Edge of the Arctic Shelf

An Unexpected Visitor
All Marshall Swartz wanted to see on this cruise was a walrus. Today he got his wish, and then some! For the last six CTD casts of the day, a young walrus turned up and entertained us with his antics. Like an expectant puppy, he followed our ship from station to station. Even though he couldn't match the ship's speed, he caught up every time we stopped for a CTD. He circled the ship, eyeing us -- could it be -- with amusement? Even as darkness fell, Marshall and I remained on the fantail, unable to pull ourselves away. The walrus pirouetted in the water and put a flipper up to his head. Marshall did his best walrus calls, and while I'm not sure what he said, I think the walrus understood because he grunted and snorted back at us from the water. We were broken hearted every time the CTD came back on deck and it was time to leave our friend behind.

Mrs. Cadwell, a fifth grade teacher at Varnum Brook Elementary School in Pepperell, Massachusetts, has some questions about seafloor mapping. In her math class, she is teaching her students how to chart the ocean depths. For a great essay on the operation of depth-finding sonars, by mapping expert Val Schmidt, read yesterday's dispatch.

Question: The shelf you are placing the moorings on, how far is it from the actual slope?
Answer: Click here to see a cross-sectional view of the mooring locations. The shallowest WHOI mooring is in 57 meters of water, on the continental shelf. This is the broad, flat plain stretching out from the shore. Moving offshore, the next bathymetric feature we encounter is the shelfbreak. The shelfbreak is defined as the point where the flat continental shelf begins to "drop off." Looking at the plot, you might guess that the shelfbreak is between mooring numbers five and six. In actuality, if you take a closer look at the plot, the first break (although not as steep) is between moorings one and two. The region just offshore of the shelfbreak is known as the continental slope. The rest of the moorings, from two to eight, are all on the continental slope. The continental slope gradually becomes less and less steep until it becomes the "continental rise." When the bottom becomes completely flat again, we have reached the abyssal plain.

Question: How far down does the slope go?
Answer: The slope ends at the edge of the Canada Basin; the depth there is roughly 3,800 meters (12,500 feet or 2.4 miles!) This is one of two deep arctic abyssal plains - the other is the Eurasian Basin, on the European and Russian side of the Arctic.

Question: At what degree?
Answer: The average angle of the slope, from shelfbreak to continental rise, is 4%. While this may not seem very steep, remember that this is the average slope over 60 kilometers. Certain segments of the slope are steeper than others. The steepest angle along the entire slope is 23%! As a reference, the New Hampshire Mount Washington auto road grade is 11%. If you've ever driven (or run or hiked) up the tallest peak in New England, you can appreciate just how steep 11% is!

Tomorrow is show time for the WHOI mooring team. The deepest mooring will be redeployed immediately after breakfast. Thanks to Ryan Schrawder's efforts (including some technical support via satellite phone!), the moored profiler is ready to go to sea. We also plan to deploy one of the three Acoustic Recording Packages.