CASCADES IN MEDITERRANEAN SUBMARINE GRAND CANYONS
CASCADES IN MEDITERRANEAN SUBMARINE GRAND CANYONS
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DOI：
10.5670/oceanog.2009.03
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发表时间：
2009-03-01
期刊：
影响因子：
2.8
通讯作者：
Sanchez-Vidal, Anna
中科院分区：
文献类型：
--
作者：
Canals, Miquel;Danovaro, Roberto;Sanchez-Vidal, Anna
在过去15年中，对地中海的水柱和近海底的水沉积过程进行了持续监测，从而对这一内陆海的运作产生了新的看法。冬末至早春期间发生的水柱分层破坏和快速、密集、富含有机物且富含沉积物的近底流，有效地将物质和能量从大陆架和上层海洋层转移到深海盆。这些电流，被称为密集的陆架水级联（DSWC），已被反复测量的系泊仪器在狮子湾（地中海西北部）和亚得里亚海（地中海中部）的并发现场实验。20世纪90年代初在东地中海进行的物理海洋学观测以及对陆架底大尺度床型的观测表明，这种现象也发生在爱琴海（东地中海），并影响到邻近的深海盆地。由于它们的位置和内陆地形，它们更容易受到寒冷、持续、强烈的冬季北风的影响，这些北风使大陆架（和近海）沃茨冷却到足以使其密度超过底层沃茨，从而一旦达到密度阈值就会引发下沉。还观察到，这些陆架上的低河流流量有利于冬末早春的级联，因为陆架沃茨变得比高河流流量下更密集。虽然离岸对流单体只给深海盆地带来“蓝水”，但DSWC事件在下沉时冲刷大陆架和斜坡海底时携带了大量的有机和无机物质。DSW的级联可能持续几个星期，级联沃茨不断下沉，直到它们找到它们的密度平衡水平，这一水平每年都在变化。据观察，特别是强烈的DSWC事件进行陆架沃茨的最深处的西地中海盆地发生在subdecadal frequency.The海底地形的影响DSWC遵循的路径是最好的说明了海底峡谷。在特定的位置，峡谷是级联陆架沃茨的主要通道，并由此发展了“冲刷海底峡谷”的概念。“如果特定事件中级联的沃茨水量太大，峡谷可能无法容纳它，因此，这些沃茨水可能会从峡谷中逸出--特别是在峡谷不那么根深蒂固的地方。也有人观察到，DSW可能级联片流，席卷大陆斜坡沿着几十公里或更长的深basin.The发现在本文中报道的只是冰山一角，在DSWC的影响，深水质量的形成和对地中海的深层生态系统。由于级联现象往往与地中海各区域春季浮游植物大量繁殖同时发生，毫无疑问，它们作为从浅海层固碳的自然机制的作用将需要科学界在今后几年予以关注。
Continuous monitoring of water-column and near-bottom hydrosedimentary processes in the Mediterranean Sea over the last 15 years has resulted in a novel view of the functioning of this land-locked sea. Destratification of the water column and fast, dense, organic-matter-rich, and sediment-laden near-bottom currents occurring in late winter to early spring efficiently transfer matter and energy from the continental shelf and the upper ocean layers to the deep basin. These currents, known as dense shelf water cascading (DSWC), have been repeatedly measured by moored instrumentation during concurrent field experiments in the Gulf of Lion (northwestern Mediterranean Sea) and the Adriatic Sea (central Mediterranean). Physical oceanography observations made in the eastern Mediterranean in the early 1990s, together with observations of large-scale bed forms on the shelf floor, indicate that this phenomenon also occurs in the Aegean Sea (eastern Mediterranean) where it impacts the neighboring deep basins.The source areas of DSWC are the northernmost shelves of the Mediterranean Sea. Due to their location and inland topography, they are more exposed to the cold, persistent, intense northerly winter winds that cool shelf (and offshore) waters enough to make them denser than underlying waters, thus triggering their sinking once a density threshold is reached. It has also been observed that low river discharge on these shelves favors late winter-early spring cascading as shelf waters become denser than they would be under high river discharge. While offshore convection cells bring only "blue water" to the deep basin, DSWC events carry huge amounts of organic and inorganic substances as they scour the shelf and slope seafloor while sinking. Cascades of DSW may last for several weeks, and cascading waters sink continually deeper until they find their density equilibrium level, which changes from year to year. It has been observed that particularly intense DSWC events that carry shelf waters to the deepest parts of the western Mediterranean basin occur at subdecadal frequency.The influence of seafloor topography on the path followed by DSWC is best illustrated by submarine canyons. At specific locations, canyons are the main conduits for the cascading shelf waters, and from this developed the concept of "flushing submarine canyons." If the volume of cascading waters in a given event is too large, the canyons may be unable to accommodate it, and, therefore, those waters may escape from the canyons-especially where they are less entrenched. It has been also observed that DSW may cascade as sheet flows, sweeping continental slopes along tens of kilometers or more before spreading over the deep basin.The findings reported in this paper are just the tip of the iceberg in terms of the consequences of DSWC on deep-water mass formation and on the deep ecosystem of the Mediterranean Sea. As cascades often occur simultaneously with spring phytoplankton blooms in the various Mediterranean regions, there is no doubt that their role as a natural mechanism for carbon sequestration from the shallow ocean layers will demand the attention of the scientific community in the coming years.