Getting to the Crux of Coring (Part 1 of 3)

Earth Observatory Blog

Getting to the Crux of Coring (Part 1 of 3)

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Coring is difficult work, requiring days of planning, specialised equipment, and no small amount of physical prowess. Unlike bathymetry, which is primarily experienced by staring at computer monitors for hours upon end, coring happens on deck — day or night, rain or shine. This combination of engineering know-how and man doing battle with the elements makes coring fascinating to observe.

These 1.5-m sections of the completed core samples are carefully labeled so that their vertical orientation in the seafloor is not lost. From the deck, these sections are moved to a refrigeration unit for the remainder of the cruise. They will also be refrigerated in transit to the Marine Geological Institute of Indonesia in Bandung, West Java Province.

Before I summarise how Mr Yvan Reaud, his colleagues at the Institut Paul Emile Victor (IPEV), and more than a dozen crewmen from the Marion Dufresne drop 30 or 40 metres (m) of steel pipe 4.5 kilometres (km) to the seafloor and haul it back up again, a few words about where we took core samples, and why.

When the survey began, MIRAGE scientists had hoped to do as many as four cores, designated C1 through C4. But a survey plan is a fluid thing, changing with circumstances on a day-by day, sometimes hourly, basis. Thus, by the time we had finished C1, it was clear that there would not be enough time to complete all four cores, so C4 was selected as the second and final target.

The Core 1 site is located in a small basin within the larger Wharton Basin, as seen in this printout from the sub-bottom profiler.

The first core site, C1, was located in a small basin within the greater Wharton Basin, resting at a depth of 4,441 m. The main objective of this core was to collect surface sediments down to about 20 m in order to learn the age of those sediments, their composition, and the rate at which they have formed. It was also hoped that upon analysis at the Marine Geological Institute (MGI) in Indonesia, where MIRAGE scientist Dr Rina Zuraida is based, researchers might be able to detect evidence of the tectonic activity that formed this feature — the result of a pull-apart fault — on the seafloor. 

The second core site, C4, was located at a depth of 4,431 m a few latitudes to the north. That puts it closer to an enormous underwater formation known as the Bengal Fan, which has been created by sediments originating in the Himalayas and delivered to the Bay of Bengal by the Ganges and other rivers in India and Bangladesh. The fan stretches 3,000 km to the south and is as much as 1,000 km wide. In fact, it’s a fairly safe bet that the same forces that have formed the Bengal Fan deposited the sediments at C4, but which sediments remains to be seen.

The Core 4 site is closer to the Bengal Fan than the Core 1 site, which is the likely source of Core 4’s sediments.

For Mr Marcus Phua, a research associate at Earth Observatory of Singapore (EOS), these questions are definitely worthwhile, but he’s a volcano guy, so when he gets his turn to test samples taken from the core, he’ll be looking for evidence of the most recent eruption of a once great volcano called Toba. The specific Toba eruption Mr Phua is interested in (there have been many) occurred roughly 75,000 years ago in what we now call Indonesia. When Toba blew its top, it deposited ash as far away as Greenland. Scientists believe the Toba eruption was so great that it probably lowered the planet’s temperature by a full degree centigrade for 1,000 years, aiding, if not triggering, an ice age. Today, all that remains of this prehistoric volcano is Toba Lake.

After the MIRAGE expedition, Mr Marcus Phua will travel to the Marine Geological Institute of Indonesia in Bandung, West Java Province, to prepare samples for analysis at the Earth Observatory of Singapore, where he will be looking for evidence of ash from a major volcanic eruption 75,000 years ago.

The Toba ash, or tuff as it’s also called, is well studied, making it easy to identify. That means when Mr Phua brings samples from the MGI back to EOS for analysis, he’ll know exactly what he’s looking for. And if he’s able to conclusively identify a layer of Toba tuff, he’ll know that all the sediments above this marker were deposited after the Toba eruption of 75,000 years ago, while those below were deposited before.

Unfortunately, at C1, the first five metres of sediments were rendered unusable when the coring pipe bent almost upon impact with a layer of rock that was tougher than anyone thought it would be. In fact, of the 40 m of pipe sent to the seafloor at C1, only 10 metres of core could be salvaged. The C4 core a few days later went better, with 18 of its 30 m of sediments saved, but like C1, the C4 pipe hit rock. The good news, though, was that the C4 pipe did not bend until 20 m down, which means Mr Phua will be able to look for signs of Toba tuff in those important first meters of sediment.

To see what bent coring pipes look like, and how the IPEV team and Marion Dufresne crew got them to the seafloor and back again, click here for Part 2.

To continue to follow the progress of MIRAGE, please check the EOS blog throughout the month of July, and spread the word using #MIRAGEcruise. 

All photographs are taken by Ben Marks, unless otherwise stated.

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