2016.06.20 All about the gravity coring !

On June 1st, the first gravity core was recovered on the northern part of the hydrothermal area. We were all ready to discover what the seafloor may deliver, staring at the monitors showing the rope tension during the penetration of the core into the seabed, and keeping our breath when the coring device came back on board. The first core gave us just over one meter of spongy and sticky beige carbonate ooze. This is a promising start!

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Gravity core deployment (Adeline Dutrieux)

The gravity corer is a device composed of a 3 meter long plastic core liner with a diameter of approximately 13 cm in a steel tube of also 3 meters. These tubes are placed underneath a head weight consisting of many large lead disks of 50 kilograms each, totalling 900kg. It is connected to an on-board winch that lowers the instrument towards the sea bottom. Due to its own weight and a relatively high speed (around 1.2-1.5 m/s) when it gets close to the seabed, the corer is pushed into the sediments. Upon hoisting a cap at the top of the corer closes down and avoids any water circulation through the core that may lead to a flushing out of the sediments. Due to the water depth in our working area (3500m) the gravity corer needs almost 2.5 hours for a round trip.

Since then, many nights were dedicated to the gravity coring, some of them returning empty or only with a few centimetres of sediment. This could be due to many reasons. As we are on the Mid-Atlantic Ridge and close to the active rifting itself where new oceanic crust in forming today, the underlying basaltic crust is very young and consequently the sediment cover in the western part of the working area is very thin (only a hundred thousand years time for sediment accumulation) and when the corer penetrates, only a few centimetres that get caught. The sediment cover increases to the east allowing for longer sediment cores. Another reason is the possibility to hit blocks of hard rock. The corer can’t penetrate, as there are no sediments, and it comes back at the surface with small fragments of volcanic glass or altered basalt. We have even recovered a piece of pillow lava with its particular hummocky shape.

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Working on the sediment sub-sampling, description (Fernando Barriga)

After several days of mixed coring results, “Mosquito” – our nickname for the gravity corer – has finally bitten deeper. In a single night, it came back to the surface with three half-full cores (which is very good regarding the sediment thickness in this area). Two cores were located at less than half kilometre from the inactive mounds. The next night, we recovered a full core (3 meters) in which we saw several distinct layers (green, yellow, black, beige, dark brown …) – this is what we are looking for and want to analyse in detail with the shipboard analytical equipment brought from the University of Lisbon as part of work package 1! We rapidly identified some sulphide layers with a shiny appearance.

After recovery, the cores are quickly transported to the “cold lab” with a temperature of only a few degrees, to meet the same environment than at 3500 meters depth. It is to minimise the weathering reactions (on the next cruise on James Cook, led by Bramley Murton, all handling will be done under a nitrogen atmosphere to avoid any oxidation). Then the pore waters are extracted from the sediments by the ‘rhizone’ method. This consists of tiny filter tubing (a rhizone) inserted, through holes drilling into the core liner, into the sediments and connected to a syringe, the syringe is then pulled out creating a vacuum, which sucks the pore water out through the rhizone. This could take any length of time from 20 minutes to 2 hours. These pore waters will be analysed for a range of chemical components and properties (such as cations, sulphides, anions, nutrients, dissolved inorganic carbon or total alkalinity) by Adeline Dutrieux and Anna Lichtschlag, part of the geochemistry team from the National Oceanography Centre.

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Pore water extraction in the “cold” lab (Florian Besson)

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Distinct dark layers of volcanic/hydrothermal origin within the cores (Fernando Barriga)

Once enough pore water has been extracted, the cores are finally split revealing the internal structures of sediments which are described and sub-sampled. Some structures are visible such as the bioturbation with light colour disturbance, or colour changes with the depth or distinct layers that are recorded with a spectral analyser. Sofia Martins and Fernando Barriga from the University of Lisbon (FFCUL) run additional on-board analyses on selected sub-samples (regarding the task 4 from the WP 1: Rapid, exploration-oriented mineralogical and geochemical study of cover rocks to sub-seafloor to conceal sulphide mineralization). We have PIMA (portable Infrared mineral analyser) which detects the major minerals such as calcite of course, but also several clay minerals. We also have a portable X-ray fluorescence that analyses the chemical composition along the cores and X-ray diffraction which gives us a precise mineralogical analysis.

Further geochemical analyses of the sediments and the pore waters are to be conducted after the cruise at both the National Oceanography Centre, Southampton and the University of Lisbon (FFCUL).