A MISSING LINK between continental shelves and the deep sea: Addressing the overlooked role of land-detached submarine canyons

这个雄心勃勃的项目将使人们对通过海底峡谷的零星但大量的沉积物，具有气候重要性的有机碳和污染物的理解发生重大变化，这些峡谷将全球大陆架与深海连接起来。超过9000个大型海底峡谷出现在世界上所有的水下边缘，往往使河流系统的规模相形见绌。这些峡谷可以转移大量的天然沉积物、有机碳和营养物质，维持重要的生态系统，并且越来越多地被认为是海底污染的热点，威胁到它们所拥有的生物多样性。沿着峡谷流动的沉积物流动速度快、密度大，会破坏支撑全球通信的电缆。因此，重要的是要了解何时以及如何触发这种流动，运输的材料量，以及至关重要的是，这些如何不同类型的峡谷。浊流监测的重点是“陆地附着”的峡谷河流或沿岸漂流，其中强大的浊流已被证明有效地运输沉积物和碳超过1000公里。尽管占全球峡谷的70%以上，但陆地分离的峡谷（远离海岸）仍然没有受到监测，暴露了我们对全球颗粒物运输的理解存在重大差距。这种偏见的结果，从一个长期持有的观点认为，陆地分离峡谷是断开从沉积物输入在今天的海平面。Whittard峡谷（位于凯尔特海，距离海岸250公里）的新测量挑战了这一范式，揭示了陆地分离的峡谷可以具有与主要陆地连接的峡谷相似频率和功率的浊流。这些令人惊讶的新结果提出了以下问题，并激发了我们的项目，该项目旨在首次确定颗粒通过陆地分离的海底峡谷向深海转移的机制和通量。如果一个峡谷源头远离现今的沉积物供应，那么频繁的浊流是如何产生的？我们将在大陆架和Whittard峡谷头部部署一系列传感器，以测量浊流之前和与浊流同时发生的情况，并将反复绘制海底地图，以确定沉积物如何以及在何处被运送到峡谷头部。然后，我们将沿着一个陆地分离的峡谷进行第一个源到汇的测量，以及世界上任何主要深海峡谷的第二个测量，因此这本身将代表一个重要的科学里程碑。有机碳或污染物的性质、浓度和埋藏效率如何，与陆地相连的峡谷相比如何？我们将分析海底和沉积物收集器样本，以确定有多少有机碳和污染物沿峡谷沿着迁移，以及它们在多大程度上因埋藏而有效地锁在海底。春季和夏季，当浊流最频繁时，Whittard峡谷的头部发生浮游植物水华，以类似于河流洪水将新鲜碳输送到陆地连接的峡谷的方式提供新鲜（海洋）有机碳。我们还观察到移动的垃圾堆积，因此将测试浊流在多大程度上运输污染物和有机碳，以及其分布与海底生物多样性热点的关系。除了构成生态威胁外，微塑料等污染物还可能有效地充当“示踪剂”，证明当代峡谷流动。有多少自然和人为物质通过陆地分离的峡谷转移？全球预算存在颗粒物运输到海洋和跨越海洋，但没有包括陆地分离的峡谷。我们将提供一阶计算来评估陆地分离峡谷的全球意义，首先评估对凯尔特边缘深海运输的贡献，然后扩大我们的结果，以确定现有全球预算中缺少什么。

This ambitious project will enable a step change in understanding of the sporadic but large flows of sediment, climatically-important organic carbon, and pollutants through submarine canyons, which connect continental shelves worldwide to the deep-sea. >9000 large submarine canyons occur on all the world's submerged margins, often dwarfing river systems in scale. Such canyons can transfer large quantities of natural sediments, organic carbon and nutrients that sustain important ecosystems, and are increasingly recognised as hotspots for seafloor pollution that threatens the biodiversity they host. The sediment flows that travel along canyons can be fast and dense, breaking cables that underpin global communications. It is therefore important to understand when and how such flows are triggered, the amount of material that is transported, and crucially, how these vary between types of canyon.Monitoring of turbidity currents has focused on 'land-attached' canyons fed by rivers or long-shore drift, where powerful turbidity currents have been shown to effectively transport sediment and carbon over 1000s km. Despite accounting for >70% of canyons worldwide, land-detached canyons (that lie far from shore) remain un-monitored, exposing a major gap in our understanding of global particulate transport. This bias results from a long-held view that land-detached canyons are disconnected from sediment inputs during present day sea levels. New measurements in Whittard Canyon (in the Celtic Sea, 250 km from shore) challenge this paradigm, revealing that land-detached canyons can feature turbidity currents of similar frequency and power to major land-attached canyons. These surprising new results raise the following questions, and motivate our project, which aims to determine the mechanisms and fluxes of particulate transfer via land-detached submarine canyons to the deep-sea for the first time. How can frequent turbidity currents occur if a canyon head lies far from present day sediment supplies? We will deploy an array of sensors on the continental shelf and within the Whittard Canyon head to measure the conditions before and coincident with turbidity currents, and will repeatedly map the seafloor to identify how and where sediment is transported to the canyon head. We will then make the first source to sink measurements along a land-detached canyon, and the second of any major deep-sea canyon worldwide, hence in itself this will represent a significant scientific milestone. What is the nature, concentration and burial efficiency of organic carbon or pollutants, and how does this compare to land-attached canyons? We will analyse seafloor and sediment trap samples to determine what quantities of organic carbon and pollutants are transported along the canyon, and to what extent they remain effectively locked up in the seafloor as a result of burial. Phytoplankton blooms occur at the head of Whittard Canyon during spring and summer, when turbidity currents are most frequent, providing fresh (marine) organic carbon in a similar manner to how river floods convey fresh carbon to land-attached canyons. We also observe mobile litter accumulations so will test to what extent turbidity currents transport pollutants as well as organic carbon and how its distribution relates to seafloor biodiversity hotspots. As well as posing an ecological threat, pollutants such as microplastics may effectively act as 'tracers', evidencing contemporary canyon flows. What volumes of natural and anthropogenic material are transferred via land-detached canyons? Global budgets exist for particulate transport to and across the ocean, but none include land-detached canyons. We will provide a first order calculation to assess the global significance of land-detached canyons, first assessing the contribution to deep sea transport across the Celtic Margin, and then up-scaling our results to determine what is missing from existing global budgets.