February 19, 2014
Over the last few weeks on this blog, the main focus has been on the sediment sampling and coring methods of the ship. While the sediment and associated analyses are an integral part of interpreting the history of the Sabrina Coast, it is only one piece of the story. Many oceanographic samples have been taken in addition to the sediment core samples. Throughout the cruise we also have completed seven CTD and seventy-five UCTD casts, and have deployed four moorings. Whereas the core samples help us to interpret past climate, the oceanographic data taken using CTD, UCTD, and moorings help us describe the present water conditions. Like the variety of core types, each type of oceanographic sample is used for a distinct purpose and under specific conditions.
Conductivity, temperature and depth, or CTD measurements, are taken by lowering a suite of sensors that are mounted on a large rosette. These sensors measure characteristics of the water column, including temperature, salinity, oxygen concentration, fluorescence, and turbidity – all with respect to depth. In addition to recording physical and chemical characteristics of the water column, the rosette can bring water samples back to the surface. Twenty-four bottles make up a large portion of the body of the CTD device. These bottles are programmed to close at specific depths, making it is easy to compare traits between depth intervals at one location. In order to properly sample, the ship must be stationary during a CTD cast so that the rosette, which is attached to a cable system, can be lowered over the aft starboard deck. Deployment time will vary depending on water depth but in continental shelf waters a CTD cast may take only an hour to complete.
The benefit of taking a full CTD cast is the variety of data it provides, as well as the water samples that can be saved and taken back to labs for further analysis. This type of sampling is thorough but only gives detailed information about the structure of the ocean at a given location. A new technology, the UCTD cast is an underway method of CTD sampling that has the benefit of taking less time to deploy, typically taking 8-15 minutes instead of an hour. In addition, the ship can continue moving at a speed of approximately 5 knots during deployment, which makes the UCTD cast convenient for use during transits. UCTD technology records temperature, turbidity, and salinity versus depth, however it does not bring up physical water samples. This makes the UCTD cast useful when we want a quick look at the physical oceanography of a region but don’t have the time for a full analysis.
Both of these technologies help to interpret the oceanographic structure at a given moment but the data collected by CTD and UCTD casts are only snapshots in time. In order to obtain seasonal to annual scale information on water column properties, our group will rely on both short and long-term moorings. Instead of lowering a system on a cable attached to the ship, moorings are lowered to the seafloor and remain stationary for greater lengths of time, sometimes years. This process allows for the collection of much more data, and can help to map changes in ocean structure over time.
We already have surprising preliminary findings from the oceanographic data collected on this cruise. Warm water temperatures were measured on the continental shelf here, perhaps an important factor driving the large amount of glacial ice loss occurring in this region. This discovery has led us to take a closer look at the location of warm water currents in the area, and the role that they play in glacial melting. In part, this will involve a more detailed analysis of the bathymetry of the region because of the influence that underwater topography can have on oceanographic currents. With all of the new and unexpected findings from our early data collection, the science crew on board is excited to continue sampling.