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

深海浊流如何形成地球上最大的沉积物堆积？

• 批准号: NE/R001960/1
• 负责人: Daniel Parsons
• 金额: $ 44.69万
• 依托单位: University of Hull
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2019
• 资助国家: 英国
• 起止时间: 2019 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FR001960%2F1
• 关键词: How; do; deep; ocean; turbidity

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.

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、营养物和淡水冲入深海，并影响其氧气水平。在地球上如此大的范围内，只有河流输送的沉积物量相当，尽管单一浊流输送的沉积物量比世界上所有河流的年通量总和还要多。在这里，我们将通过对流入深海（2-5公里）的浊流进行首次详细测量，对浊流及其更广泛的影响的理解做出重大改变。几十年来，对形成主要海底扇系统的浊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首次详细测量相媲美。该项目尤其及时，因为最近成功测试了测量浅水（小于2公里）水中浊流的新方法和技术，现在可用于深水、大型海底风扇设置。我们选择研究西非附近的刚果峡谷，是因为在 2010 年和 2013 年收集了一组特殊的初始测量值。这些测量值在 2 公里水深处是迄今为止浊流最深的测量值。令人惊讶的是，他们发现个别浊流持续了近一周，占据了20%的时间。这是令人惊讶的，因为之前测量的所有海洋浊流只持续了几个小时或几分钟，并且发生时间小于总时间的 0.1%。它表明，流入深海形成主要海底扇的浊流可能在关键方面与其浅水表亲不同。这些初步测量显示了刚果峡谷监测的可行性。他们帮助我们设计了一个项目，该项目现在将展示这些水流如何流入更深的海洋。我们将沿着刚果峡谷在 2 至 5 公里的水深处部署 8 个系泊装置，用于测量多个水流的频率、持续时间和流出距离；及其速度、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。我们的总体目标是通过首次直接测量来展示深海浊流的行为，并了解这种行为的原因和更广泛的影响。我们将回答以下有关流动行为的关键问题：（1）什么控制流动持续时间，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，当移动速度更快的流体“热点”远离事件的其余部分时，水流将急剧拉伸，从而对海底峡谷产生近乎连续的冲刷。 (2) 什么控制跳动以及流动是否变得更强大？我们将测试浊流是否倾向于两种不同的行为模式之一，即它们侵蚀和加速（称为点火的过程），或沉积沉积物并消散。 （3）存款如何记录流动行为和特征？这很重要，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中陆地有机碳的转移和埋藏？最近有人提出，将陆地有机碳埋藏在深海中非常有效，并且是对长期大气二氧化碳水平的重要控制。这一假设意味着陆地有机碳在海底运输过程中几乎没有发生分馏。近海水流埋藏的有机碳成分与河流供应的有机碳成分相似。我们将通过分析沿近海路径的流量和沉积物中有机碳的数量和类型来检验这一假设。