Vast underwater canyons revealed

The multibeam acoustic mapping was conducted by NIWA, with funding from the Foundation for Research, Science & Technology.

NIWA marine geologist, Dr Helen Neil, says the existence of the canyons has been known for a long time, but until now they've been drawn as more or less straight lines on the map.

'What we've found is that the canyons and channels are incredibly complicated. Near shore, the upper reaches meander over a 'river' bed up to 20 kilometres wide. Further out to sea, the channel is more than 1000 metres below the surrounding seafloor in places. We mapped over 650 kilometres and didn't get to the end of the system,' says Dr Neil. To put this in perspective: New Zealand’s longest meandering river (Waikato) is 425 km long, and Auckland’s SkyTower is 328 metres high.

Sediment periodically flows through the Hokitika and Cook canyons in what’s called a 'turbidity current'. These are like undersea avalanches: a dense 'river' of material cascades down the continental slope. Eventually, the turbidity current loses energy, dropping heavier material first until only very fine silt is left to reach the 'abyssal plain' – the dark basin of the deep seafloor.

'We were stunned as the images came in on Tangaroa. You can see how the river of mud changes shape over time. It meanders across the sloping seafloor depositing sediment, forming pools and ripples. Periodically it breaks through its banks to cut off a meander, creating an ox bow lake, only for the straightened section to be undercut, new material to be deposited, and the meandering to start all over again,' says Dr Neil.

The NIWA scientists hope to conduct a series of voyages to the West Coast canyon region. 'We plan to study the relationship between the sediment and past climate, including abrupt climate change, which will help us understand more about what could happen with present global warming,' says Dr Neil. 'The Hokitika & Cook canyons are also important spawning areas for hoki and hake, providing an essential food supply for larvae, so better knowledge of the canyon system will improve our knowledge and understanding of these fisheries.'

Background

New Zealand’s sediment load

New Zealand has just 0.18% of the world landmass, but is responsible for 1% of the sediment going into the world’s oceans.

On the North Island east coast, the unstable hinterland, frequent heavy rain, and deforestation have dramatically increased erosion and mud supply to the marine environment. The East Cape, including Waiapu and Waipaoa, produces about 33% of New Zealand’s total contribution of sediment to the oceans.

On the South Island west coast, sediment supply to rivers is driven by rapid tectonic uplift of the Southern Alps and a vigorous wet climate. South Westland, including the Hokitika, Cook, and Haast canyon systems produces about 29% of New Zealand’s total sediment contribution.

The South Island west coast’s sediment load

More than two-thirds of all the sediment flowing into the oceans from the South Island comes from South Westland. An estimated 50 times more sediment is currently injected into the Hokitika, Cook, and Haast canyon systems than into the Bounty Trough on the east coast of the South Island. This is a consequence of the closer proximity of the Southern Alps and the lack of large natural lakes which trap up to 15% of the sediment yield to the east of the Alps.

Acoustic seabed mapping: how it’s done

NIWA uses a multibeam echo sounder mounted on the hull of one of its research vessels. In this case, RV Tangaroa was fitted with the Simrad EM300. This produces a fan of 135 sonar beams, sent out every 2-4 seconds, which can map a swath of the seafloor up to a width five times the water depth, up to a maximum of 5 km wide.

The time taken for a sound pulse to travel from the ship and reflect from the seafloor back to the ship gives the water depth. The system corrects for the following factors:

• the seawater separates into layers of different temperature and salinity, and sound travels through the layers at different speeds,
• the sound is refracted as it goes from one layer to another on the way out and on the way back,
• the ship is tossing about on the surface.

The system not only records the time (which gives the depth) but also the strength of the sound energy reflected from the seafloor in decibels. This strength of reflected sound wave is called back-scatter and tells the scientists about differences in seafloor substrate and composition. More sound energy is reflected off hard young lava flows than gooey mud, for example. NIWA uses this difference to map areas of the seafloor with different substrate types – lava flows on submarine volcanoes, gravel fans near the coast, and mud channels on the bottom of the deep ocean.

NIWA also uses this type of information to produce marine habitat maps. Physical variables, like water depth, substrate type, sea-surface temperature, and tidal and bottom current flow all have a profound effect on the distribution and type of marine ecosystems and biodiversity.

Comparison with the Grand Canyon

The Grand Canyon is 446 km long, up to 29 km wide, 1500 m deep.

The Hokitika and Cook canyon system is at least 650km long, up to 20 km wide, and up to 1000 m below the surrounding seafloor (reaching depths below the sea surface of about 3750 m).