23. January 2017, 4 weeks and 4 days at sea.
Today was a remarkably day for the ISOTAM-Team of the University of Göttingen:
For the first time measurements of the near surface waters showed values at 0.3 to 1.0 µmol/liter of nitrogen-ammonia, above the detection limit (Fig. 1). These measurements complement observations of concentrations and of stable N-isotope ratios of gaseous ammonia and particulate ammonium in the atmosphere. Ammonium (NH4+) in Antarctic ice cores could either originate from oceanic or continental sources. However, it is yet unclear which dominates and how its signal changes on its way to the Antarctic ice shield. To resolve these questions, isotope-signatures of NH4+ (in aerosols and surface water) and gaseous ammonia (NH3) were and are analyzed during the previous, this, and the one but next Polarstern expedition from the Norther Hemisphere to the Antarctic continent as tracers for ‘Antarctic‘ NH4+.
24 January 2017, 4 weeks and 5 days at sea
The final spurt, literally. The work on deck starts with a mooring deployment, to continue with a mooring recovery during noon-time, another deployment in the afternoon and a sound source calibration in the evening. To capture the narrow flow of Antarctic Bottom Water down the continental slope, the stations of this hydrographic section ahead of us are packed closely, with regulation on working hours necessitating an intense beat of mooring work during day, while the night is filled with CTDs and Bongo net hauls. To be at the right place at the right time, Polarstern performs a finely tuned dance back and forth along this expedition’s final section.
25. January 2017, 4 weeks and 6 days at sea
The foremost colour that comes to your mind when thinking about an expedition into the Southern Ocean, is probably white. However, the Phytooptics Group is interested in a different colour: green! If phytoplankton, unicellular algae, are present in the water, it is usually coloured green due to their photosynthetic activity. A pump system delivers surface ocean water directly into the lab, where it feeds an instrument which continuously measures the optical properties of the sea water 24/7. Additionally, every three hours, water is sampled and filtered (Fig. 2) to measure the absorption spectra of phytoplankton and yellow substances. A portion of the filters is shock frozen to analyse the phytoplankton pigments back home at the AWI, striving to unravel the phytoplankton community with respect to functional groups. The same analysis is applied to water taken from the CTD from six different depths. The data will be used to validate or initialize the calculation of phytoplankton maps of the Southern Ocean from satellite images as well as models, and aim for a better understanding of the phytoplankton community in the Southern Ocean and the change it undergoes in response to climate change.
26. January 2017, 5 weeks at sea
The morning starts by deploying a mooring, a task which is more plannable with regard to the time it will take, than any recovery. Late in the afternoon we attempt another try to recover AWI-207-8. The acoustic beacons reveal its underwater position, yet it fails to surface. Fiona, our ROV, is on stand-by for deployment, equipped with a thick recovery rope this time. The ship takes position, Fiona is deployed and our ROV pilots (Fig. 3) manage the approach within the shortest of time: the uppermost measurement device, an upward looking sonar measuring ice thickness, is afloat above the ROV in the middle of the screen against the blue backdrop of the sea surface (Fig. 4). Yet our attempts to latch onto the mooring line are obstructed: While the mooring rope is caught by the carabiner, it does not close. Then it closes, but at the same time the rope slips out. Our ROV pilots show utmost resilience. Over and over they press the carabiner against the rope until suddenly the latch snaps open, catching the mooring rope for keeps. We are docked! But as if this would not have been enough suspense already, while surfacing the recovery line and the ROV’s tether get entangled. Ascending most slowly and carefully we finally succeed in recovering Fiona. The next step is to haul the mooring back on board. Gently the ship pushes away from the mooring, straining the recovery rope, which seems to hold. Suddenly the tension on the rope eases, yet not as completely as last time. What has happened? Over the intercom the bridge relieves us: the acoustic beacons show the rising of the mooring, apparently the clutch of the release must have been stuck (no surprise after 6 years) and required a good pull to let loose of the anchor.
For our mooring team, it’s time to act: within 2 hours the mooring is recovered, yet time presses nevertheless: another recovery is scheduled for the later this evening.
27. January 2017, 5 weeks and 1 day at sea
Two more difficult mooring recoveries are ahead of us today. Mooring AWI-206-7, deployed in 2010, and AWI-206-8, deployed in 2012, shall be recovered. Late last night we had already passed and acoustically localized AWI-206-7, being pleased to find it where it was dropped 6 years ago. By morning, however, thick ice drifted on top of its location and the wind has freshened, unfavorable conditions for a recovery, prompting us to give 206-8 a try first. Arriving at its location, the sea has calmed somewhat and no ice is around. We contact the mooring acoustically, release it, wait, yet nothing, no reaction whatsoever. Searching the waters with the ROV would be a waste of time, the range of its acoustic sonar is about 50m, we rather invest this time in recovering 206-8. There we know at least where the topmost floatation is located. Yet releasing it fails as well - 6 years are a long time after all - and the ROV is deployed. Meanwhile we start establishing a routine and our ROV pilots manage the approach of the mooring, the attaching of the recovery rope and resurfacing within 20 minutes. Having the ROV back aboard, the mooring team - comprised of members of the crew and the AWI’s oceanography department - carefully tugs at the recovery rope. Nothing moves - more tension - this time via the fully extended crane to pull as vertically as possible (Fig. 5) - and suddenly we get feedback from the bridge: The acoustic beacons are slowly being lifted - the mooring is being pulled up including its anchor. Less than two hours later the mooring is aboard and another 6 years of deep sea data feed our data bases and foster our understanding of the ocean. The recovered pair of releases, however, provide a most pitiful view (Fig. 6).
28. January 2017, 5 weeks and 2 days at sea
Early in the morning a short mooring is deployed - routine work compared to yesterday’s recovery. Thereafter we heed a long standing trading - polar baptism. In the evening, during this cruise’s last barbeque - the baptists enjoy the spectacular performances of the newly-baptized who participate in the PSSS contest (Polarstern searches for the super star) while the newly-baptized joyously receive their certificates. The crossing of Drake Passage - renowned for its many storms - may commence.
29. January 2017, 5 weeks and 3 days at sea
The one but last station day finds Polarstern just off Gibbs Island. There, we shall recover a GPS station for our colleagues from the Technical University Dresden. However, fog, low hanging clouds and winds of up to 30 knots render an attempt to land there by helicopter as too risky, and we have to abort this station. We steam towards a location northwest of Elephant Island, Sir Ernest Shackleton’s renowned refuge. There we moor two underwater recorders to acoustically “observe” the fin whales, which frequent these waters, year-round. Deploying the mooring demands quite some intuition by the nautical officer, as it shall be positioned on a little plateau amidst steep slopes all around.
30. January 2017, 5 weeks and 4 days at sea
Our last station day. Only two CTD and four Bongo Net hauls are today’s tasks, providing our biologists with their final samples from the southern Drake Passage. Once finished, our equipment needs to be packed as quickly as possible. Each box requires its list of contents, each container its list of boxes. The fact that marine scientists need to incessantly export and import their equipment to be able to do their jobs has not been considered benevolently in the customs regulations so far. Hence we spend entire days on this rather unproductive red tape, valuable hours lost for scientific work.
This brings our weekly reports on Polarstern expedition PS103 to an end. It was, in spite of the occasional adversities, most successful and scientifically yielding, due to, last but not least, the scientists’ enthusiasm and the crew’s dedication and support. We all are looking forward to return home, even though some 20 hours on the plane are lying ahead of us.
Olaf Boebel
