PS110 - Weekly Report No. 2 | 3 - 11 January 2018
From the Equator to the Cape-Basin
Also along the second part of our journey, the top priority remains to reach our destination on time in order to avoid delays in the tight schedules of the forthcoming Antarctic expeditions. Thus, during one of the shortest transits from Bremerhaven to Cape Town, there is very little time for station work.
To begin with, Polarstern is sneaking off the northern hemisphere in the morning of January 4, almost as if Neptune should better not notice that we are crossing the equator.
After two prior attempts to test the new small underwater vehicle ROV (Remote Operating Vehicle: BlueROV2) our mission was accomplished on January 6. Using a rubber boat, we launched it into the sea and tracked its position with the help of an underwater positioning system attached to the boat. The performance of the small ROV to move and hold position against significant currents was tested successfully.
With the sea-trials we assess the ability of one or a number of coordinated small ROVs to move and hold position relative to a drifting and moving ship. Such a manoevre of three ROVs allows an exact positioning of a small calibration sphere, for example, thereby providing a strong simplification of the procedure to calibrate the scientific echo sounders on board FS Polarstern, which is a very demanding task due to the size of the icebreaker.
The small ROV is able to cope with the currents. With sincere thanks to the engineers on board FS Polarstern actively involved in the test, the ROV was named "Seebiber".
The last spot before Cape Town to carry out station work is at a position about 6 nm north-west of St. Helena. The small island belongs to the British Overseas Territory and is reached at dawn of January 8th. We acquired ship-based geomagnetic data using the magnetometer system installed on Polarstern along two turning circles, each of which 2 nm in diameter. This procedure is needed for calibrating the re-engineered magnetic system by quantifying the induced magnetizing effects of the earth’s magnetic field onto the iron-rich ship along all possible moving directions of the vessel. This allows eliminating these secondary effects from mapping duties of the earth’s magnetic field to be performed by Polarstern during future expeditions. A geomagnetic observatory operated on the island of St. Helena by Geoforschungszentrum Potsdam (Germany) can provide near-by geomagnetic reference data of the earth’s magnetic field needed for the calibration.
On this morning, quite a few people on board may have wished to visit the island of emperor Napoleon’s banishment for only a few hours despite the cloudy weather. Unfortunately, even with the temptation of being so close, landing was not possible due to the very tough timetable of our expedition.
On January 11th, we crossed the Walvis Ridge off Namibia which, in places, rises up to only 38 m below sea level followed by sailing through parts of the 5000-m-deep Cape Basin towards the south. Along this part of the track, we tested again the hull-mounted scientific echo sounders.
Polarstern is equipped with a modern scientific echo sounder (Simrad EK-80), which can be operated in a broadband frequency mode for surveying fish and zooplankton. In addition to the ship-mounted echo sounder, a mobile broadband echo sounder (EK80 WBT tube) is used during this cruise as described in the previous weekly report.
Backscattering of acoustic signals by fish and zooplankton strongly depends on the sound frequency. Compared to echo sounders using one or a set of very small frequency bands, broadband echo sounders allow a significant improvement of species identification and a specific survey on their abundance and behavior. The mobile and ship-mounted echo sounder systems are operated on station and during ship’s transit, testing new features and performance after mayor system updates.
Further, the sediment echo sounder Teledyne PARASOUND PS70 received a technical update prior to PS110. The new features can transmit a broader band of frequencies as chirped pulses. This results in a better acoustic performance of recording the internal geometry of the uppermost 200 m of sediments below the sea floor. Longer waves of the frequency bands penetrate better while shorter waves provide a better resolution of the stratification of the deposits. We are testing which settings are best to gain optimal results under different sedimentary conditions.
Sediments are the memory of the ocean. Environmental changes leave an imprint on the deposits of the sea floor such as influenced by ocean currents, suspension input from rivers, dust from the atmosphere, and organisms settling through the water column. Geosciences take care of reading the information stored in this natural archive to reconstruct and understand the natural variability during Earth’s history. Investigations using sonic waves are only one of many methods used in this context.
When leaving the inner-tropical convergence zone on January 4th and entering the area of south-easterly trade winds, the weather conditions remained within its normal range of wind forces around 4 Bft and swells around 2 m. The austral summer brings us increasingly more daylight. In about 48 hours, Polarstern will reach Cape Town on time.
All on board are well and in a good mood.
We are sending our best wishes to all our families and friends at home.
Frank Niessen (with contributions from Sören Krägefsly about the EK-80 and ROV)
(Polarstern, 28° 04' S, 9° 35' E, 12.1.2018)