Quantifying transport of sediment off of continental shelves
This activity requires students to manipulate oceanographic data
Skills and concepts that students must have mastered
Students should have some background in Excel, including the use of functions and the making of plots. Prior to, or in conjunction with, this exercise students should be introduced to the ways in which tides, currents and waves impact sediment on the continental shelf.
The exercise presumes that students either understand geopotential anomaly, or are instructed in it as part of the lesson. A brief description is given below in Teaching Notes and Tips.
How the activity is situated in the course
This activity can follow another SERC activity, https://serc.carleton.edu/teachearth/activities/178061.html. In that activity students see that understanding the physics of water motion helps in understanding sediment transport. In this activity students quantify the impact of a process on sediment removal.
If instructors choose not to use the SERC activity mentioned above, this exercise can be used as an example of a deep ocean process that can be important in removing sediment from the shelf. Then this activity would nicely compliment material on shelf edge processes and the interaction of those with deep ocean processes.
The goal in either case is the same, to have students manipulate data, make some calculations and come to a conclusion.
Content/concepts goals for this activity
This is primarily a data analysis exercise and the goals are:
- Evaluate the correlation between two measured variables.
- Manipulate a measured variable to obtain a desired variable.
- Become comfortable with back-of-the-envelope calculations as a way to evaluate the importance of a process. In this way it follows observational science, where information collected in one study points to a process that requires more study.
Higher order thinking skills goals for this activity
Other skills goals for this activity
Description and Teaching Materials
Instructors should provide students with a hard copy of the student handout, preferably printed in color, a hard copy of either the handout that explains how particle beam attenuation coefficient is derived from light transmission data or the light attenuation challenge handout. Additionally, instructors should make the Excel data file available either by email, a course management system or a thumbdrive.
Depending on the student background instructors may want to describe the formation of eddies or rings. See the reference section below for sources of information. The generation of anticyclonic eddies is well understood, they are the result of Loop Current meanders pinching off of the current. Cyclonic eddies (in the Gulf of Mexico), such as the one examined in this exercise, are less well understood but at least sometimes seem to be spun up by the anticyclonic eddies. This exercise focuses on the manipulation of oceanographic data, so a quick simple explanation of the oceanographic processes should suffice.
- This is the student handout that explains the exercise. Quantifying transport of sediment off of continental shelves (Microsoft Word 2007 (.docx) 2MB Nov6 17)
- This downloadable Excel file includes all of the data needed to complete the exercise. Data file for Quantifying sediment removal (Excel 2007 (.xlsx) 22kB Sep21 17)
- This handout explains how particle beam attenuation coefficient is derived from light transmission data. Explanation of light attenuation (Microsoft Word 2007 (.docx) 15kB Nov6 17)
- This handout is for students with more advanced math backgrounds. Use it instead of the previous one. It challenges them to derive the formula for particle beam attenuation coefficient from light transmission data. Light attenuation challenge (Microsoft Word 2007 (.docx) 15kB Nov6 17)
Teaching Notes and Tips
Students may need some tips on programming a function into Excel. The conversion of light transmission data into particle beam attenuation coefficient requires several operations, all of which can be accomplished in a single cell in the spreadsheet. However, some students will find it easier to treat the function as multiple steps (one step for each operation), and use a different column in the spreadsheet to achieve each step (see associated figure). Check data have been given in the spreadsheet so students can confirm that they have correctly entered the function.
Instructors may want to use the plots as an opportunity to emphasize the choice of appropriate scaling and the use of descriptive titles. The resulting plots should be understandable by a larger audience, not just those who have completed the exercise.
Students may 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 current direction based on a balance between the horizontal pressure gradient and the Coriolis force. In the case of the cyclonic eddy in this exercise, the surface and mid-depth currents parallel the geopotential anomaly contours, carrying water around the eddy in a counterclockwise direction. Friction slows the currents at the seabed. This decreases the Coriolis force making the current veer down the pressure gradient. This is called bottom Ekman veering. In the case of the cyclonic eddy in this exercise, bottom Ekman veering carries water, and the associated suspended sediment, off of the shelf.
We assess student mastery of the tools (i.e. Excel) and the content of this exercise by giving them another data set, having them plot the data, and then compare the results to the station profile they made in the exercise. This file contains both the data and the instructions for this assessment.
Assessment question (Excel 2007 (.xlsx) 12kB Sep22 17)
References and Resources
This link has more information on the inherent optical properties of seawater including an explanation of why the units for beam attenuation coefficient (and therefore particle beam attenuation coefficient) are m-1. The link also has information on the several processes that serve to attenuate light as it passes through water. http://www.oceanopticsbook.info/view/overview_of_optical_oceanography/inherent_optical_properties#fig:C3_defineIOPs
This reference describes the formation of Gulf Stream rings. The cyclone examined in this exercise was probably spun up by an associated anticylone. That anticylone had a genesis just like a warm core Gulf Stream ring. https://marine.coastal.edu/gulfstream/p5.htm
The formation of cyclones in the Gulf of Mexico is different than those formed as cold core rings from the Gulf Stream. This journal article discusses the characteristics of these cyclones and documents their travel from east to west in the Gulf.
Hamilton, P., Lower continental slope cyclonic eddies in the central Gulf of Mexico, Journal of Geophysical Research, 97(C2), 2185-2200, 1992.
This reference defines nepheloid layer, and has a brief description of the nepheloid layer on the Texas continental shelf. https://en.wikipedia.org/wiki/Nepheloid_layer
For those who want to delve more deeply into this topic, this journal article discusses mechanisms for the formation of intermediate nepheloid layers similar to the one examined in this exercise. https://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/17037/Pak_et_al_JGR_1980.pdf?sequence=1