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

浮游植物是水生的单细胞植物，位于全球大气和海洋之间碳循环的核心。像其他植物一样，浮游植物需要阳光和营养才能生长和繁衍。然而，在海洋中，阳光仅限于上层几十米，而营养物质的浓度在海面很低，在一公里或更深的地方最高。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充足的表层水中的过程。在中纬度地区，获取营养物质的问题显得尤为严峻。中纬度的风提供了广泛的向下输送水，这阻止了营养丰富的深层水向上转移到阳光充足的表层水域。因此，人们可能会认为，由于缺乏营养物质，中纬度海洋的大部分地区将成为沙漠。然而，中纬度海洋中浮游植物的增长超出了最初的预期，而且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清除大气中的碳。海洋学家已经计算了支持这种生长所需的营养量，答案与基于表层海洋中惰性示踪剂浓度的总营养供应估计值一致。然而，将已知的营养供应加在一起，远远达不到总的营养需求。因此，中纬度海洋上的生物生长如何持续存在着一个难题。如果我们想要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。在这个建议中，我们解决了如何将深层营养物质输送到中纬度地区的表层水域的问题。我们建议测试一种新的观点：通过大西洋中部海脊的潮汐产生增强的湍流和混合，这反过来为上层温跃层水域提供营养供应。然后，这些营养物质沿着盆地西侧的密度面水平输送，可能会沿着墨西哥湾流被席卷，最终进入冬季混合的表层。当这个表层在春天变浅变暖时，营养物质就可以提供给浮游植物了。工作计划包括两个主要组成部分。我们将实施一项实地方案，收集大西洋中部海脊及其附近的湍流和营养物质浓度的测量数据。这项实地工作将包括从海脊上一组新的长期停泊的仪器中收集数据，以及一次专注于5周的研究巡航。我们的工作包括以足够快的速度进行采样，以便能够解决洋流中的潮汐变化和海脊上的混合：这是以前从未做过的，我们将在浅海陆架海进行潮汐测量的专业科学家与其他擅长深海混合和运输的科学家聚集在一起，以便完成这项工作。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野外观测的更广泛的影响，使我们能够确定在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北大西洋中纬度的浮游植物生产。

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 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.