Brandt, Peter

OceanRep

Prof. Dr. Peter Brandt  METEOR cruise 145  1st Weekly Report M145, Mindelo-Recife  13.02.-18.02.2018  The METEOR cruise M145 from Mindelo to Recife started on Tuesday, February 13, 2018, at the height of the Cape Verdean Carnival. On the one hand, we were able to gain an insight into the great festivities and processions through the city, on the other hand, also the preparations for the journey luckily ran smoothly. A smaller part of our equipment was already delivered to Catania, the departure port of M144-2, and thus was already on board when METEOR arrived in Mindelo. The remaining containers were then brought to the pier and the part of the scientific crew that arrived early was able to start unpacking the containers directly after METEOR’s arrival. Already on Sunday almost the entire scientific crew was on board. The laboratories were set up, equipment was installed and the mooring work, which was to take place on the afternoon of the day of departure, was prepared. With METEOR’s departure from Mindelo began the last research cruise of the Kiel Collaborative Research Center 754 "Climate-Biogeochemical Interactions in the Tropical Ocean". From 2008-2019, 18 SFB754 research cruises to the tropical Atlantic and 13 cruises to the tropical Pacific were conducted. Many fundamental insights were gained on natural and anthropogenic changes in the oceanic oxygen content as well as on the underlying ocean dynamics, biogeochemistry and biology of the tropical oceans. This project has taken the interdisciplinary cooperation within GEOMAR and with our national and international partners to a new level. This can also be seen for our research cruise M145, in which researchers from 6 subprojects of the SFB754 as well as various national and international groups participate. The remaining time until the end of the SFB754 project in December 2019 will be dominated by the synthesis of observed data and model results. Furthermore, the foundation has been laid for a continuation of important time series that document climatic changes in the ocean beyond the SFB754 period. GEOMAR will continue to operate the interdisciplinary Cape Verde Ocean Observatory (CVOO) mooring. Additionally, in cooperation with the EU project AtlantOS and the international PIRATA program, oxygen and flow measurements will be carried out increasingly on PIRATA moorings. The evolution of oceanic oxygen content in the future and in particular the long-term deoxygenation and its consequences will certainly remain an important field of research in the coming years to decades. Prof. Dr. Peter Brandt  METEOR cruise 145   Fig. 1: Departure of METEOR from the port of Mindelo on February 13. Mooring materials such as floatations and wires are already prepared for the first mooring deployment on the following day (Photo: PB).  After leaving the port of Mindelo (Fig. 1), our first activity was the service of the CVOO mooring. Shortly after successful recovery, the first time series have already been analyzed. In line with our previous studies, they show various eddies passing the mooring on their way from the northeast African upwelling area to the west. They are characterized by significantly reduced oxygen concentrations (Fig. 2). During the night, some CTD stations were used for the calibration of moored instrumentation and in the morning of the next day, the CVOO mooring could be deployed successfully.  The hydrographic section along 23°W took us from 15°N to 11°N. There, just east of the actual section, the first SFB754 oxygen mooring was recovered. Here in the center of the oxygen minimum zone, typically lowest oxygen values are observed at depths of about 400 m. We are looking forward seeing the calibrated oxygen values in comparison to previous mooring periods.  Prof. Dr. Peter Brandt  METEOR cruise 145   Fig. 2: Meridional velocity measured with the ADCP from the CVOO mooring (upper panel) and oxygen (red) and meridional flow (blue) at about 200m water depth (bottom panel). The depth of the oxygen sensor is shown in the upper panel as a black line. Eddy passages are marked by gray bars in the bottom panel (Fig. RH & JH).  After the first days, that have been exciting as always, the work routine with CTD probe, incubations and water analyzes has started. All scientists immediately felt at home on board METEOR. In particular our cruise participants who are traveling with METEOR for the first time were impressed by the technical capabilities and also the quality of life that this ship has to offer. Unfortunately, a severe hand injury of a crew member requires a stopover in Praia and thus a two-day interruption of our research work. We wish him all the best. Get well soon!  Greetings from the tropics, Peter Brandt and the cruise participants of M145  

