Why a mooring?
The purpose of the Regional Circulation Mooring (RCM) is to measure currents within the axial rift valley of Endeavour Ridge
at the regional scale, and to gauge the effect of hydrothermal venting on those regional currents. The energy released by the five high-temperature hydrothermal vent fields and the diffuse venting in the axial valley has been estimated to be about 1800MW, that of a mid-size nuclear power plant. This significant energy input in the form of heated water drives an inward circulation of about 5 cm/s at both ends of the valley, not unlike that which occurs when a sea breeze develops over heated land at mid-day. Our northeast RCM is the first of four moorings designed to constrain (or accurately estimate) the flow into the valley where, among other uses, it can be used as a proxy measurement of the total hydrothermal venting at the ridge.
Studying regional currents will help us understand how species inhabiting hydrothermal vents colonize new sites, sometimes separated by hundreds of kilometres. The larval stage of most species inhabiting the seafloor (benthic species) develop and grow in the water column, they are called planktonic. Those larvae metamorphose once they settle on the seafloor if they find a suitable habitat. We think that those planktonic larvae travel via the axial valley currents, or they are sometimes trapped in huge vent plumes created after a volcanic event, before finding a new site.
How does it work?
The mooring is composed of 3 different types of instruments affixed at varying depths along a cable extending from the seafloor upward 267m. Together, these instruments measure deep-sea current velocity in 3-dimensions as well as the physical properties (temperature and salinity) of the water. The top of the mooring line (267m from the seafloor and 1890m below the ocean surface) is lifted by a large buoy, keeping the line vertical at all times. The upper-most instrument, positioned at 1904m, is an Acoustic Doppler Current Profiler (ADCP). The ADCP can estimate currents up to 800m above the sea floor. Below this, four instrument pairs are positioned at 1954m, 2029m, 2104m, and 2149m in depth down the mooring line. Each pair includes a Conductivity Temperature and Depth (CTD) sensor and an Acoustic Current Meter (ACM). The RCM is anchored to the sea floor by a 650kg weight at 2154m in depth.
Installation of the mooring: a two-ship dance
The first step was to find a suitable location for the mooring system. It needed to be located on relatively flat ground close to our North RCM Instrument Platform. ROPOS was launched from the R/V Thompson to survey the seafloor. Once a suitable location was found, the installation commenced. This operation required very close collaboration between two ships: the R/V Thomas G Thompson and the CCGS John P Tully.
The CCGS Tully arrived with the RCM on the morning of 20 September 2010, after spending the previous day in the sheltered waters of Barkley Sound assembling the mooring and preparing it for deployment. Lining up in a parallel heading just abaft the port beam of the R/V Thompson, they began to stream out the mooring. By shifting floats, they were able to lower the 270m mooring from the top of the navigation and recovery package.
Meanwhile, ROPOS dove, navigated out of the way and waited 300m above seafloor. In order to precisely place the mooring on the ocean floor the same ultra short baseline (USBL) navigation system that navigates the sub was used. This enabled us to guide the mooring down by radio instructions from the lab of the R/V Thompson to the bridge of the CCGS Tully. The coordination between the two ships was phenomenal. To further fine-tune the placement, ROPOS followed the mooring downward and visually guided the last few metres.
The control of a 267m mooring hanging from a 2.3 km cable in a complex flow area such as Endeavour Ridge is not a simple task. The CCGS Tully does not have dynamic positioning and the mooring was placed by the formidable ship handling skills of the CCGS Tully’s bridge. Truly an amazing feat: the mooring was placed within a metre of its designated position.
Then the Tully ghosted away, like a rock star after the gig.
