Assignment: Ocean Sediments & Bathymetry

1. Sediments (to be done in lab)

(1.1) At each table should be a set of sediment slides containing various sediments from around the world. Examine the prepared sediments slides and determine their characteristics by filling out the table below.

(1.2) After you have completed the table, check your results against a key provided by your instructor. Which sediments are more influenced by land processes (erosion, rivers, etc). Explain.

(1.3) Given what we've learned about sediments and their sources, would you guess that the sediments of Puget Sound, which has 16 major and many minor rivers draining into it, are more terrigenous, biogenous, hydrogenous, or cosmogenous? Explain.

2. Core Sample

(2.1) By studying the sediments on the ocean floor, we can gather clues about what surface conditions were like at various times in the Earth's past when the sediments were deposited. Using those clues, we can try to reconstruct what might have happened during the Earth's past. As the particles pile up on the ocean floor, they begin to exert pressure on the sediments below. What do you think will happen to these sediments over time? Explain.

(2.2) Below is a 6 m long core sample taken from the deep sea floor off shows fine grained mud layers alternating with sandier layers. The volcanic ash at the bottom is dated as 7700 years old (after Adams, 1990). How many alternating sand-mud layers do you count?

(2.3) How can you explain these alternating course, sandier layers with fine particle mud layers? (HINT: Note that this sample core was taken in a submarine canyon)

(2.4) Given the location of the core sample and its proximity to the Cascadia Subduction Zone, these alternating sandy-mud silt deposits are probably caused by megathrust earthquakes. A megathrust earthquake occurs where one tectonic plate is forced under (subducts) another.
Due to the shallow dip of the plate boundary, these earthquakes are among the world's largest, with moment magnitudes (Mw) that can exceed 9.0. So how many megathrust earthquakes have we had in this region over the past 7770 years?

3. Hood Canal Bathymetry

INTRODUCTION: If you haven't heard the news lately, Hood Canal is in trouble. Large inputs of non-point and point source runoff and have lead to low oxygen environments especially in the deeper basins of Dabob Bay and Hoodsport. Let's examine the bathymetry of Hood Canal to see if it can lend any clues as to why this particular part of Puget Sound is so impacted.

(3.1) Obtain a map of Hood Canal. What is the scale on the map?

(3.2) What unit are the depth measurements?

(3.3) Obtain a straight edge and a pencil. Starting out near the opening of Hood Canal (near Admiralty Inlet) draw a light pencil line down the center of the canal using a straight edge, changing direction as the canal varies. Continue drawing a center line the entire length of Hood Canal all the way to Lynch Cove. How many nautical miles long was your line?

(3.4) If you were to divide your line into 30 increments, how many nautical miles would each increment be?

(3.5) Estimate the depth at each of the 30 points and fill in the table below. Remember to make depth numbers negative. Using Excel or other spreadsheet program, plot the bathymetry of Hood Canal along your line with a 30 increment setting (i.e. divide your line into 30 equal points and plot each point with the depth in fathoms). Include a print out of the graph with this lab making sure you have a title, description and properly labeled axes.

(3.6) Describe the overall shape of Hood Canal's bathymetry.

(3.7) What is the origin of the Hood Canal Basin (i.e. how did it form?)

(3.8) Are there any features of Hood Canal's bathymetry that your group can see that might influence its circulation? Explain.

(3.9) Locate on your bathymetric drawing of Hood Canal a inlet named Dabob Bay. This area has been in the news recently as a place in trouble because of pollution and nutrient runoff from the surrounding farmlands and urban areas. Is there anything that you can see from its bathymetry that might indicate it has a particular susceptibility to these problems? Explain.