How do deep-ocean turbidity currents behave that form the largest sediment accumulations on Earth?

被称为浊流的海底流动形成了地球上最大的沉积物堆积（海底扇）。它们将全球大量的沉积物、有机碳、营养物质和淡水冲入深海，并影响其氧气水平。只有河流能在如此广阔的地球上运输如此多的泥沙，尽管一条浊流运送的泥沙比世界上所有河流每年运送的泥沙总和还要多。在这里，我们将通过对流入深海（2-5公里）的浑浊流进行首次详细测量，对浑浊流及其更广泛的影响的理解做出一步改变。几十年来，这种对形成主要海底扇系统的浊度流的直接监测一直是沉积学、海洋学和海洋地质学的“圣杯”。它大致上可以与主要河流系统或其他在地球上移动沉积物的一级过程的第一次详细测量相媲美。由于最近成功测试了测量浅水（小于2公里）浊度流的新方法和新技术，该项目特别及时，现在可以用于深水，大型海底风扇设置。我们之所以选择研究西非的刚果峡谷，是因为2010年和2013年收集了一组特殊的初始测量数据。在水深2公里处进行的这些测量是迄今为止对浊度流进行的最深测量。令人惊讶的是，他们发现单个浊度流持续了近一周，占据了20%的时间。这是令人惊讶的，因为所有以前测量到的海洋浊度流只持续了几个小时或几分钟，而且发生的时间不到总时间的0.1%。它表明，流入深海形成主要海底扇的浊流在关键方面可能与浅水表亲不同。这些初步的测量结果表明，对刚果峡谷进行监测是可行的。它们帮助我们设计了一个项目，现在将展示这些水流是如何流入更深的海洋的。我们将沿着刚果峡谷在水深2至5公里处部署8个系泊，测量多个水流的频率、持续时间和流出距离；以及它们的速度、湍流和含沙量结构；以及水、沉积物和有机碳排放的变化。我们的总体目标是使用第一次直接测量来显示深海浊流的行为，并了解这种行为的原因和更广泛的含义。我们将回答以下关于流动行为的关键问题：(1)是什么控制了流动的持续时间，流动拉伸是否会导致近乎连续的峡谷冲刷？我们将测试一个新的假设，该假设预测，随着快速流动流体的“热点”从事件的其余部分流出，流动将急剧扩大，从而产生近乎连续的海底峡谷冲刷。(2)是什么控制着跳动，流动是否变得更强大？我们将测试浊度流是否倾向于两种不同的行为模式之一，即它们侵蚀和加速（一个称为点火的过程），或沉积沉积物并消散。(3)沉积物如何记录流动特性和特征？这一点很重要，因为沉积物是大多数浊度流的唯一记录。(4)流动行为如何影响深海陆相有机碳的转移和埋藏？近年来有人提出，深海中陆生有机碳的埋藏是非常有效的，并且是长期大气CO2水平的重要控制因素。这一假设表明，在海底运输过程中，陆地有机碳的分馏很少。近海流埋藏的有机碳组成与河流提供的有机碳组成相似。我们将通过分析沿离岸通道流动和沉积的有机碳的数量和类型来检验这一假设。

Seafloor flows called turbidity currents form the largest sediment accumulations on Earth (submarine fans). They flush globally significant amounts of sediment, organic carbon, nutrients and fresher-water into the deep ocean, and affect its oxygen levels. Only rivers transport comparable volumes of sediment across such large expanses of our planet, although a single turbidity current can transport more sediment than the combined annual flux from all of the World's rivers combined. Here we will make a step change in understanding of turbidity currents, and their wider impacts, by making the first detailed measurements of turbidity current that runout into the deep (2-5 km) ocean. Such direct monitoring of turbidity currents that form major submarine fan systems has been a 'holy grail' for sedimentology, oceanography, and marine geology for decades. It would be broadly comparable to the first detailed measurements of major river systems or other first-order processes for moving sediment across the planet. This project is especially timely due to recent successful tests of new methods and technology for measuring turbidity currents in shallower (less than 2 km) water, which can now be used for deep-water, large-scale submarine fan settings. We choose to study the Congo Canyon off West Africa due to an exceptional set of initial measurements collected in 2010 and 2013. These measurements at 2 km water depth are the deepest yet for turbidity currents. Surprisingly, they showed that individual turbidity currents lasted for almost a week, and occupied 20% of the time. This was surprising because all previously measured oceanic turbidity currents lasted for just a few hours or minutes, and occurred for < 0.1% of the total time. It suggests that turbidity currents that runout into the deep ocean to form major submarine fans may differ from their shallow water cousins in key regards. These preliminary measurements show how monitoring is feasible for the Congo Canyon. They help us to design a project that will now show how these flows runout into the deeper ocean.We will deploy 8 moorings along the Congo Canyon at water depths of 2 to 5 km that will measure frequency, duration, and run-out distance of multiple flows; together with their velocity, turbulence and sediment concentration structures; as well as changes in water, sediment and organic carbon discharge. Our overall aim is to show how deep-sea turbidity current behave using the first direct measurements, and understand causes and wider implications of this behaviour. We will answer the following key questions about flow behaviour:(1) What controls flow duration, and does flow stretching cause near-continuous canyon flushing? We will test a new hypothesis that predicts flows will stretch dramatically as a 'hot spot' of faster moving fluid runs away from the rest of the event, thereby producing near-continuous flushing of submarine canyons. (2) What controls runout and whether flows become more powerful? We will test whether turbidity currents tend towards one of two distinct modes of behaviour, in which they erode and accelerate (a process termed ignition), or deposit sediment and dissipate. (3) How is flow behaviour and character recorded by deposits? This is important because deposits are the only record of most turbidity currents.(4) How does flow behaviour affect the transfer and burial of terrestrial organic carbon in the deep-sea? It was proposed recently that burial of terrestrial organic carbon in the deep sea is very efficient, and an important control on long-term atmospheric CO2 levels. This hypothesis implies little fractionation of terrestrial organic carbon occurs during submarine transport. Composition of organic carbon buried by the offshore flows is similar to that supplied by the river. We will test this hypothesis by analysing amounts and types of organic carbon along the offshore pathway in both flows and deposits.