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

浮游植物是水生的单细胞植物，在大气和海洋之间的全球碳循环中处于核心位置。和其他植物一样，浮游植物也需要阳光和养分才能生长茂盛。然而，在海洋中，阳光仅局限于海平面以上的几十米，而营养物质的浓度在海面上很低，在一公里或更深的深处最高。因此，浮游植物的生长从根本上依赖于将深层营养物质转移到阳光照射的表层水域的过程。在中纬度地区，获取营养的问题似乎尤为突出。中纬度地区的风为水提供了广泛的向下运输，这抑制了营养丰富的深层水向阳光充足的表层水的转移。因此，人们可能会认为，由于缺乏营养，中纬度海洋的大部分地区将成为沙漠。然而，浮游植物在中纬度海洋的生长比最初可能预期的要多，并且在全球范围内非常重要，因为它驱动了大约一半的海洋生物从大气中去除碳。海洋学家已经计算出了支持这种生长所需的营养物质的数量，其答案与根据海洋表面惰性示踪剂浓度估计的总营养供应一致。然而，将已知的营养供应加在一起，远远低于这一总营养需求。因此，如何维持中纬度海洋上的生物生长是一个难题。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，我们需要回答这个问题。在这个建议中，我们解决了中纬度地区深层营养物质是如何运输到地表水中的问题。我们提议测试一种新的观点：经过大西洋中脊的潮汐产生了增强的湍流和混合，这反过来又为上层温跃层的水提供了营养供应。然后，这些营养物沿着盆地西侧的密度面水平运输，可能沿着墨西哥湾流被扫过，最终进入冬季混合表层。当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组成部分。我们将执行一项实地计划，收集大西洋中脊上空及其附近的湍流和营养物质浓度的测量数据。这项野外工作将包括从一个新的长期系泊在山脊上的仪器阵列收集数据，以及为期5周的重点研究巡航。我们的工作包括足够快的采样，以便能够解决海流和山脊上混合的潮汐变化：这是以前从未做过的，为了做到这一点，我们召集了具有浅海陆架潮汐测量专业知识的科学家和其他具有深海混合和运输专家的科学家。我们工作的第二个组成部分将使用大西洋环流的计算机模型来探索野外观测的更广泛含义，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来解释中纬度北大西洋浮游植物的生产。

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 deep nutrients up to the sunlit surface waters.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, and the answer agrees with estimates of total nutrient supply based on the concentrations of inert tracers in the surface 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.