游植物生产。

Phytoplankton are aquatic, single-celled plants that lie at the heart of the global cycling of carbon between the atmosphere and the oceans. Like other plants, phytoplankton require sunlight and nutrients to grow and flourish. However, in the ocean sunlight is confined to the upper few tens of metres, while nutrient concentrations are low at the sea surface and greatest at depths of a kilometre or more. The growth of phytoplankton is thus fundamentally dependent on processes that transfer nutrients from depth up to the sunlit surface.Over the mid latitudes the problem of acquiring nutrients appears to be particularly stark. The winds at mid latitudes provide a widespread downward transport of water, which inhibits the transfer of nutrient-rich deeper waters up into the sunlit, surface waters. Thus, one might expect much of the mid latitude ocean to be a desert due to a lack of nutrients. However, phytoplankton growth in the mid latitude ocean is more than might initially be expected, and is globally very important as it drives about half of the oceans' biological removal of carbon out of the atmosphere. Oceanographers have calculated the amount of nutrient required to support this growth, based upon the concentrations of inert tracers in the upper ocean. However, adding together the known nutrient supplies falls significantly short of this total nutrient requirement. Hence, there is a conundrum as to how the biological growth over the mid latitude ocean is sustained. If we want to understand how carbon is cycled between the atmosphere and oceans, and how it affects our climate, we need to answer this problem.In this proposal, we address the problem of how deep nutrients are transported into the surface waters in mid-latitudes. We propose to test a new view: tides passing over the mid-Atlantic ridge generate enhanced turbulence and mixing, which in turn provides a nutrient supply to the upper thermocline waters. These nutrients are then transported horizontally along density surfaces over the western side of the basin, probably being swept along the Gulf Stream and eventually passing into the winter mixed surface layer. When this surface layer shallows and warms in spring, the nutrients are then available to the phytoplankton. The work plan involves two main components. We will carry out a field programme collecting measurements of the turbulence and nutrient concentrations over and adjacent to the mid-Atlantic ridge. This fieldwork will involve collecting data from a novel long-term moored array of instruments on the ridge along with a focused 5 week research cruise. Our work involves sampling sufficiently quickly to be able to resolve tidal changes in currents and mixing over the ridge: this has never been done before, and we have brought together scientists with expertise in tidal measurements in shallower shelf seas with others who are expert in deep ocean mixing and transports in order to do this. The 2nd component of our work will use computer models of circulation in the Atlantic to explore the wider implications of the fieldwork observations, allowing us to decide whether or not mixing over the mid-Atlantic ridge really does provide enough nutrients to explain the phytoplankton production in the mid-latitude N Atlantic.