JC142 MarineE-tech cruise to Tropic Seamount, north-east tropical Atlantic...by Paul Lusty

Autosub 6000 being launched from the RRS James Cook to undertake a
24 hour multibeam, sidescan sonar and sub-bottom profile survey. 
We are now about two weeks into the RRS James Cook cruise, which departed from Santa Cruz, Tenerife on the 29 October. The current cruise forms part 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 ‘E-tech’ element cycling and concentration in natural systems, and determine how to minimise the environmental impacts of extraction. The ‘E-tech’ elements are those mineral raw materials critical to environmental technologies (e.g. cobalt, tellurium, selenium, gallium, indium and the heavy rare earth elements), and for which concerns about security of supply exist. MarineE-tech (http://prj.noc.ac.uk/marine-e-tech/) aims to improve understanding of E-tech element concentration in seafloor mineral deposits. The project partners involved in this cruise are the British Geological Survey (BGS), National Oceanography Centre (NOC), HR Wallingford, and the University of São Paulo, who are running a parallel research programme on Fe-Mn deposits on the Rio Grande Rise, funded by the São Paulo State Research Foundation (FAPESP).

Some of the E-tech elements are highly concentrated in hydrogenous ferromanganese (Fe-Mn) crusts, which develop on oceanic seamounts. The MarineE-tech study area is located in the north-east tropical Atlantic, about 650 km south of the Canary Islands (about 2 days sailing on the RRS James Cook), and at the south-west end of the Canary Island Seamount Province. The islands and more than 100 seamounts, which extend over a distance of some 1000 km form one of the most significant volcanic provinces in the Atlantic Ocean. Our research focuses on the Tropic Seamount, the southernmost in the archipelago (23.5° N, 20.4° W). Tropic Seamount has an area of about 770 square kilometres, comparable to the Island of Anglesey in North Wales. Previous, limited work has identified Fe-Mn crusts on this and a number of the other seamounts to the south of the Canary Islands.

Clockwise from top left: Typical exposure of Fe-Mn crust broken by pelagic sediment cover on the summit of Tropic
Seamount; ROV Isis collecting a Fe-Mn crust sample to be placed in the basket; A cut sample of Fe-Mn crust showing
distinctive layering that develops from accumulating on the seafloor over millions of years; The first batch of Fe-Mn crust
samples collected by Isis laid out in the laboratory an-board the James Cook. 
During the six week cruise we plan to assess the distribution and thickness of the Fe-Mn deposits, through a combination of high-resolution seafloor imaging and detailed sampling across the range of heterogeneous environments that exist on the seamount. The new samples will be used to characterise the gross geochemical composition of the crust, and the micro-scale mineralogical, textural, geochemical and isotopic composition of individual growth layers. This data will be used to assess the significance of the temporal and spatial environmental factors, and processes (including palaeo and modern oceanographic conditions e.g. seabed morphology, sedimentation rates, upwelling rates and microbiology) that control the compositional, textural and thickness characteristics of seafloor Fe-Mn deposits. We will also be collecting data on the seamount ecosystem, and conducting plume generation and modelling experiments to investigate the potential environmental impacts from exploring for and extracting Fe-Mn deposits. We plan to acquire bulk samples of Fe-Mn crusts, which will be used to assess the potential for the recovery of E-tech elements from these resources using novel, low energy (and low-carbon) extraction technologies such as bio-processing, ionic liquids and hollow fibre membranes. This work is being led by the University of Bath.

New ship-board multibeam swath bathymetry acquired over Tropic Seamount
that will be used to select areas for detailed follow-up mapping and sampling
To date we have been focusing on acquiring new bathymetry data across the seamount to inform subsequent mission planning. We are initially mapping the area using ship-board multibeam swath bathymetry (25 m resolution) and geoacoustic sub-bottom profiler. These data will be used to identify areas of rock outcrop (from acoustic backscatter), and sub-areas will be identified for detailed follow-up surveying using the NOC autonomous underwater vehicle (AUV) Autosub 6000, and the remotely operated vehicle (ROV) Isis. We have already conducted five AUV missions, principally to the test the Autosub 6000 for acquiring higher resolution swath bathymetry (1 m resolution), sidescan sonar (5 cm resolution), sub-bottom profiles (10 cm resolution), colour still images and magnetic data. We have 24 hours of operation on the ship, divided between two science shifts (17 scientists in total), so we are almost continuously acquiring new data.

The primary objective of the first ROV dive was deploy a seafloor lander platform on the summit of the seamount. This has been designed by HR Wallingford, with the objective of collecting time series data on the hydrographic regime and sediment movement in the water column across the seamount. During the cruise we plan to generate sediment plumes (in an attempt to simulate potential disturbance associated with seafloor mining) on the seafloor by sucking up pelagic sediment with the ROV and blowing it into the water column. The movement of the sediment plume will be monitored for several hours by a range of sensors on the lander and compared with results of previous modelling.  Additional hydrographic data will be obtained from three moorings that we have strategically positioned across the seamount. When we recover these at the end of the cruise they will have been deployed for at least 30 days, and provide a time series dataset on currents, temperature and conductivity (which can be corrected with temperature to give salinity) of the water column around the seamount. Additional data on the water column is being collected by numerous CTD deployment (we have undertaken 16 to date), which measure conductivity, temperature and depth (pressure), and can collect water samples for subsequent analysis.

From L-R: Manipulator arm of ROV Isis holding a tube from the sediment pump and generating a plume in the water column
to be detected by the lander; Seafloor lander placed on the seabed ready for monitoring and testing of plume. 
The majority of Fe-Mn crusts samples collected from seamounts globally have been acquired by dredging. This indiscriminate technique provides little or no spatial control on the location or depth from which material is acquired. The high resolution geochemical and isotopic research we are planning at BGS requires the acquisition of a new sample suite with good understanding of the spatial relationship between samples, in terms of their location on the seamount and water depths. In order to accomplish this we plan to collect samples from specific locations using the manipulator arms on the Isis ROV and a new core drill attachment, which NOC have specifically built for this project. The second Isis mission tested the core drill and obtained a 20 cm rock sample. During the ROV missions we are also mapping the distribution of sediments, Fe-Mn crust, other rock types and seafloor fauna using the high resolution cameras on Isis.

Paul Lusty

Co-Chief Scientist JC142