Moored Dye Observations

Even though we are using a variety of modern (a few of them truly novel) instruments on board the Discovery, at times measuring what is going on deep down in “our” canyon in the Rockall Trough feels like looking for a needle in a haystack. Understanding measurements of an ocean that varies rapidly in space and time can become truly difficult. Think of our dye survey for example. We raced around with the ship to map out the fluorescein as it was moving with the tide, at times barely able to catch up with the dye blob. If only we could be in more than one spot at the same time to obtain a more coherent picture of the near-bottom dynamics!

While we cannot split the ship in two halves, we can lower instruments into the water and leave them sampling near the seafloor while collecting measurements from the ship elsewhere. This is what we did as we deployed an oceanographic mooring for the duration of the dye experiment. Moorings are structures built from anchors, line, and buoys. The anchor keeps the mooring line in one spot at the bottom, the buoy pulls the upper end upwards, thereby keeping the line taught. Depending on the application, a mooring can reach from just a few meters above the seafloor to thousands of meters tall. Oceanographic instruments attached to the mooring line record measurements while us scientists can use precious ship time to carry out other measurements. In some cases we even travel home, go about our daily lives and come back a year or two later to hopefully still find the mooring sitting at the ocean floor and pick them up.

Left: The moored profiler in the lab with its various sensors labeled. Photo credit: Matthew Alford. Right: Moored profiler attached to the mooring line and about to be lowered into the water during deployment. Photo credit: Zoltan Nemeth.

We deployed the dye experiment mooring on June 28^th a few kilometers down-canyon from the dye release site in about 2000 m water depth. We attached a moored profiler equipped with a number of sensors to the mooring line. The moored profiler has a motor that clamps onto the mooring line, moving the whole instrument package over a pre-programmed depth range. In our case, we set the profiler to go as close to the bottom as possible and to cover a depth range of about 500 m above the bottom. We carefully adjusted the density of the profiler to match the density of the water inside the canyon, thereby making it neutrally buoyant to reduce the motor load needed to drive the instrument package up and down. At a vertical speed of about 0.35 m/s, the profiler was set to record a vertical profile about every half hour. To gather measurements, we attached a current meter to measure horizontal flow velocities; a CTD to measure temperature, conductivity, and pressure, which are needed to calculate ocean density; turbulence sensors to estimate how water is being mixed; and a fluorometer with the hope that we would detect dye if it were to flow down the canyon from where we were to inject it.

As usual before a mooring recovery, we were a little nervous as we got ready to retrieve the mooring back on July 5^th. Will the release mechanism near the bottom of the mooring work to leave the anchor behind at the seafloor and let the rest of the mooring float back to the surface? Did all the instruments measure as they were supposed to? Will there be any interesting features in the measurements? And one of the pressing questions of this specific experiment: will we see any dye coming down the canyon?

Moored profiler observations in height above bottom and time. Top: Temperature time series. Bottom: Down-canyon velocity time series.

A first look at the moored profiler data revealed exciting features of the near-bottom flow. As the tide moved up and down the canyon, the near-bottom temperature and associated density at the mooring site changed drastically. Periods of intense down-canyon flow were followed by short but even stronger pulses of water moving up the canyon. And then, around midnight on July 2^nd, about 20 hours after injection further up the canyon, a peak in fluorescence – the dye patch arrived at the mooring!

Fluorometer data from the moored profiler in height above bottom and time. A higher voltage indicates stronger fluorescence in the water. The dye patch is clearly visible around midnight on July 2nd.

The initial pulse of dye showed up during down-canyon flow in water temperature of about 3.8°C, very similar to where we injected the dye. An hour or two later, the mooring observed a second, broader pulse of dye, this time in slightly warmer water, indicating mixing of the dye with waters above. A tidal phase or about 12 hours later another dye signal, at diminishing amplitude. Towards the end of July 2^nd the battery of the fluorometer ran out, but we are very happy it lasted long enough to observe the dye down-canyon – all the while we were measuring from the ship further up the canyon. Stitching together the observations from these different platforms to obtain a coherent picture of how the dye was moving around and being mixed will take a while and is a difficult task, but we are hopeful that our dataset is sufficient for a basic understanding of the dynamics at play.