Ocean Drilling Programme

The world’s techno­logically most advanced research vessel, JOIDES Resolution, recently spent two months probing the secrets of the deep ocean floor off the east coast of the South Island.

The research cruise was the 181st leg of the interna­tional Ocean Drilling Programme, which has been examining the rocks on the floors of the world’s oceans for the past 34 years.

Why drill at sea, you may wonder. The main reason is that seas are where sediments accumulate, and sediments provide a treasure trove of geological informa­tion about the activity of landmasses nearby.

Sea-floor rocks potentially contain a more complete geological record than do terrestrial rocks. This is because when land is above water it erodes. No new sedimentary rock is being laid down, so existing rocks are destroyed.

Many landmasses—New Zealand is a good example—have had a very up-and-down existence, for some of the time being terra firma and in other epochs being beneath the waves. Many of the shallower sea floors close to the current coastline of a landmass could also have spent periods as dry land, but this is less likely in deeper waters further from shore. On the other hand, drill too far from land and there may be little in the way of sediment sources, so rock build-up could be very slow and uninformative as to the history of a particular landmass.

The Ocean Drilling Programme is a scientific project of international collaboration, aimed at unlocking the secrets of our Earth and the workings of the global oceans. No other scientific research project is as globally inclusive or has been running so successfully for so long. Among its many achievements, the pro­gramme has been instru­mental in proving the theory of plate tectonics.

Over 1100 sites from every corner of the world’s oceans have been drilled to depths of up to 1 km below the sea‑floor. The  programme is run by scientists and is largely free from government and political interference. Scientists from almost every country in the world have participated on board, and the vast storehouse of core samples is freely accessible to anyone wishing to undertake bona fide research on them.

The $NZ80 million per year programme is funded by the United States National Science Foundation (60 per cent) and 20 other countries: Australia, Canada, China, France, Germany, Japan, South Korea, Taiwan, the UK and 11 countries of the European Science Founda­tion Consortium.

On board the Joides Resolution when it visited New Zealand waters, with a complement of 63 crew and 50 scientists and technicians, were three New Zealanders: Lionel Carter (marine geologist, National Institute of Water and Atmospheric Research), Gary Wilson (paleomagnetist, Institute of Geological and Nuclear Sciences) and Bruce Hay­ward (paleontologist, Univer­sity of Auckland).

Carter has been studying the deep ocean current systems and sediment deposits off the east coast of New Zealand for over a decade, using New Zealand’s ocean research vessels. From these he has been able to seismically image the structure of sediment layers beneath the sea floor and take sea-floor samples and short cores up to several metres deep, which docu­ment sea-floor processes that have been occurring over the past 50,000 or so years.

The choice of the seven sites drilled during the JOIDES Resolution visit was based largely on Carter’s pioneering work. The sites were located in water depths of 400-4500 m. Drilling at such depths requires the ship to put down an enormous length of 20 cm diameter drill pipe just to reach the sea-floor, before coring can even begin.

The ship does not anchor, but maintains its position above the site using satellite navigation and 12 thruster motors distributed around the hull. Drilling is done through a “moon pool,” or well, in the middle of the boat. A heave compensator on the 80 m-high drilling rig allows the drill bit to remain at the bottom of the drill hole even when swells are lifting the vessel 10 m, permitting drilling to continue through all but the largest storms that the Southern Ocean can (and does) throw up.

One of the reasons the Ocean Drilling Programme chose to drill the ocean floor east of New Zealand was to better understand the huge deep-ocean current system that circulates like a giant conveyor belt through all the world’s oceans. The local manifestation of this current (the Deep Western Bound­ary Current, DWBC) flows northwards into the Pacific Ocean around the flanks of the continental slope east of New Zealand, carrying cold water from Antarctica with a flow 100 times that of the Amazon River.

Active erosion of New Zealand’s mountains and hills has delivered a voluminous mud supply into the path of this current. This mud is winnowed and modified by the current as it builds up drifts on the sea-floor, and in its layers is preserved a unique record of the history of the composition, tempera­ture and strength of the current stretching back to its inception 32 million years ago following the break-up of Gondwanaland.

South of Bounty Trough (a trench running out to the Bounty Islands to the south­east of the South Island) the Antarctic Circumpolar Current has been found to be even more powerful than the DWBC, producing extensive scouring of the seabed. This erosion seems to have commenced about 10 million years ago, and has provided sediment for the huge mud drifts to the north, which may be several hundred kilometres in length and a kilometre deep.

On board, Gary Wilson and Bruce Hayward played an important role in helping to determine the age of the mud layers as coring pro­gressed deeper and deeper, by examining fossils within current stretching back to its inception 32 million years ago following the break-up of Gondwanaland.

South of Bounty Trough (a trench running out to the Bounty Islands to the south­east of the South Island) the Antarctic Circumpolar Current has been found to be even more powerful than the DWBC, producing extensive scouring of the seabed. This erosion seems to have commenced about 10 million years ago, and has provided sediment for the huge mud drifts to the north, which may be several hundred kilometres in length and a kilometre deep.

On board, Gary Wilson and Bruce Hayward played an important role in helping to determine the age of the mud layers as coring pro­gressed deeper and deeper, by examining fossils within the core. They discovered that one hole penetrated mud that was accumulating on the ocean floor 65 million years ago—the date when a giant meteor is thought to have slammed into the Earth, contributing to the extinction of the dinosaurs and 80 per cent of all life forms on land and in the sea.

The most exciting drill hole, from a world science perspective, was located 450 km north-east of Chatham Island. Quite unexpectedly, it produced a core from a 600 m-thick unbroken record of mud and ooze accumulation on the sea-floor extending back 20 million years. This core is unique in that it contains a full record of all the times the Earth’s magnetic pole has switched back and forth during this period.

Also, for the first time we have core that contains a complete sedimentary record of all the astronomically driven hot and cold cycles of the world’s climate over that time period. Changes in climate affect the rate of erosion as well as the flow of currents, and information on temperatures can be gained from the arrays of diatom and foraminifera microfossils present in different strata.

The 100,000-year and 41,000-year climate cycles of the Ice Ages over the past two million years are now well known, and are driven by the “Milankovitch clock,” regular fluctuations in the tilt of the Earth’s axis and in the shape of its orbit around the sun. Now we have gained the ability to recognise and study them in cycles of light and dark mudstones going back much further in time, when the climate extremes within each cycle were not so great.

Perhaps the most exciting hole for New Zealand scientists was that drilled in 4500 m of water 600 km east of Gisborne. Its core contains some 140 distinct volcanic ash layers extending back over 12 million years. For the very first time, we now have a complete record of all the largest volcanic eruptions that have taken place in the northern North Island—initially from the Coro­mandel region, then the Kaimais and, in the past million or two years, from the Taupo-Bay of Plenty area.

None of these ashes come from andesite volcanoes such as Ruapehu or Egmont. They all come from truly giant ignimbrite-type eruptions from rhyolitic caldera volcanoes, of which only the youngest can still be easily recognised in Lakes Rotorua, Okataina and Taupo. One of these ash layers was almost a metre thick.

The cores were studied on board the ship, and prelimi­nary reports have been prepared, but most of the detailed work is only just beginning. Most of the 4 km of recovered core has been shipped back to a refrigerated store in Texas, and will provide material for detailed studies by scientists from all over the world, including New Zealand, for many decades to come.