1. Wochenbericht M145

Prof. Dr. Peter Brandt  METEOR cruise 145  2nd Weekly Report M145, Mindelo-Recife  19.02.-25.02.2018  The second week of METEOR cruise M145 from Mindelo to Recife focused on the southern part of the oxygen minimum zone of the tropical North Atlantic. Oxygen minimum zones are formed in regions with weak currents, i.e. low oxygen supply by currents and mixing, and relatively high oxygen consumption associated with high biological production and the associated remineralization of the sinking biological material. We find lowest oxygen levels at a depth of about 400 meters, well below the surface mixed layer, which is almost always saturated with oxygen, and above the deep ocean, that is only reached by small amounts of sinking biological material and where accordingly the oxygen consumption decreases rapidly. The work concentrates on measurements with the CTD rosette, which is lowered into the ocean down to depths of up to 5000 m. The CTD Rosette is equipped with a variety of instruments, some of which deliver their data online via the wire to the deck unit aboard METEOR, others record data internally, and with water bottles that take water samples from different depths for biogeochemical analyzes and experiments in METEOR’s laboratories or on deck. Our participants from the MPI for Marine Microbiology sample water from the euphotic zone in order to measure biological nitrogen fixation and primary production rates. Nitrogen is an essential nutrient to all living organisms. However, only certain nitrogen species can be taken up and made use of by most organisms. Dinitrogen gas, which is the most dominant nitrogen species in the atmosphere, cannot be used by the majority of all organisms. Nevertheless, there are certain prokaryotic microorganisms that are able to “fix” dinitrogen gas, meaning that they are able to react it to more favorable nitrogen species. This process called nitrogen fixation is a major source of nitrogen for photosynthetic primary producers. Primary producers, e.g. microalgae, have the ability to fix the greenhouse gas carbon dioxide into biomass. Therewith they build the basis of the overall foodweb. In order to investigate and quantify these two important biogeochemical processes water samples from different depths along the 23°W section are incubated with stable nitrogen and carbon isotopes. Eventually the incoorporation of these isotopes into biomass can be measured and analyzed using mass spectrometry.  Prof. Dr. Peter Brandt  METEOR cruise 145   Fig. 1: Anchor drop at the end of the mooring deployment of our equatorial mooring at 23°W (Photo: TF).  The second week of the METEOR M145 trip ended with the mooring work at the equator, 23°W. Since December 2001 a current meter mooring has been serviced to measure the equatorial velocity field. This mooring was first deployed in the framework of French research projects and then continued in cooperation between the international PIRATA program and various German projects (BMBF RACE, SFB754). Since 2006, this mooring, which is globally the only deep sea mooring at the equator measuring the velocity field from the surface to the seabed, has been recovered and redeployed approximately every one and a half years during our voyages. Many publications result from this dataset, which initially document strong currents with multi-year fluctuations, as well as in combination with other data and models discussed the importance of the currents for the distribution of oxygen and tracers in the ocean or the climate in the tropic al Atlantic sector. After each recovery, the first view on the data of the deep-profiling instrument is eagerly awaited. This time, it has returned an almost complete dataset of deep equatorial circulation. The mooring work on the equator and the previously recovered moorings in the center (at 11°N) and at the southern edge (at 5°N) of the oxygen minimum zone of the tropical North Atlantic ends the mooring work of the SFB754. Almost all instruments of our SFB754 moorings have provided complete datasets and will thus make a significant contribution to the synthesis of the results in the last phase of the SFB754. Prof. Dr. Peter Brandt  METEOR cruise 145   Fig. 2: Zonal (upper panel) and meridional velocity (lower panel) measured at the equator, 23°W with an MacLane profiler profiling up and down the mooring wire every six days. Note east- and westward current bands marked by red and blue colors, respectively, in the upper panel. The phase of these current bands slowly propagate downward, which is associated with upward energy propagation. The meridional velocity is dominated by monthly fluctuations most clearly visible at the end of 2017 (Fig. FPT).  From the equator we continue south, before turning west towards Brazil in the middle of next week. Meanwhile, we are almost back on track with our research program thanks to the above-average speed of METEOR, but also due to the very smooth and professional work on board. Minor technical issues were solved immediately, and in particular the excellent cooperation between the captain, crew and science team allows for very effective recovery and redeployment of our deep-sea moorings. Here we would like to thank especially the crew for the professional work and the consistently very positive working atmosphere on board.  Greetings from the tropics, Peter Brandt and the cruise participants of M145  

2. Wochenbericht M145

http://oceanrep.geomar.de/42017/2/Weekly_Report_2_M145.pdf

2. Wochenbericht M145

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