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.

海底流量称为浊流，构成了地球上最大的沉积物积聚（海底风扇）。它们在全球范围内将大量的沉积物，有机碳，养分和新鲜水冲入深海，并影响其氧气水平。只有河流在我们地球的如此大的膨胀范围内运输可比较的沉积物，尽管单一的浊度电流可以运输比世界所有河流的结合年度通量更大的沉积物。在这里，我们将通过对进入深处（2-5 km）海洋的浊流的第一个详细测量来对浊流及其更广泛的影响进行逐步改变。数十年来，这种对构成主要海底风扇系统的浊流的直接监测一直是沉积学，海洋学和海洋地质的“圣杯”。这将与主要的河流系统或其他一阶过程的第一个详细测量大致相提并论。该项目尤其及时，这是由于最近成功测试了用于测量较浅（小于2 km）水的浊流的新方法和技术，现在可以将其用于深水，大规模的海底风扇设置。由于在2010年和2013年收集了一系列特殊的初始测量值，我们选择研究西非范围的刚果峡谷。这些测量值在2 km的水深下是最深的浊流。令人惊讶的是，他们表明单个浊流持续了将近一周，并且有20％的时间。这是令人惊讶的，因为所有先前测量的海洋浊流持续了几个小时或几分钟，并且发生在总时间的<0.1％。它表明，进入深海形成主要海底风扇的浊流可能与他们在关键方面的浅水表亲不同。这些初步测量表明，刚果峡谷的监测是如何可行的。他们帮助我们设计一个项目，该项目现在将显示这些流程如何进入更深的海洋。我们将在2至5公里的水深沿刚果峡谷沿着刚果峡谷进行8个系泊设备，从而测量多个流量的频率，持续时间和跑步距离；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。我们的总体目的是展示使用第一个直接测量的深海浊度电流的行为，并了解这种行为的原因和更广泛的含义。我们将回答有关流动行为的以下关键问题：（1）哪些控制流程持续时间，流动伸展会导致几乎连续的峡谷潮红吗？我们将测试一个新的假设，该假设预测流将急剧扩展，因为更快的移动液与活动的其余部分延伸，从而产生接近连续的海底峡谷。 （2）哪些控制跳动以及流量是否变得更强大？我们将测试浊流是否趋向于两种不同的行为模式之一，在这种行为模式中，它们侵蚀和加速（称为点火的过程），还是沉积沉积物并消散。 （3）如何通过存款记录流动行为和特征？这很重要，因为沉积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葬？最近有人提出，深海埋葬地面有机碳非常有效，并且对长期大气二氧化碳水平进行了重要控制。该假设意味着在海底运输过程中，陆地有机碳的分馏很少。沿海流埋葬的有机碳的成分与河流提供的有机碳相似。我们将通过分析流量和沉积物沿近海途径的有机碳的数量和类型来检验这一假设。