ANT-XXV/2, Weekly Report No. 3
Fig.1: A pressure sensor equipped inverted echosounder (PIES) is being deployed from RV Polarstern via the ships telescoping beam.
© realnature
19 December - 26 December 2008
It’s Christmas day, eight o'clock in the morning. The deck crew, a nautical Officer and two scientists are waiting on deck to start deploying a deep-sea mooring. Professionally and quickly the job is done. Anchor, rope, instruments, and floatation, one after the other, are lifted into the water and lowered to depth. A last mooring recovery is scheduled for the afternoon. After this, all mooring operations for the cruise will have been successfully completed. This last recovery, however, bears uncertainty: deployed on the 23rd of December 2002, it has been at sea for six years and two days, a daring time span for battery powered instruments. Will the batteries of the acoustic release perform? They have not been used for all this time, and the theoretical calculations result in plenty of remaining charge. However, this battery type tends to become passive if unused for a long time and could refuse to let go of the anchor, in spite of all the energy still stored. We are lucky and briefly after triggering the acoustic release command, the mooring's floatation elements surface next to an ice floe - which by the way have become rare by now - and the mooring is promptly recovered.
This is not the first mooring on this cruise. During our approach to Neumayer Station, three PIES (Pressure sensor equipped Inverted EchoSounder) were deployed. These instruments record a time series of weight and height of the water column above them. They are compact and combine batteries, electronics, release system, pressure gauge, and an acoustic transducer in a single pressure and corrosion resistant glass sphere. The PIES were lifted by the telescoping beam into the water surface and let go, from where they fell freely to the seafloor, typically at 5,000 m depth. The three instruments deployed complement a six-instrument array, which extends from Cape Town to south of the Antarctic Circumpolar Current. Supplemented by a similar array across Drake Passage, operated by American and French colleagues, the data will serve to determine how much water and heat is exchanged between the Atlantic, Pacific and Indian Oceans, processes which, as part of the global overturning circulation, have significant influence on the strength of the Gulf Stream and the northern European climate, in spite of a distance of more than 10,000 km.
Fig.2: The MURU (double bubble) underwater recorder is lowered into the water over the stern of RV Polarstern.
© Hans Gerber
The PIES deployments were followed by the deployment of two Marine Underwater Recording Units (MARUs) - also known as double-bubbles due to their unique shape. Their task is to record the "songs" of whales and seals to unravel the seasonal migration pattern of these species. The two moorings – a collaborative effort between AWI and Cornell University (USA) - complement two similar systems, deployed farther south in April of this year and the PALAOA (PerenniAL Acoustic Observatory in the Antarctic Ocean) listening station near Neumayer station at Atka bay, Antarctica. There, underwater acoustics are recorded round-the-clock and fed as live-stream to the internet via a satellite link: at www.awi.de/acoustics everyone may listen to the underwater soundscape of Antarctica.
Fig. 3: Preparation of a PALAOA-s recording unit for underwater sound on an ice-floe at a distance of approximately 130 km (1 flight hour) from the ship.
© Olaf Klatt
Further recordings were collected by mobile listening stations, termed PALAOA-s (s for satellite), which were first deployed on the fast-ice and later on drifting ice-floes. The additional fast ice recordings, when synchronized with the PALAOA recordings, will allow for the estimation of position of acoustic sources whereas the ice-floe recordings will provide a sample of acoustic environment at locations remote from the ice-shelf. Two months ago, AWI acousticians traveled via the DROMLAN airlink to Neumayer to place several of the PALAOA-s units on the fast-ice of Atka bay. With the arrival of RV Polarstern, these activities came to an end as the team exchanged their fiberglass igloos for the warmer cabins of Polarstern and will return to Germany via Cape Town.
While berthed next to the fast-ice and later to the ice-shelf, extensive unloading operations commenced onboard Polarstern to provide the old and new stations with food, material and fuel for the coming Antarctic winter. Members of the wintering team disembarked to move into their new home on the ice-shelf. After four days, Polarstern bid farewell to the wintering and constructions teams at Neumayer and took up its return journey to Cape Town, not without having had a memorable Christmas reception on Christmas Eve in Polarstern's blue saloon.
Best wishes and greetings,
Olaf Boebel