Transporting sediment off of the continental shelf

This activity is used in an undergraduate Marine Geology course. The course is designed for majors in Marine Science, but any student who has taken Physical Geology may take this course. It is placed in the part of the course where we study sediment transport across and off of the continental shelf.

Students should be familiar with geostrophic flow, lutite flows, cascading and turbidity currents. If they are not, those concepts can be taught as part of this exercise.

This exercise is completed in class because the concept of bottom Ekman veering is new to them, and this exercise is a nice example of a geological application for what may seem like an esoteric topic.

This activity gives students the opportunity to apply their knowledge of geostrophy to derive ocean currents. Since they have previously learned about turbidity currents and lutite flows they apply that knowledge as they decide how to classify the intermediate nepheloid layers.

This activity requires that students identify that sediment is drawn off of the shelf in the cyclone and the seaward directed limb of Eddy Vasquez. They should attempt to address why this is so. In most cases, this provides the instructor with the opportunity to introduce the concept of bottom Ekman veering.

This activity is preceded by a lecture on continental shelf sedimentation. In this lecture we discuss the transport and deposition of shelf sediments in times of high sea level. Relict sediments are discussed as is the bypassing of the outer shelf by modern muds. Shelf edge processes capable of suspending muds are discussed which naturally leads into this exercise, an examination of a process that draws these muds off of the shelf.
Student handout for off shelf sediment transport (Microsoft Word 2007 (.docx) 354kB Apr20 17)

Students will need to be reminded of how to interpret a geopotential anomaly map. They can treat is as sea surface height and assume geostrophic flow. That will allow them to derive currents based on a balance between the horizontal pressure gradient and the Coriolis force.

Be prepared with a clear description of bottom Ekman veering. This is a nice follow on from the concept of geostrophic flow. Near the sea floor friction slows the current. This reduces the Coriolis force, since it is proportional to current speed. As a result the current turns in the direction of the pressure gradient. Since the pressure gradients in the cyclone and in Eddy Vasquez are in opposite directions, flows and sediment transport are in opposite directions. The onshore flow associated with Eddy Vasquez carries clear, slope water. The offshore flow associated with the cyclone carries the shelf bottom nepheloid layer out into slope waters.

A short discussion of particle bean attenuation coefficient may be necessary. It is sufficient to say that low values correlate with low suspended sediment concentrations and vice versa.

Students are given a similar problem on an exam. They are asked to explain the processes responsible for continental slope intermediate nepheloid layers.

Ocean Chemistry and Deep-Sea Sediments (1989) by the Open University Team is a good resource for lutite flows and turbidity currents.

This link gives an explanation of geostrophic currents
http://oceanmotion.org/html/background/geostrophic-flow.htm