Going for the top!

8 March, 2010

Yesterday we came across a remarkable ocean feature – a seamount. Seamounts are common in the world’s oceans, and their origin is diverse. The seamount we mapped yesterday is a part of a submersed ridge (North Brazilian Ridge) of tectonic and volcanic origin.

We spent several hours mapping the seamount, which was located in approximately 3,500 meters of water. Mapping the ocean floor is a long process; the ship’s equipped with a Seabeam, which can ‘see’ at that depth about 6 kilometers on each side. That means that with each transect, the Seabeam covers an approximate footprint of 12km. We also use a high resolution sub-bottom profiler, which allows us to determine how the bottom sediments are, whether they are soft and layered, or made of harder rock. In order to cover the entire seamount (24 km wide, 48 km long) we spent approximately 12 hours. However long, mapping the ocean floor is a fundamental necessity of coring – without knowing what’s down there we cannot make an asserted decision of where to take ocean floor samples. There is also the added risk of sending our instrumentation against hard rock, which would probably damage it irreparably.

After the mapping was completed, two coring locations were chosen and a gravity core was taken at each (see map). A box core was also taken at the second location. We were quite surprised at the sediment we recovered in the box core – there appeared to be three distinct layers – an upper one of calcareous sand, a second one of consolidated calcareous silts of light brown color, and a third one of compacted mud of a darker shade of brown. Our expert geophysicist Cleverson pointed out that the seamounts in this area are thought not to receive much material from the continental shelf, as they are relatively isolated, and the majority of the sediment from the shelf reaches the deepest portions of the ocean floor via slumping, or mass wasting. The sand and silts we observed in the core were likely of pelagic origin, raining down and accumulating on the seamount from the water column above it. It provides an ideal location to understand what processes have occurred in the water column, away from the coast, in the past few hundred years.

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