Friday, 2 November 2018

Hunting for critical metals in the south-west Atlantic (Part I): RSS Discovery research cruise to the Rio Grande Rise…by Paul Lusty

Location of the Rio Grande Rise and the MarineE-Tech
project research area (red box).
We departed Santos, Brazil on the RRS Discovery (DY094 cruise) on the 20 October heading to the Rio Grande Rise, which is about 1300 km offshore, in international waters. This is the second British-led cruise of the of the ‘Marine ferromanganese deposits - a major resource of E-tech elements (MarineE-tech)’ project. The project is funded by the NERC Security of Supply of Mineral Resources (SoS Minerals) Research Programme, which aims to understand ‘critical’ metal cycling and concentration in natural systems, and determine how to minimise the environmental impacts of extraction. Critical metals are mineral raw materials (e.g. cobalt, tellurium, niobium and the heavy rare earth elements) vital to technologies (e.g. components of electric vehicles: motors/batteries; renewable energy generation: photovoltaic cells, wind turbines) for transition towards a low carbon future, and for which concerns about security of supply exist. Demand for some critical metals is expected to grow by orders of magnitude as manufacturing of green or low-carbon technologies expands globally. All these materials are ultimately derived from the Earth and MarineE-tech aims to improve understanding of the geological and oceanographic processes controlling the concentration of critical metals in deep-ocean mineral deposits.

A typical example of a ferromanganese crust: the dark layer,
deposited on a sedimentary rock substrate
Some critical metals, notably cobalt and tellurium, are highly concentrated in hydrogenous ferromanganese (Fe-Mn) crusts, which form directly from seawater, on virtually any hard substrate in the oceans. They are particularly common on deep-ocean plateaus and seamounts. During 2016 the MarineE-tech project undertook the most comprehensive study of Fe-Mn crusts on a single seafloor edifice, at Tropic Seamount in the north-east tropical Atlantic. As a follow-up to the Tropic study the current expedition is investigating Fe-Mn crusts on the Rio Grande Rise (RGR) in the south-west Atlantic.

The RGR is the largest bathymetric feature on the oceanic part of the South American plate and is located about 2000 km west of the Mid-Atlantic Ridge. The RGR is divided into a number of sub-regions and is intersected by a major north-west south-east-trending submarine graben structure. The current expedition focuses on a small area on the western side of the RGR, investigating the 25 km wide graben and its adjacent Fe-Mn crust covered plateaus. The origin of the RGR is the subject of continued debate. It is an aseismic, volcanic oceanic plateau that is likely to have formed on or close to the Mid-Atlantic Ridge, and may have a common origin with the Walvis Ridge, which extends off the coast of Africa. Basalt ages suggests these deep-sea plateaus principally formed between 89 and 78 million years ago. 

Extensive Fe-Mn crust deposits are known from the RGR and the area is the focus of commercial mineral exploration. The study area was selected to improve understanding of the local-scale controls on Fe-Mn crust formation and metal concentration, and associated marine ecosystems. The Discovery cruise involves researchers from the National Oceanography Centre, British Geological Survey, University of Edinburgh and the University of São Paulo. We have a diverse range of scientists on board, including geologists, biologists, geochemists and oceanographers. Pierre Josso from BGS, a MarineE-tech post-doc researcher, is also attending the cruise. Pierre’s research focuses on improving age models for Fe-Mn crusts by combining a range of techniques including cobalt-chronometry, LA-ICP-MS studies and Os isotopes.

From L-R: The RRS Discovery preparing to depart from Santos Brazil. The autonomous underwater vehicle Autosub 6000
 is installed in its launch and recovery system on the back deck; The screens in the main laboratory on the RRS Discovery
 from which HyBIS is controlled and the mission is run.
A joint University of São Paulo and National Oceanography Centre research cruise to the RGR in January 2018 provides important background information for planning the current expedition, including ship-based bathymetry (25 m resolution) and data obtained from a number of dredge lines and gravity coring. During the current cruise we plan to examine two areas identified by the previous work in much greater detail. We will be mapping the seafloor to identify areas of Fe-Mn crust using the autonomous underwater vehicle Autosub 6000. This will provide high resolution swath bathymetry (1 m resolution), sidescan sonar (5 cm resolution), sub-bottom profiles (10 cm resolution) and magnetic data. This data will be interpreted on the ship and used for planning dives with the robotic underwater vehicle (HyBIS). We will undertake 12 hour missions with HyBIS to ground truth the autonomous underwater vehicle data, create geological and ecosystem maps from the high definition camera observations, and collect rock and biological samples with its manipulator arm. Seafloor dredging will be used to collect additional samples across the mapped areas. All rock and biological samples are initially processed on the ship. For the geological material, this involves photographing the samples, cutting the rocks to expose the Fe-Mn crusts and making initial sample descriptions.  We are likely to have about 14 days on station at the RGR and have 24 hours of operation on the ship to maximise the use of our time at sea and the amount of data collected. This study will provide new insights into crust formation on the RGR and allow the MarineE-tech team to make comparisons with the crusts they have studied in the north-east Atlantic.

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