CD 157: Investigating the submarine canyons off Portugal


 


The Portuguese submarine canyons

Did you know that only a few km offshore Portugal there lies a network of submarine canyons and gullies comparable in size to the Grand Canyon in the USA?

Just 8 km from the shoreline at Lisbon, in water less than 100 metres deep, lies the head of the Lisbon-Setubal Canyon. This canyon is a vast gorge cut deep into the seabed, believed to have formed by the currents and sediment spewed into the Atlantic by the Tagus and Sado rivers.

Above: Map showing the Setubal-Lisbon canyon system. Click to enlarge.

Submarine canyons often act as chutes for sediment to travel from the continental shelf down to the deep sea basins. In the case of the Lisbon-Setubal Canyon, the sediment comes from the Tagus and Sado rivers, continental shelf sediment stirred up by storms, and sediment which is naturally moved along the coast by currents. All this sediment gathers at the head of the canyon (the part nearest to the coastline), but little is known about how the sediment then moves down towards the deep sea. During the cruise, the team will be looking for evidence that the sediment moves downhill due to slope failures - a very similar process to snow building up on the sides of a mountain until it becomes unstable, causing an avalanche to occur.

We know that this type of sediment transport process occurs in the area, because turbidite deposits are found on the Tagus Abyssal Plain to the east of the canyon. Turbidite deposits are the end result of a sediment 'avalanche' flowing down the canyon and out onto the abyssal plain. The deposit forms as the cloud of sediment particles carried by the turbidity current settles out of the water column and builds up on the seafloor [click here to find out more about turbidity currents and turbidites]. The most recent turbidite deposit present on the Tagus Abyssal Plain can be accurately dated at 1755, when a major earthquake hit Lisbon. This earthquake triggered a sediment avalanche which eventually settled out to form a turbidite deposit on the seafloor [click here for more on the Lisbon earthquake].

In general, many of the turbidite deposits in the deep sea are too large to be sourced from just one canyon: single canyons simply don't hold enough sediment to create a deposit that big! In these cases, marine geologists suspect that some of the sediment comes from more than one canyon, and may include some sediment from the areas in between the canyons too. On the Tagus Abyssal Plain, it is unclear how much sediment comes from the Setubal-Lisbon Canyon alone and this is one of the problems we will be investigating on the cruise. The end of the canyon is marked by a small sandy fan deposit, which has probably been formed by small, frequent sediment flows down the canyon over time. Therefore, this sediment fan holds the complete history of all the sediment that has moved down the canyon!

The plan of action

So, how exactly do we plan to investigate how the sediment moves down the canyon? We have already collected sidescan sonar data from the area around the sandy fan at the canyon mouth, and this information will be used to target specific sites for taking sediment cores within the fan.

We will also be taking sediment cores (like these on the left) along the length of the canyon so that we can identify places where recent sediment flows have occurred. To compare the canyon sediments with those outside the canyon, we will also take a few cores from the continental shelf adjacent to the canyon.

A special underwater camera system (called SHRIMP - pictured right) will be used in the broader parts of the canyon to look for sedimentary features such as ripples which indicate areas where currents are active. We will then compare this information to data collected by other scientists in the area to see if the data match. The camera will also be used to record the biology found in the canyon, and how this relates to the type of sediment present.

In addition to the coring and camera work, a seismic system will be used to look at the structure of the underlying geology at the canyon head.

This page is still under construction...please bear with us!

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© Challenger Division for Seafloor Processes
April 2004
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