JC010: Mud volcanoes and submarine canyons


Meet...Raquel Arzola, PhD student

Raquel is a PhD student at the NOCS looking at the geology and sedimentology of the Portuguese Canyons. Her main aims are to identify how sediment is transported, including timing and the effect on the animals that live in the canyons. She uses piston coring to obtain long sedimentary records (up to twenty metres long). By looking at different types of sediments she can determine where they come from, how they entered the canyon and how they transformed into sedimentary flows. An example of a flow is a turbidity current. Turbidity currents are formed when sand and mud mixed in the water has higher density (is heavier) than sourouding water, so it flows due to gravity and only runs out when all the sediment content has been deposited.

To determine the timing of these flows she finds the deposits of the turbidity currents and dates them. She does this by finding shells of little marine organims that were deposited at the same time and then dates them by finding the amount of radioactive carbon isotope present. She will send the shells for radiocarbon dating to a laboratory in Scotland. Once she has the results she will have more information about the sedimentary transport and the activity of the canyons.

Raquel explained that the cores can reccord different periods of time, depending on how active the canyons are. But she expects to reach many hundreds of thousands of years in one piston core. She will mainly study Quaternary sediments but she expect to obtain some that are older.

Left: Muddy sediments with organisms. Right, erosion of sediments on the Nazaré Canyon

During the past there have been lots of climate changes, and these are recorded in the sediments. During the cold periods more sea water was held in ice caps so the global sea level was lower. This created bigger coastal areas. Rivers could erode more material and take it to the sea. Because of this, we find many more sediment flow deposits in the canyons during cold periods. On the other hand, higher sea levels during warm periods cause smaller coastal areas so less material enters the canyon. So we can say that canyons were much more active during cold periods than during warm periods like today. During cold periods the sedimentation rate can be much higher (up to 1cm per year) but during warm periods sedimentation rate tends to be around 1 mm per year or less. In the Nazare canyon this sedimentation rates are much higher than in the adjacent Portuguese Canyons.

I asked Raquel how the sedimentation rate of the canyons can affect organisms. Raquel told me that it will affect them in several ways. Firstly, knowing where the sediments enter the canyon will control where the organisms live because the flow can be very destructive. Secondly, different organisms like different types of sediments - for example worms like to burrow in soft mud but corals like to be attached to hard surfaces or rocks. And finally, she can study how long it takes for the animals to recover in an area that has been destroyed by a sediment flow.

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