Quantifying transport of sediment off of continental shelves

Lauren Sahl, Corning School of Ocean Studies, Maine Maritime Academy

Denis Wiesenburg, Department of Marine Science, The University of Southern Mississippi

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Summary

This activity requires students to manipulate oceanographic data

to derive new variables, make plots, interpret plots and make back-of-the-envelope calculations to weigh the importance of eddies in shelf edge sediment removal. It provides a chance for students to use some of the function and plotting power in Excel. It also requires that they use information from their plots to make calculations to estimate magnitude.

Context

Audience

This activity is used in an undergraduate Marine Geology course in the part of the course that addresses modern continental shelf sediment dispersal processes. It could also be used in a graduate level Marine Geology course. We precede this activity with another SERC activity (https://serc.carleton.edu/teachearth/activities/178061.html) which introduces students to the concept of bottom Ekman veering and the impact on sediment transport. In that activity students learn about the process. In this activity students quantify the effects of that process. So using both activities teaches both the process and the product. However, this activity can be stand alone as a data analysis activity. In that case just use the supplied data to achieve the exercise goals.

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.

Goals

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

This activity focuses on the analysis of data and concludes with students having to describe a further study that would allow them to proceed from back-of-the-envelope calculations to measurements that would allow them to quantify the impact of the studied process. This requires reflection on both the data set presented, and on the other tools available to oceanographers.

Other skills goals for this activity

Students may work in pairs to determine useful further studies. This process requires that they understand the ultimate goal of a future study, and understand how oceanographic observations may be combined and manipulated to achieve that goal. Instructors may have them search the WWW for studies similar to the one they describe. They may evaluate those studies, to determine if they do achieve the goal they have identified. This part of the activity lends itself to an energetic class discussion.

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 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.

Assessment

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

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