Current thinking in sea floor mystery
Giant depressions and smaller pockmarks on the sea floor off the South Island puzzled scientists. They have now probably solved the mystery. Will Harvie reports.
When the really large depressions in the sea floor came to light a few years ago, New Zealand scientists were puzzled.
The holes were among the largest recorded in the world and concentrated on the Chatham Rise, east of Banks Peninsula.
They’d been discovered accidentally, when a research ship happened to pass over and record their presence, says Jess Hillman, who earned her PhD in marine geophysics at the University of Otago by studying the New Zealand sea floor depressions.
One of the depressions was 11 kilometres by 6km and 100m deep. It was big enough to enclose all of Wellington City. Others were only slightly smaller.
What could have caused these unusually large features?
One early theory said they may have been created by meteorites that crashed into Earth aeons ago.
Given the size of the depressions, the space object must have been large and the impact probably explosive.
But the depth of the holes and their roughly oblong shape didn’t match conventional space object craters. Water depth, even when sea levels were much lower, also worked against the meteorite theory. It was ruled out.
A second theory proposed that the big depressions were caused by the sudden release of methane gas, a sort of volcano of gas rising quickly from under the sea floor.
In 2010, Bryan Davy of GNS and colleagues calculated that a ‘‘single, sudden release of large amounts of methane’’ from one of the largest depressions would have been equal to about 3 per cent of the current annual global methane release from natural sources.
Three per cent from one event is considerable.
There is evidence that these ‘‘explosions’’ have occurred elsewhere on the planet. ‘‘Massive blow-out craters’’ left behind after methane pops have been found in the Barents Sea off Norway. Craters up to 1km wide are believed to have been created at the end of the last Ice Age.
When ice sheets were present, they put enormous pressure on the sea floor. As the ice retreated, the pressure lessened and the methane built up and formed mounds. These ‘‘abruptly released’’ methane and formed the craters, Norwegian scientists reported in the journal
It should be noted these scientists are talking geological time, says Hillman. The events could have lasted days, months or a year. ‘‘It would be instantaneous on a geological scale,’’ she says in an interview.
Craters and depressions have been observed on continental shelves around the world in the past 15 years or so.
New technology called multibeam imaging allowed researchers to direct high-frequency sound pulses from a ship to the seabed. The sound bounces back to receivers and results in highly detailed maps of the sea floor, says Dr Andrew Gorman, Hillman’s PhD supervisor.
In a 2013 voyage with German and US scientists, Hillman and colleagues got new data and compared the NZ sea floor features with those off Norway and elsewhere. They didn’t seem to match.
Some of the New Zealand examples were much larger.
Furthermore, they didn’t find the ejecta – sediments and rocks forced out by an eruption – that would have been expected if there had been a ‘‘sudden’’ expulsion.
They also collected sediment samples and analysed them chemically. They didn’t find the quantities of methane and other substances that were expected if there had been a significant pop.
‘‘Based on the new data we acquired, we can’t find any evidence of methane hydrates in the subsurface that could have caused the depressions to form,’’ says Hillman, who now works for GNS.
OK, so no sudden expulsion. What happened?
In recent months, Hillman, Gorman and colleagues such as Dr Ingo Klaucke of the Geomar Helmholtz-Centre for Ocean Research in Kiel, Germany have published a suite of scientific papers that seem to solve the mystery.
It turns out the large New Zealand depressions are aligned with the direction of powerful currents flowing across the Chatham Rise.
There are several currents associated with the Subtropical Front, which flow north along the east coast of the South Island, passing Dunedin. When they hit the Chatham Rise – an underwater plateau extending 1400 kilometres east from Bank’s Peninsula – the currents are largely deflected east and north east.
‘‘Our results reveal a direct correlation between the dominant current flow direction, and the modification and alignment of depressions,’’ reported Hillman and colleagues in a 2018 paper in the
In a different paper, Klaucke writes ‘‘the depressions are interpreted as the result of scouring by strong bottom currents’’.
The new papers show the currents are strong enough to displace and carry the kinds of sediments found on the Chatham Rise, Hillman says.
But wait. Currents by themselves probably would not scour an otherwise featureless sea floor, says Hillman. It’s more likely that some initial feature existed that the currents could work on.
There may have been a smaller depression, or ‘‘pockmark’’. Hundreds and perhaps thousands of pockmarks have been found along the east coast of the South Island in recent years. They are typically 10m or 20m across.
They are probably caused by small-scale ‘‘out-gasing’’, says Gorman.
When plants and other biological materials are covered by sediment, they eventually break down into methane and similar compounds. A back garden compost works in a similar manner. These gases rise.
Further out on the Chatham Rise, additional fluids are squeezed out of the sea floor when it gets compacted under more sediments, says Klaucke by email. This is the process described by Davy, but on a smaller scale.
An alternative explanation is that small faults exist under the pockmarks and depressions. Deep fluids and gases rise through the fault and vent at the surface.
The jury is still out on the initiating process, Hillman and Gorman say. More expeditions, probably with a drilling rig ship like the Joides Resolution, would be required.
Nonetheless, mystery mostly solved: The currents did it.
"Our results reveal a direct correlation between the dominant current flow direction, and the modification and alignment of depressions."
Jess Hillman