Health & Science
Oceans away
by Marilyn Head
A highly successful two-week scientific expedition that extracted 29 sediment cores from New Zealand’s continental shelf.
It’s still giving me nightmares,” confesses marine geologist Geoffroy Lamarche, who shudders recalling the shattered giant steel corer belonging to one of the world’s largest oceanographic research vessels, France’s Marion Dufresne. The 10mm-thick steel pipe bent and broke after several abortive attempts to extract a “soft” sediment core from the seabed 200km off the East Coast. “We knew it would be a bit hard to pene-trate because of the tephras [deposits of volcanic ash which have been compacted over thousands of years], but we didn’t expect to break the equipment!”
Fortunately, it proved to be a minor setback in a highly successful $2.6m two-week scientific expedition that extracted 29 sediment cores from New Zealand’s continental shelf. This was the first stage of an international research collaboration with the French institutes Géosciences Azur and Géosciences Rennes in a project researching past records of catastrophic environmental change. Two initial targets were submarine avalanches off the East Coast and changes in foraminifera, the fossilised shells of microscopic creatures buried in the sediment of the Hokitika canyon system off the West Coast.
Being in charge of 40 international scientists from 20 different institutions was quite a responsibility, says Lamarche, who, despite his French nationality, led the New Zealand consortium aboard the Marion Dufresne, together with fellow National Institute of Water and Atmospheric Research (NIWA) scientist Helen Neil. “With so many different nationalities, there’s always the potential for tension,” he acknowledges, “and it’s so intense – we work around the clock.”
The scientists, including 10 New Zealand students, were assigned two four-hourly watches, which among other tasks involved using specialised research equipment like the 10-tonne deep piston corer Calypso and a multibeam sonar mapping instrument. Other work was fitted in around the watches, making for an average 16-hour working day.
“It was exhausting, but also very exciting,” says Lamarche. “The students were great – they’re fantastic workers, and it was an incredible opportunity for them to work with such scientists at the beginning of their careers.”
Neil, an ocean-going researcher for 18 years, agrees. “Most scientists form collaborations that are personal – you can’t collaborate if you don’t get on with people. Here we know each other well – often we’ve been to primary school or university together or played in the same orchestra, and these different interactions really help. But you do have to have some time out, otherwise it becomes too repetitive and frustrating and you run the risk of tensions.”
Good weather, excellent recreational facilities and a few informal lectures contributed to a relaxed atmosphere – and the odd drop of French wine helped, too.
Onshore, however, the survey’s success is measured by the mountains, or rather metres, of mud collected, divided into 1.5m lengths and stored in giant refrigerators in both New Zealand and France.
“Standard protocol with physical samples such as these,” says Neil, “is that only one half is used for testing. The other half is archived, because in a decade that mud will still be in fantastic condition and there’ll be new techniques we can use to get more information from it. It is a bit daunting when you open the fridge, though!”
New Zealand’s continental shelf lies at a critical junction between the tropical climate influences of the north and the cool Antarctic influences of the south; it also straddles the Australian and Pacific tectonic plates. Here, therefore, are clues to the complex interactions between Earth’s atmosphere, oceans and land. The high resolution of the samples should enable the scientists to date past cataclysmic events associated with sudden climate change – eg, avalanches, polar ice melts or volcanic eruptions – to within a few hundred years, perhaps even decades.
“Through time we can see peaks of rapid warming due to slight switching in the circulation of currents,” explains Neil, “but what we don’t quite understand is how the climate is able to change so rapidly and then change back. Is one event triggering the other or are two discrete things happening? These changes are not intuitive – it’s such a complicated system, with so many things feeding into it.”
One of the big climate drivers is the thermo-haline circulation of the oceans. “Warm ‘light’ [less salty] water will float on top of cold, dense salty water and when tropical and arctic currents meet, the warm water is pulled around by the cold water sinking beneath it,” says Neil. Changes in the composition of the water brought about by sudden ice melts or freezing (for example) can be detected by measuring different isotopes of carbon and oxygen found in the shells of microfossils: Neil will use these to track climate changes between warm and cold periods.
Changing sea levels also impact on Lamarche’s research into huge submarine avalanches possibly triggered by earthquakes associated with the collision of tectonic plates. “A rise in sea level increases the weight on the sediment significantly – that could also trigger an avalanche,” he says. He will be looking for indications of abrupt climate change in the nature of sediment brought down by erosion.
“Right now, we can identify the layer of sediment deposited by Cyclone Bola, which was only a few years ago. These cores will give us an edge in estimating the age of events over hundreds of thousands of years and seeing how often they occurred. We have heaps of fantastic data that will keep us going for years.”
