November 24, 1999

We rounded Cape Horn this morning and have set our course into the Drake Passage, for the South Shetland Islands. The event was marked by a distinct increase in the swell and we have all been struggling to get our sea legs beneath us. Pintado petrels and black-browed albatrosses wheel about the ship and gather in our wake: our escort. The petrels are predominantly black and white with a brown neck and head and black legs and feet. Their undersides are white and flash dramatically as they turn away from the sun, riding the winds above the waves. The albatrosses, fewer in number, have long, narrow wings, like one sees on a glider. They occasionally loaf and settle in the churning foam of our wake, keeping a wary eye on us. The black brow accentuates the eyes of these rather large birds, conveying a disquieting intensity in their gaze. I have often thought of the "Rhyme of the Ancient Mariner..."

Diagram of an XBT.

The first science task came as the ship neared 54 degrees 50' S. From this latitude, an expendable bathythermograph (XBT) would be cast at regular intervals, 24 hours a day. In order to obtain useful measurements, the sampling would take place on the hour starting at a sounding of 200 metres (or bathymetric contour) and then on the half hour as the ocean temperatures change more dramatically through the Subantarctic, Polar and Circumpolar Current Fronts. As we approached the South Shetland Island, at the other side of the Drake Passage, the frequency of casts would be adjusted to every 45 minutes until we reached 62 degrees 50' S.

Marine Technicians (MT), Beth McAndrews and Trent Sanamo, had organized volunteers in groups of three who would stand a 3-hour watch. Generally, it takes two for a launch: one person on deck with the probe and launcher and one in the electronics lab below deck to track the probe on a computer during its quick descent. Peterson's interest is in upper ocean temperature gradients--in this case to depth of 750 metres.

Beth McAndrews checks with the bridge prior to launching an XBT.

The XBT is a small, missle shaped probe comprising a lead weight and streamlined body which houses a thermister bead and a spool of copper wire. This filament is attached to another reel within the launch canister which stays on board. In this way, once the XBT is cast, there is little tension on the wire (see diagram*). The copper wire in the shipboard casing and is connected to the computer data recorder.

With the probe loaded, Beth checked with the bridge and the E-Lab and then cast the probe over the side of the ship. Meanwhile, the descent was monitored on the computer. The resistance in the thermister changes with depth in such a way that that a temperature profile can be generated: resistance plotted against time, or temperature reckoned with depth. At 750 metres, the wire is cut and the data is logged and archived. About 60 of these samples are obtained on each southbound crossing of the Drake Passage.

Beth McAndrews with XBT.

While the XBT teams worked through their rotations, the ship’s crew busied themselves cleaning, while the science teams checked on their laboratory set ups, watched videos, chatted read and played cards. Through the looking glass…

* Diagram from "Principles of Underwater Sound," (3rd Edition) by Robert J. Urick. (Peninsula Publishing, Los Altos, 1983), p.115.