March 29, 2010

Overcoming adversity

Sometimes things don’t turn out the way you plan, but you always have to make the best of what you have. We had planned to do a number of vertical casts and tows during this cruise with our CTD – an instrument system that measures the basic properties of seawater (conductivity, temperature and depth) with special sensors to detect the particle and chemical plumes coming from volcano, and several large bottles for collecting water samples. We wanted to map the extent of plumes from the eruption and measure their chemical impact to the surrounding ocean environment. Also, if there had been any recent landslides, there would probably be layers of turbid water deeper around the flanks of the volcano. But these plans were thwarted early in the cruise when the ship’s new crane and winch for handling the CTD developed problems that could not be repaired at sea. This was a very unfortunate setback.

CTD Instrument

However, when the going gets tough, the tough use MAPRs (an acronym for Miniature Autonomous Plume Recorder). MAPRs are small, self-contained instruments that can easily be attached to wires or lines of many sizes and have the same plume detecting sensors as the CTD. They do not have the rapid sample rate of the CTD, and they have no way to collect water samples, but the one MAPR we had on board did give us the opportunity to get at least some valuable information about the plumes over the volcano. Several MAPRs had been deployed on the moorings that were swept away by the landslide, so despite the volcano’s apparent appetite for MAPRs, we deployed the one we had as many times as we could. The versatile little MAPR was strapped to Medea for nearly every Jason dive, clamped to the line with every plankton net tow, and even deployed all alone at the end of the 700 meter line (the longest length of line we could come up with on board) with a 400 pound weight at the end. We even attached the small hydrophone to it to ride piggy-back for a few casts.

Data from the MAPR showing the plumes.

By being flexible and adaptive, we managed to create quite a few sampling opportunities. We were even able to repeat a line of casts south of the summit several times to show the short-term variability of the plume. While ROV dive observations showed just how variable and dynamic the conditions near the volcanic vents are, the MAPR profiles show both consistency and variability in the plume dispersing above the volcano.

One Jason dive went deep, downslope along the slide area to about 1850 m. The MAPR profile from this dive showed there were no lingering turbid layers around the flank of the volcano. This implies the landslide occurred long enough ago for the fine particles that were surely stirred up during that event to have either settled back to the seafloor or be carried away by local currents.

Despite the unexpected total loss of the use of our CTD, we are not coming home completely empty-handed.

This image shows a cross-section of the plume as defined by suspended particle concentration. A series of 5 locations south, and down-current of the eruptive vents, was sampled on four consecutive nights.

Photo of CTD on deck after a successful test cast at the very start of the cruise. Water was being rinsed through the bottles in preparation for the sampling program we anticipated.

MAPR being prepared for deployment with hydrophone attached.

Hydrophone strapped to the MAPR.

Sharon Walker
NOAA Pacific Marine Environmental Laboratory
Vents Program