Harriet Cole
A hydrographic study on the effect of Rumble III on the thermohaline structure and circulation
Rumble III is an active submarine volcano located on the Kermadec Arc, north of New Zealand at -35.75°N, 178.5°E. During March 2009 a hydrographic survey was conducted to measure temperature, salinity and density changes with water depth near the volcano. Water samples were collected to measure nutrient and oxygen concentrations. The speed and direction of the currents were measured using a 75kHz Acoustic Doppler Current Profiler (ADCP). Using this data, evidence of upwelling and downwelling was found. Upwelling occurs when water from deeper in the water column is pushed up towards the surface. Downwelling is the opposite of this. By plotting temperature, salinity and density changes with depth the uplift of isotherms, isohaline and isopycnals (levels of the same temperature, salinity and density) was seen throughout the water column up to 75 meters below the surface. The distribution of nutrient and oxygen concentrations agreed with this pattern with higher nutrient concentrations seen where the isotherms, isohalines and isopycnals were uplifted and lower concentrations seen where they dipped down. The distribution of oxygen was the inverse of this because oxygen concentrations decrease with depth whereas nutrient concentrations increase with depth. From the ADCP data, internal tides were found. Internal tides are waves within the water column that repeat over the same time period as the normal tide. These are generated when the normal tide passes over a topographical feature such as a ridge, seamount or volcano, vertically displacing the water. This provides a mechanism by which the upwelling of water can occur on short timescales. Another mechanism that would result in upwelling is the formation of an eddy; a circular flow pattern that is tens of kilometers wide. An eddy above the volcano would result in water being pulled down over the summit and then spread out horizontally. This then causes water to be upwelled further out from the volcano to replace the water being downwelled. Evidence for this type of circulation was found at Rumble III. Nutrient rich water is upwelled to 75 meters depth above Rumble III which is in the zone where light penetrates. This may increase primary productivity in the surface waters by providing a nutrient source. Photosynthetic organisms will use these nutrients to grow by taking carbon dioxide out of the atmosphere and turning it into organic matter. When the organisms die this organic matter sinks to the sea floor where it may be buried. A high rate of carbon export to the sea floor means a high rate of carbon dioxide removal from the atmosphere. This has global climate implications as it is a means by which levels of atmospheric carbon dioxide can be decreased.