Student Cruise Two
Falkor at Honolulu. Credit: Mark Schrope
The gravimeter, installed in the main lab, will be a key tool for the cruise's first leg. Credit: Mark Schrope
Gravimeter in the lab
On Friday, Falkor will depart Honolulu for the second time with something other than the standard complement of seasoned ocean scientists. While the expedition will produce valuable data, the focus will be on inspiring student participants to move toward careers in ocean science, and for those already headed that direction, to provide much needed training for future expeditions.
This second week-long student cruise, done in collaboration with University of Hawaii at Manoa (UH), will be split into two separate legs, doubling the number of students involved and allowing exploration of two different research questions using some of the most advanced oceanographic technologies available.
Leg One: Hidden Structure
The target for the first leg will be the southwest quadrant of the partially submerged structure known as Maui Nui. This is the geological complex that includes the islands of Maui, Molokai, Lanai, and Kaho’Olawe. Those are just the pieces that rise above the surface. If you add the below-the-water view, you see that all these islands are part of what looks like one large underwater mountain. “But we don’t really know the order in which things were put together,” says Robert Dunn, a UH marine geologist who will lead the cruise, “It’s a bit of a puzzle.”
Solving the puzzle will expose students to Falkor’s high-resolution sonar mapping system. But seafloor maps can’t answer all the relevant questions, mainly because as sea level has risen and fallen, huge amounts of coral have grown and died on Maui Nui’s slopes, obscuring the underlying geological structure.
To get around the challenge the team will also use a gravimeter and a magnetometer. The gravimeter takes very precise measurements of slight fluctuations in gravity caused by the differences in the weight of the rocky features below the ship. The magnetometer similarly measures small changes in magnetism.
Processing the data from these two instruments requires corrections made using data from the sonar maps. Ultimately, researchers can tease out other influences to produce data about parameters like the density of rock layers below all that obscuring ancient coral. This will allow them to identify relatively narrow, denser areas known as dikes where, during the area’s volcanic history, magma intruded out into various segments of Maui Nui as it grew, helping to form the main structures we see today. The patterns of those magma dikes will help scientists figure out the timing and pattern of the different islands’ formations.
During the second leg of the cruise, a separate group of students will get a taste of how biology and ocean physics interact. The team’s study site will be the submerged ridge that extends out from Kaena Point on Oahu’s west side, and their focus will be a phenomenon known as the internal tide.
When normal tides push water up features like this ridge, they can create submerged waves known as internal tides. The Hawaiian Islands are more or less perpendicular to the lunar tide flow, so this effect is particularly pronounced. The currents and vertical motions associated with internal tides have significant effects of their own, including a major role in the ocean’s energy budget. Although the energy from internal tides can spread across thousands of miles, the team will focus on localized, high-energy processes at the ridge.
The team will be looking mainly at turbulent mixing around the top of the ridge created as internal tides move up and over. The hypothesis is that this turbulence mixes nutrients from deeper waters together with plankton in surface waters to support more biological productivity than would otherwise be possible.
While exploring that idea, as well as the physical processes involved, the students will be working with two key pieces of equipment. The first is a microstructure profiler, which measures turbulence. It will be deployed off Falkor at different times during tidal cycles to create detailed maps of the small-scale current fluctuations that collectively lead to turbulent mixing above the ridge.
The students will also be working with Falkor’s CTD rosette to collect additional data about the waters below, including larger-scale current information. Water temperature will be another key parameter because it is a good tracer of turbulent processes. The team will also gather water samples at multiple depths. These will be processed aboard the ship and ultimately analyzed on shore to determine what animals are present, and to look fro chemical clues about where organisms get their food. By comparing against samples collected away from the ridge, the team will be able to assess whether the turbulence does in fact drive more productivity.
Most students have to wait until much later in their careers before having such an opportunity, says Glenn Carter, a UH oceanographer who will be leading the second leg. “This is a really unusual opportunity and the students are extremely excited,” says Carter, “I think it will solidify their interest in oceanography.”
--by Mark Schrope