JC36: Geology & biology of the Whittard Canyon


Cruise diary

22 July 2009
Location: Whittard Canyon, 48º 16' N / 10º 09' W

Henry writes:

Even in the deepest parts of the ocean the combined influences of the solar and lunar driven tides are plainly seen. As any coastal enthusiast knows most tides have a semi diurnal cycle. That is to say they have two high tides and two lows daily with just less than 12 hours between the high tides. These tides are caused by the gravity of the sun and moon pulling on the oceans water causing it to bulge in some places and be dragged from others. These tides vary as Earth turns and the position of the sun and moon relative to Earth change.

These tides can have a major impact on currents at the seabed. Below is a figure showing how the water depth (black line) changes twice daily in relation to the semi-diurnal tide in the Whittard Canyon. Also shown is the temperature (blue), salinity (red), and potential density (green) of the water at the seabed nearly 3,700 m depth (about 2.3 miles deep), which are correlated to the changing tide. Changes in these water properties are often indicators of changes in ocean currents.

As part of our studies, we’re measuring currents in the canon. We expect that these changes in water properties will relate to changes in current speed and direction. These currents can be magnified by complex topography like that found in canyons. Variable currents have interesting impacts on things like the transport of food, but also occasionally make working at the seabed very challenging. During the solar eclipse yesterday, when the moon and sun were both ‘pulling’ from the same direction, the tides were particularly strong. These currents can reduce visibility and increase the time it takes to conduct seafloor activities. Fortunately, though, the most intense periods last only a couple of hours. But as the graph shows the tide turns again soon.

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