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

被称为浊流的海底水流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。只有河流才能在我们星球如此广阔的地区输送相当数量的沉积物，尽管一条浊流可以输送的沉积物比世界上所有河流的年流量总和还要多。在这里，我们将通过对流入深海（2-5公里）的浊流进行第一次详细测量，从而在理解浊流及其更广泛的影响方面取得一步进展。这种对形成主要海底扇系统的浊流的直接监测几十年来一直是沉积学、海洋学和海洋地质学的“圣杯”。这将与主要河流系统或其他一级沉积物在地球上移动过程的首次详细测量大致相当。该项目特别及时，因为最近成功地测试了测量浅水（不到2公里）浊流的新方法和技术，现在可用于深水、大规模海底扇环境。我们选择研究西非的刚果峡谷，是因为在2010年和2013年收集了一组特殊的初始测量结果。在2公里水深处的这些测量是迄今为止浊流最深的测量。令人惊讶的是，他们发现单个浊流持续了近一周，占了20%的时间。这是令人惊讶的，因为所有以前测量的海洋浊流只持续了几个小时或几分钟，并且发生在总时间的0.1%以下。这表明，流入深海形成主要海底扇的浊流可能在关键方面与浅水表亲不同。这些初步测量表明，监测刚果峡谷是可行的。我们将沿着刚果峡谷部署8个沿着水深2至5公里的系泊设备，测量多股水流的频率、持续时间和流出距离，以及它们的速度、湍流和沉积物浓度结构，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显示深海浊流的行为，并了解这种行为的原因和更广泛的影响。我们将回答以下关于水流行为的关键问题：（1）是什么控制着水流持续时间，水流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，随着快速移动的流体的“热点”远离事件的其余部分，流动将急剧伸展，从而产生近乎连续的海底峡谷冲刷。(2)是什么控制了跳动，流量是否变得更强大？我们将测试浊流是否倾向于两种不同的行为模式之一，在这两种模式中，浊流侵蚀和加速（称为点火的过程），或存款沉积和消散。(3)沉积物是如何记录流动行为和特征的？这一点很重要，因为沉积物是大多数浊流的唯一记录。(4)流动行为如何影响深海陆地有机碳的转移和埋藏？最近有人提出，陆地有机碳在深海中的埋藏是非常有效的，并且是对长期大气CO2水平的重要控制。这一假设意味着在海底输运过程中，陆地有机碳的分馏很少发生。离岸流埋藏的有机碳组成与河流提供的有机碳组成相似。我们将通过分析流动和沉积物中沿着近海路径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.