CD 157: Investigating the submarine canyons off Portugal


What does the mud tell us?

Research cruises are expensive to what is it about deep sea mud that's so important?

Part of the reason for the cruise is to collect sediment samples from various, carefully selected points on the sea bed around the underwater canyons off Portugal. When the corer penetrates it collects a column of mud and sand which provides a historical record of past events. These seabed samples are taken at known depths and very precise locations using seafloor maps and profiles as a guide. By putting all of this information together it is possible to create a description of events on the sea floor.

A typical sediment core showing turbidite layers

It takes an expert to interpret the detail but you can see from the sample we recovered on 1st June 2004 that there are some definite patterns. This particular sample is interesting because it shows a series of underwater ‘avalanches’, called ‘turbidity currents’. These events happen very quickly; for example in a few minutes several centimetres of sandy sediment can be laid down. This will have come from further up the slope or even from the continental shelf, and is usually carried into the sea by rivers.
The turbidites may be separated from each other by layers of fine mud or silt. These layers represent a period of stability with very gradual deposition of fine material sinking through the ocean waters. In these layers you often see circular or tube like patterns. These are made by worms (similar to a lugworm) which burrow through the near-surface sediments seeking food. This activity mixes the layers.

[read more about turbidites]

We also look for the skeletons of small organisms called Foraminifera in the sediments. The presence of these tiny microfossils indicates a period of low sedimentation rate. Identification of these back at SOC will also enable us to date selected layers quite precisely. By dating the layers the geologists can construct a model of what has been happening to the sediment in shaping the canyon.

This work will be useful to several audiences. Oil companies are interested in the pattern of sedimentation, especially sandy layers which can act as oil reservoirs. They also like to know if it is safe to put underwater pipelines along certain routes. [This is not the case in our canyon]. Analysis of the sediments tells us about the movement of pollutants from man’s activities on the land and also helps us to account for the movement of carbon in the carbon cycle. This is important in understanding aspects of global warming.

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April 2004
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