The German GEOTRACES GI07 cruise is sampling in the South Indian Ocean
By Eric Achterberg,
GEOMAR Helmholtz Centre for Ocean Research Kiel/
University of Kiel
RV Sonne at sea 25°36 S/56°36 E
The research cruise SO308 on RV Sonne along GI07 section is more than 2 weeks underway, and we are now sampling at station 23, to the south of Reunion. Over the last 2 weeks we have sampled shelf and slope stations along the Mozambique coast, crossed the Mozambique Channel and sampled along the EEZ of Madagascar. We are in an excellent routine and smoothly deploying the equipment at the stations.
Each station they sample in detail the water column from the surface ocean to the seafloor, and collect waters and particles. We use a titanium CTD rosette frame (Fig. 1) for contamination prone elements. The stainless steel CTD frame is used for non-contamination prone sampling of elements and isotopes like radium, thorium, uranium, rare earths and neodymium. This CTD is also used for sampling of microbial communities, metagenomics and proteomics. The CTDs frames are full of biogeochemical sensors, and cameras to observe zooplankton and sinking particles. Every 2 or 3 days, at our superstations, we also add an additional stainless steel CTD cast, and deploy 9 in situ pumps to a maximum depth of 800 m for particle collection.
In addition we sample sediments with a mini multicorer (MUC). To operate in a time efficient manner, we hang the mini MUC underneath the CTD frame. The combined deployments have been very successful and we have managed to collect sediment samples at all the deep stations. One of the 4 cores is used for pore water extraction to allow determination of benthic fluxes to the bottom waters.
The cruise will cross the South Indian Ocean along about 23°S from Mozambique to Fremantle (Australia). The research topic of the cruise is to determine in detail the distributions, sources and sinks of trace elements and their isotopes (TEIs) in the water column along a zonal section in one of the least studied ocean regions on earth. We aim to investigate the biogeochemical cycling of TEIs, and their interactions with surface ocean productivity and the carbon and nitrogen cycles (incl. N2 fixation) given that some TEIs act as micronutrients. The supply pathways of TEIs to the South Indian Ocean from ocean boundaries will be investigated, including inputs from the atmosphere (east African and northwest Australian dust), continents (Zambezi river), sediments (on continental shelves/slopes and deep seafloor), and ocean crust (hydrothermalism). The TEI transport within water masses will be determined with a focus on the flow of hydrothermally derived TEIs towards the Southern Ocean but also the deep inflow of Southern Ocean waters into the SIO. The TEI transport assessment along the cruise track will also allow a more reliable use of some TEIs as paleo circulation proxies. We will deploy 19 ARGO floats from German and US institutions along our cruise track, which also include Biogeochemical floats.
The cruise departed from Durban (South Africa) on 31st October 2024 and will return port on 22nd December 2024 in Fremantle (Australia). It involves a range of national and international research groups and we have 15 different nationalities on board which creates an excellent multi-cultural environment. The cruise is led by GEOMAR, and we have scientists involved from Constructor University, ZMT, the Universities of Tasmania, Xiamen, Zhejiang, Minnesota, South Florida, Chicago, Stanford, University College London, the Alfred Wegener Institute, Woods Hole, Max Planck Institute for Marine Microbiology (Bremen), IAEA Monaco, Helmholtz-Zentrum Hereon.
