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

Lead Research Organisation: Durham University
Department Name: Geography

Abstract

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.

Planned Impact

Beneficiaries of this work are unusually wide-ranging. They include the oil and gas sector, the telecommunications sector, modellers of organic carbon cycling, benthic biologists, physical oceanographers, and assessment of subsea geohazards for both government and industry.

Geohazards and Pipelines: Turbidity current pose a substantial threat to expensive and strategic oil and gas pipelines, or associated seabed infrastructure. For example, preliminary flow monitoring data from the Congo Canyon caused a pipeline to be routed beneath the canyon floor by Chevron and partners, at a total cost of $2 Billion. Our project will help to understand how frequency, duration and power of flows varies with distance along canyon, and hence where pipelines can be routed.

Oil and Gas Reservoir Characterisation: Submarine fans built by turbidity currents host valuable subsurface oil and gas reservoirs in locations worldwide. This project will produce perhaps the most complete dataset from across a large submarine fan, and the only one where recent deposits can be related to directly measured flows. This will be a key data set for understanding how flows are recorded by deposits, which underpins the generation of most turbidite reservoir models. For example, flow measurements and cores can show whether (and why) sand accumulates in submarine canyons.

Numerical and Experimental Flow Modellers: Laboratory-scale experiments and numerical models underpin many reservoir models, and understanding of these flows more generally. There is a compelling need to test these models against direct measurements from full-scale oceanic flows. This project will produce a timely and robust test of such models, as predicted flow evolution can be compared to the observed changes in flow measured at multiple locations along the canyon.

Source Rock Location and Characterisation: This project will help to understand how different types and ages of organic carbon are fractionated by turbidity currents, and how this determines their final location and burial efficiency. It will be an exceptional dataset for understanding how flow processes influence the location and types of source rocks deposited across large submarine fans.

Telecommunication Cable Networks: Seafloor cables have strategic importance because they carry >95% of global data traffic, including the internet and financial markets. It is important to understand turbidity current frequency and behaviour because they can break multiple cables. This is one of the few ways to seriously disrupt internet connections over large areas, as multiple breaks prevent mitigation by local rerouting of traffic. This project will seek to understand the general behaviour of these flows, and thus controls on the duration and magnitude of impacts on cables.

Organic Carbon Cycle Modellers: We seek to understand the role that turbidity currents play in transfer and burial of different types of organic carbon. Our results will be important for the wider community with interest in organic carbon sequestration, and its effects on atmospheric composition.

Benthic Biologists and Ecosystem Functioning: Turbidity currents supply important nutrients to submarine canyon ecosystems, which are hot spots of biological diversity. Degradation of the organic carbon sediment that they rapidly deposit can result in widespread oxygen minima. Our aim is for field data to be used by those interested in how valuable benthic ecosystems function in the deep sea.

Physical Oceanographers: This is valuable opportunity to quantify how turbidity currents mix different water masses, and how their importance compares to better known processes such as internal tides. Our water column data are thus of significant interest to physical oceanographers.

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