Temperature: 28 F   Weather: Partly sunny, 15 knot winds

Overnight we did a deep conductivity-temperature-depth (CTD) cast, sending the instrument package down to the bottom of the Canada Basin, greater than 3,500 meters deep (about 2 miles). It's about time I explain the CTD, one of the main functions of this cruise on the Healy.

(Photo: The incredible shrinking styrofoam cup! Keep reading to learn about deep-water research.)

See more photos and track the Healy's course on this map.

The CTD is a collection of sensors attached to the bottom of a frame (called a rosette) that contains 24 tall, thin, cylindrical containers. The whole instrument is about 7 feet tall. It is attached to a winch on the starboard side of the ship, and is slowly lowered into the water each time we stop the ship. As it heads down, the sensors continuously report data back to a computer on the ship, including temperature, salinity, oxygen, fluorescence, and depth. The researchers watch for points of interest along the way, then use the cylindrical containers to collect water samples at those depths while the rosette is on its way back up.

Back in the lab, the water samples are analyzed by research teams interested in various aspects of the water's chemistry. One man from Florida measures four basic parameters of the seawater carbon dioxide (CO2) system: pH, alkalinity, dissolved inorganic carbon, and partial pressure of CO2. This information contributes to an ocean monitoring system to look at how much CO2 is absorbed by seawater. Cold air sinks rapidly into the ocean in polar regions, increasing the ocean's uptake of CO2 here, making this a good place to monitor the maximum change in the CO2 system.

A woman from California measures dissolved organic carbon to carbon-date the age of seawater. She finds that the deep water here contains carbon that is thousands of years old, which indicates the very slow speed of the circulation pattern at the bottom of the Arctic Ocean.

The ship runs CTD lines that typically extend from the shelf to the basin, where we stop every 2 to 3 nautical miles to gather CTD data and water samples. Collectively, these data points are analyzed and plotted together to represent two-dimensional transects of the water column. By scrutinizing these plots, oceanographers can discover the source of major water currents, and determine how those water masses circulate.

A couple days ago we decorated Styrofoam cups to send down on the deepest cast. Last night we put the cups into a mesh laundry bag, then attached the bag to the rosette and sent them to the bottom of the ocean. The cups went down 3,840 meters, which is almost 2.5 miles. Water is very heavy, and the deeper you go, the more water pressure there is. At 2.5 miles below the surface, the pressure is about 4,000 pounds per square inch. Our cups came back shrunk to just larger than a thimble!

Melanie Smith, Landscape Ecologist, Audubon Alaska