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Drilling for Sub-Seafloor Life

The Japanese deep-drilling vessel DV Chikyu can core up to 4000 metres below the seabed and in areas where there is a potential danger of striking oil or gas. Photo courtesy of the Japan Agency for Marine–Earth Science and Technology

The Japanese deep-drilling vessel DV Chikyu can core up to 4000 metres below the seabed and in areas where there is a potential danger of striking oil or gas. Photo courtesy of the Japan Agency for Marine–Earth Science and Technology

By Chris Yeats

Extreme sub-sea temperatures, noxious fumes and broken drilling rods made life difficult onboard a scientific expedition that set out to sample life deep beneath the sea.

The International Ocean Discovery Program (IODP) is an international marine research collaboration that employs drill ships to recover samples of sediment, rock, fluids and living organisms from deep beneath the seafloor. In 2010 I was a member of IODP Leg 331, which drilled five sites at the Iheya North hydrothermal field in the Central Okinawa Trough back arc basin south of Japan.

Leg 331 was unusual for a couple of reasons. First, the 34-day expedition aboard the drill ship DV Chikyu was significantly shorter than a typical 2-month IODP expedition. Second, and more significantly, it was the first IODP expedition, and still one of only a handful of cruises, to focus principally on biology rather than geology.

The impact of this focus on the scientific party was profound: membership was dominated by microbiologists, biochemists and fluid chemists, for whom the recovered core was a means not an end. Consequently, logging of the 312 metres of drill core recovered during the expedition was left to a small team of three to four geologists, most of whom had been attracted to the cruise by the possibility of drilling metal-rich massive sulphides – but more on that later.

Due to its proximity to the Asian continent, the Okinawa Trough contains significant quantities of organic-rich sediment. We therefore anticipated (and in some cases got) drill core containing levels of hydrogen sulphide (H2S) that could be harmful to humans, so the initial collection of core was conducted wearing breathing apparatus until it was declared safe. This meant that some members of the science party, including myself, and all of the Chikyu core technicians needed to be trained in safe operating procedures for H2S before departing from Shimizu. Wearing heavy overalls, breathing apparatus and a full face mask with temperatures in the mid-30s and humidity close to 100% is a pretty good substitute for sauna therapy!

Previous work by Japanese scientists at Iheya North, including manned and unmanned submersible dives and extensive seismic surveys, had given the cruise participants a fair idea of what to expect at the site. Hydrothermal alteration and massive sulphide mineralisation at Iheya North is hosted in a geologically complex sequence of sediments: basically gravel and mud overlying a volcanic substrate.

The principal aim of the expedition was to sample the hydrothermally altered sediments and prove the existence of a functionally active, metabolically diverse biosphere associated with sub-seafloor hydrothermal activity in the Iheya North field. This goal wasn’t achieved because things got hot a lot faster than we expected. The initial strategy was to drill using perspex core liners, but we soon abandoned that strategy at our first site and switched to aluminium when it became obvious that the plastic couldn’t survive the temperatures we were encountering.

Subsurface temperatures at the three hydrothermally altered drill sites rapidly exceeded those that could support life, effectively meaning that the biologists were sampling sterile material. However, each of these three sites successfully drilled into hydrothermal fluid reservoirs and created artificial hydrothermal vents, which have been monitored and sampled by Japanese researchers for the past 7 years.

As an ore deposit geologist, what excited me most about Leg 331 was the proposal to drill directly into a large actively venting massive sulphide mound – the 20-metre high North Big Chimney (NBC) mound – which has venting temperatures of 311°C. I had been a shipboard scientist for the Ocean Drilling Program’s previous attempt to drill an active hydrothermal field in a back arc basin environment in the Bismarck Sea in 2000 (ODP Leg 193), but that expedition deliberately sought to avoid drilling known massive sulphides.

Leg 331’s first attempt to drill NBC, about midway through the cruise, ended disastrously. The hole was spudded in the top of the mound and did not drill vertically. Consequently, when the Chikyu drillers attempted to withdraw from the hole to recover the core (they were using a non-wireline system adapted from the oil and gas industry), the rods became jammed and ultimately broke. The drill bit remained lodged in the hole and all of the core was lost.

Our second attempt at the very end of the cruise was much more successful. The hole was drilled adjacent to the base of the NBC and successfully penetrated 45 metres. Unfortunately, after the experience of the first hole at the NBC, the decision was made to minimise equipment risk at the site so the hole was cored without a core catcher and only 1.7 metres of core was recovered. However, the material recovered included an interval of black zinc-rich massive sulphide. Although other research cruises using dredges, submersibles, remotely operated vehicles and various types of drills have been successful in recovering massive sulphide mineralisation from seafloor hydrothermal sites around the globe, the interval recovered from NBC at Iheya North has the same mineralogy and textures as kuroko black ore – one of the main ore types of kuroko-style deposits that are mined for zinc, copper, lead, silver and gold in Japan.

Hydrothermally altered volcanic rocks recovered from further down the hole also resembled those associated with this type of mineralisation on land, meaning that this site is an outstanding modern analogue for kuroko-style mineralisation, and provides near-indisputable evidence confirming the long-held theory that these terrestrial deposits formed on an ancient seabed associated with volcanic activity. This is arguably the most important scientific result to come out of IODP Leg 331.

I have been fortunate enough to participate in more than 20 research expeditions during my career on Australian, American, Japanese, British and Indonesian research vessels. In terms of available analytical facilities and drilling capability, DV Chikyu is undoubtedly the most impressive facility I’ve sailed on. The vessel is enormous and, unique among the ships on which I’ve sailed, incredibly stable under normal weather conditions. It really doesn’t feel like you’re at sea at all. The crew are friendly and helpful, the food is amazing (but I do love sushi), and the accommodation is first class.

This incredible research facility has not been available to IODP as much as could be hoped, but new possibilities have started to open up. I can only hope that in coming years other Australian scientists will have the opportunity to sail on what was and should be one of the IODP’s most valuable assets.


Chris Yeats participated in IODP Expedition 331 in his role as Research Program Leader of Mineral System Science at CSIRO Earth Science and Resource Engineering, and is currently the Executive Director of the Geological Survey of NSW. This is an edited extract from Exploring the Earth Under the Sea: Australian and New Zealand Achievements in the First Phase of IODP Scientific Ocean Drilling (ANU Press, 2017), which is available for free download at https://press.anu.edu.au/publications/exploring-earth-under-sea