Ancient iceberg scours found on Chatham Rise

As a flotilla of modern-day icebergs heads away from New Zealand, scientists from NIWA have identified what appear to be ancient iceberg scours on the seabed off New Zealand’s east coast.

The large scours showed up on maps of the seabed produced from surveys of the Chatham Rise in August as part of the Government’s Ocean Survey 20/20 research programme.

‘Grooves measuring about 2–2.5 km long, 200 m across and 10 m deep, can be seen in the seabed on the eastern Chatham Rise, at a water depth of about 450–470 m’, says NIWA marine geologist Scott Nodder, who led the Chatham/Challenger Biodiversity Project onboard NIWA’s deepwater research vessel Tangaroa in August.

‘They were probably caused by an iceberg that carved off the Antarctic icesheet during the last Ice Age 18 000 to 20 000 years ago’, says Dr Nodder. ‘The sea level was 100 m lower then, so the iceberg would have sat in about 350–400 m of water. The scours look very similar to ones we recorded in the Ross Sea in 2004’, he says.

‘The curved grooves were likely caused by the iceberg grounding on the seabed, then moving around with the currents. Three gouge marks to the north of the grooves may have been where the keel of the iceberg hit the seafloor’.

‘The iceberg that made these scours would have been somewhere in the range of 2 to 5 km long, similar in size or slightly larger than the largest berg recently spotted south of Invercargill’, says NIWA oceanographer Dr Mike Williams. ‘Those icebergs would have carved off a much larger berg in Antarctica, measuring about 160 km by 30 km’, he says.

The scours are about 170 km west of the Chatham Islands and 725 km from Christchurch. Similar scours were found further west on the Chatham Rise in 2001 and on other transects on the 2006 Ocean Survey 20/20 voyage less than 500 km from Christchurch.

Boulders and pebbles from Antarctica have previously been found on the seafloor around New Zealand, probably carried there by icebergs.

The scientists hope to take sediment samples from the scours to confirm their origin during the next Ocean Survey 20/20 voyages in April and June 2007.

Image

A colour image of the seabed showing the scours seen on the Chatham Rise is available for download here (right click and ‘save as’).

Background information

• The images were obtained from multibeam swath bathymetry surveys onboard RV Tangaroa. This technique uses a sophisticated sonar to send out 254 beams of sound in a swath up to 5km wide across the seabed. The time for the reflected sound to travel back to the ship indicates water depths, while the strength of the signal indicates the type of substrate (e.g. rock or sand). The information is processed to produce detailed images of seabed topography and substrate type.
• The surveys were carried out in August in the first of three voyages on RV Tangaroa to map and compare biodiversity and seabed habitats on the Chatham Rise (off the east coast of the South Island) and Challenger Plateau (off the west coast of the North Island). The 23-day voyage involved staff from Land Information New Zealand (LINZ), the University of Otago, and NIWA.
• Further voyages will be made in April and June 2007 as part of the Chatham/Challenger Biodiversity Project. The Chatham/Challenger Biodiversity Project (part of Ocean Survey 20/20) is a collaborative effort led by the Ministry of Fisheries, with major input from the Department of Conservation, LINZ, and NIWA.
• Ocean Survey 20/20 is a multi-agency Government initiative to extend our knowledge of New Zealand’s ocean resources, and is being coordinated by LINZ. Further information can be found on LINZ’s website at: http://www.linz.govt.nz/home/news/items/20062007-ocean-survey-2020/.