Forty years of vent exploration – Four weeks of sampling

Four weeks at sea and we’ve got into the steady rhythm of the ship; we log the time passed from the menu in the galley (fish-Friday, curry-Saturday) and the weekly linen change.

I have studied this hydrothermal site called TAG, nearly 4km below us on the seafloor, for nearly 30 years. First for my PhD, then on and off over the years. TAG is now one of the most well studied, deep-sea vent sites anywhere on the seafloor. The nations that explore the deep have mapped every square metre of the active hydrothermal mound, we have drilled deep beneath the seafloor into the stockwork that lies underneath the mineral deposit, we have mapped out the older inactive mounds littered over the rift valley floor, marking past sites of venting.

The International Seabed Authority has granted IFREMER (France) a 15-year exploration licence for a suite of 100, 10x10km blocks that include this area of the mid-ocean ridge. This remote and extremely deep mineral deposit is one of the largest in the Atlantic but I question whether mining will ever happen here despite the significant levels of copper, gold and other metals present right at the seafloor. We don’t know what the impact of extraction is on specialist vent fauna and the extreme depths and associated pressure makes operations extremely risky compared with shallower (or land-based) sites.

We are here to assess the impact of these vents on the wider ocean chemistry. To test how leaky they are – which metals, sulphide and other species are dispersed up into the water column and how far this enriched plume of essential micro-nutrients can be tracked into the deep ocean interior.

Our physics team track the hot buoyant fluid up over several hundred metres into the overlying water; zig-zagging the sampling frame through the plume to measure instabilities and mixing. We overlay the chemistry onto these physical observations to look at iron oxidation, sulphide complexation, particle exchange reactions and the fate of these elements. We repeat sample along the ridge; close to vents, far from vents to track the water as it disperses.

One of the most important elements of this work is the way we integrate our results into the global GEOTRACES programme – our measurements are made to the same rigorous protocols and carefully cross-calibrated so we can contour our data straight into the global database. We are looking forward to adding our piece of the puzzle to the global understanding of how the geology of the seafloor controls the chemistry of the ocean.