Kaikoura deep sea ecosystem earthquake recovery showing hopeful signs
Deep sea life in the Kaikōura Canyon is recovering faster than expected from last November's massive earthquake but it's too early to say whether the rebound will continue at the same pace.
The 7.8 magnitude Kaikōura Earthquake caused huge changes to the geology of the canyon. Some places where the volume of organisms living in the mud had been among the highest in the world, were left devoid of life.
Changes caused by the quake were "a catastrophic event for the ecology of the canyon", Niwa marine ecologist Dr Dave Bowden said earlier this year.
But the latest research trip to the canyon, which finished on Sunday, has found some hopeful signs. Scientists sampled the seafloor scoured or buried in the earthquake to begin to work out the potential for recovery of the deep sea ecosystem. The research is also highlighting how violent the undersea disturbance was.
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Niwa ecologist Dr Daniel Leduc said there was evidence that juveniles of animals that once dominated the head of the canyon have begun colonising the seafloor.
"The deep sea communities might be recovering faster than we originally thought, with high densities of small organisms such as urchins and sea cucumbers in some areas of the canyon, as well as large numbers of rattail fishes swimming immediately above the seabed."
Niwa Kaikōura deep sea project leader Dr Ashley Rowden said evidence from elsewhere in the world had suggested smaller organisms were likely to respond relatively rapidly after the submarine mudslides.
But it had been thought larger organisms such as the urchins and sea cucumbers could take many years to recover, and may never recover to their previous composition.
"So to come back after 10 months and begin to see some of the juveniles of those species was somewhat of a surprise to us," Rowden said. Essentially those species so far only had one potential recruitment period, in spring and summer, following the quake.
"It's very early days. It's positive in the sense there are indications it's faster than we might have expected, but at the same time what the ongoing trajectory is, is still unknown."
It might be the animals initially had enough food to grow to a certain size, but not enough continuous food supply beyond that. It was also possible there could be further, although smaller, submarine landslides.
The edge of the Kaikōura Canyon is at a depth of about 150 metres, and drops down to around 3000m. The head of the canyon comes within a few hundred metres of the coastline just south of Kaikōura.
Rowden said the Niwa scientists had some information from the edge of the canyon but most of it was from the floor at the base of the canyon, at depths of hundreds to thousands of metres.
As well as signs of recovery, researchers had also found areas within the canyon that did not appear to have been affected at all by landslides.
"That's important in the sense that the functioning in those areas can carry on," Rowden said. " Populations of organisms that live there can supply new recruits for disturbed areas."
Geologists have also been able to confirm the earthquake caused a significant loss of material from high up in the canyon.
The collapse and loss of material at the canyon head triggered a violent and vigorous current moving a dense, mud-laden cloud, Niwa geologist and voyage leader Dr Alan Orpin said.
"We've had an event that has passed through the canyon at significant speed - many tens of kilometres an hour. That's been of sufficient speed and vigour that it's re-shaped the seabed."
Much of the material passed out into the deep ocean, while evidence of the power of the current could be seen in the way gravel waves on the floor of the canyon had been re-shaped.
"It's pretty staggering stuff. This is big geology," Orpin said.
The gravel waves on the floor of the canyon were a few metres high and hundreds of metres long. It was thought the gravel may have been delivered to the base of the canyon thousands of years ago when sea levels were lower.
"What the movement of these gravel waves shows is that the big dense cloud of material that moved down through the canyon, moved at a sufficient speed and was sufficiently violent that it was moving gravel, just like a river in flood," Orpin said.
"The violence of it was sufficient that the fine grain mud couldn't settle. It couldn't fall to the seabed because the current's too strong. A good proportion of that fine grain mud then would have moved beyond the canyon."
A dense mud cloud at least 200m high then passed along the Hikurangi Channel, with evidence showing it travelled along the channel at least as far north as offshore Gisborne.
It was thought that in some parts of the canyon metres of sediment had been removed, while in some areas material was deposited.
On top of the areas of eroded seabed and of the gravel waves a blanket of soft, soupy mud 5-15cm deep had formed, Orpin said. He was reasonably confident that layer was no older than the earthquake. It would have been formed from the tail end of the sediment cloud caused by the earthquake, and by normal background sedimentation since the quake.