by Ryan McElroy, Student Intern
Coral reef mapping efforts aboard the NOAA Ship Nancy Foster this past week have been exciting for everyone involved. By night, survey specialists “mow the lawn,” collecting multibeam data they compile into bathymetry images. Geospatial scientists take the data from there and run mathematical analyses that produce products delineating habitats. Just after day breaks, ROV operations begin to ground truth these maps and any echograms collected. Ecologists sit on the edge of their seats watching the live feed and noting, among other things, coral abundance, ground cover, and fish species present. Repeat this process everyday for twelve days, and you’ve got yourself a very robust data set with which a variety of questions can begin to be answered.
As an intern on this cruise, I have been fortunate to experience a bit of everything going on. Deploying the ROV throughout the morning, organizing files, working on maps, learning my reef fish, and tracking the autonomous glider are just a couple highlights. It is fascinating to see all the data get collected and to begin to understand how much prep work, organization, and effort goes into the project from so many different people.
I am midway through undergraduate study of geology. Most of my field experience up to this point had been in the Green Mountains of Vermont, with a little time spent on out Lake Champlain. Getting the opportunity to see some marine geologic features only before discussed in class is amazing. Here are some of my favorites from the trip so far:
We can visually explore the ocean floor with the remotely operated vehicle all the way down to 300 meters. Off the coast of St. Croix and St. Thomas, this means shallow reef features and shelf breaks.
Rhodaliths are small round “rocks” formed by living red algae. Calcium carbonate secreted by the organisms builds up into balls that cover the ocean floor. Over time, this calcium carbonate can be buried and will contribute to forming limestone that may one day be brought again to the earth’s surface
Coral reef is a rocky biogenic structure with complex three dimensionality. Hard corals secrete calcium carbonate and attach themselves to a pre-existing hard surface. As each of the tiny polyps in the colony grow and reproduce, so does the rocky home the live on and in. Topography attracts fish of all sizes, as protection and food abound here.
I have seen coral fossils in 400+ million-year-old Vermont limestone!
Furrows are a sedimentary bedform. Sands and particulates of the seafloor are sorted and distributed by currents. Helical (spiraling) flow along the floor brings finer material up and out of the troughs and deposits it on ridges. The troughs become coarser (heavier pieces remain) while ridges grow ever finer. On the dive where the above image was taken, the troughs were about a half meter deep and nearly one meter across. The ridges continued linearly for tens of meters.
In waters deeper than 300 meters, the only way to “see” the ocean floor is with sound. Acoustic beams are sent down under the boat, and based on the time it takes for the beams to return after reflecting off the bottom, a deeper bathymetry map is created.
Submarine canyons are long, deep incisions in the slope off the shelf break. Conduits for moving sediment out further to sea via turbidity currents, they can extend more than ten kilometers out off the shelf break, dropping about one kilometer over this distance. The canyon walls in the above image measured to 200 meters high at some points. Evidence of canyons and the fans of sediment that accumulate at their base can be found in old sea floor rocks exposed at the current surface. I have seen some of these “turbidite sequences” in Vermont rocks.
A slump is an underwater landslide. Slope failure can been triggered by overburden, earthquake, or active faults. This slump seen above is HUGE–over seven kilometers across! The scarp wall starts at a depth of 370 meters. Over 90% of the released material is likely out in deeper water beyond the range of the mapped area (more than 1500 meters down). With sonar lines that can penetrate the ocean floor, core samples, and a lot more time, three-dimensional maps of sediment and bedrock could be made to trace the extent of the debris flow generated by this slump.
A failure this big can trigger tsunamis – massive waves. One of my professors is working on better understanding slumps, what triggers them, and their effects on sea state. He logs data in Lake Champlain and models waves generated by events like this. The main difference is that there everything is scaled down quite a bit!
Seamounts are unusually high points of relief given the gradual slope of surrounding sea floor. The image above is a potential seamount, which measures 195 meters in diameter and rises roughly 60 meters from its base at 880 meters depth. However, we don’t know much beyond that! One possibility is that it is a cone emitting something—possibly gas or mud. Material may be precipitating out and “growing” the cone. If we had passed over while collecting “water column” data with the multibeam, we may have been able to detect emitted material. Unfortunately it is out of range of our ROV, although there are vehicles that could investigate features this deep. Other possibilities could be that this feature is a relic reef or even the remains of an old slump.
It amazes me that even with top of the line technology and brilliant minds at work, there is still so much that we do not yet know about our planet. With every new discovery, more and more questions come up. Years of work could be spent measuring, sampling, and observing these features, but at the end of the day, there’s something wonderful about a mystery. What is so cool to me is that we would never have known these things were down there unless we looked! There is beauty and complexity at every level. For a curious kid like me, it’s great to know I can always learn something new.