Why there is a major tsunami risk to the east coast of New Zealand.
Scientists have uncovered evidence of a massive tsunami that slammed into the North Island’s west coast some 600 years ago, raising intriguing questions about what might have caused it.
The event, thought to have occurred in the mid-15th century, is among dozens of prehistoric “paleotsunamis” around the country that researchers have documented through ancient geological deposits.
In the latest case, a team of researchers were working on dune systems on the Kāpiti Coast when they found evidence to suggest sand had suddenly and violently been shifted inland at some point in the past.
After measuring the extent and height of the dunes, the team dug trenches to explore material beneath it, before taking samples of old tree stumps submerged in a nearby wetland formed when the sand blocked drainage.
Further analysis and radiocarbon dating allowed them to reconstruct the environment – including a large podocarp forest and sand dunes that offered Māori inhabitants a refuge from the wind, as indicated by old shell middens - as it would have appeared in the 1400s.
More interestingly, they were able to show how this area was instantly transformed by an event that drowned the forest, wiped out the dunes and seemingly caused Māori to abandon the coastal area, with no further signs of occupation after that date.
Given how far inland the deposits had been traced, the researchers concluded they weren’t dealing with a small, local event, but a possible region-wide tsunami that could have hit coastlines stretching up to Taranaki.
“Locally, this would have caused widespread coastal abandonment by Māori, causing a movement both inland and uphill, something that we know happened because of other research done in the region - but this confirms it for the Kāpiti Coast.”
This map shows sites of paleotsunamis - or those that occurred prior to written records - around New Zealand. Green sites indicate where the geological record is excellent, yellow moderate, and red, poor. Source / Niwa
For scientists investigating huge tsunamis that hit our shores many centuries ago, Goff said the study came with some further interesting takeaways.
One was that changes in local geomorphology and human activity - as well as sediment buried in the geological record - could offer a useful window to the past.
Another was the need to understand just what might have triggered the tsunami.
New Zealand’s largest paleotsunamis - including metres-high surges caused by giant “megathrust” earthquakes - have often been linked to the Hikurangi margin, stretching along the opposite side of the country.
Partly by drawing on some of those events, scientists recently estimated a 26 per cent chance of a subduction zone quake 8.0 or larger striking beneath the lower North Island within the next 50 years.
But Goff said there was still much to learn about the tsunami threat to the west coast.
“We do not know how big and how often these events happen.”
While these west coast events could have produced tsunami measuring up to 70m high - and sending waves as far as eastern Australia - they were extremely rare, occurring about once every million years.
In another just-published study, scientists have found evidence of ancient fires - some well pre-dating Aotearoa’s settlement - buried at the bottom of South Island lakes.
The centuries-old deposits add a new dimension to scientists’ understanding of human-related burning deep into New Zealand’s past.
To reconstruct changes in activity over more than 10,000 years, the international study team drew on traces of refractory black carbon - highly-condensed soot particles created from burning biomass and fossil fuels.
“It’s important to understand black carbon through time, because it’s a key aerosol that modifies atmospheric conditions - and may induce climate feedbacks on local to global scales,” said the study’s lead author Dr Sandra Brugger, a paleoecologist at the Swiss-based University of Basel.
While New Zealand’s fire history has been relatively well studied - mainly from charcoal records stored in lake sediments - Brugger said the new study was the first to directly measure local deposits of refractory black carbon over time in New Zealand.
Normally sampled within ice cores, a new methodology developed at the United States-based Desert Research Institute meant that black carbon could be analysed in sediments drawn from four lakes in New Zealand.
Their analysis turned up some fascinating findings.
Cores taken from Diamond Lake, near Wānaka, pointed to spikes in black carbon at different times over the last 12,000 years.
“Before people arrived in New Zealand, fires were very infrequent and charcoal levels correspondingly low, but in sediment cores taken from the South Island, we showed significant spikes in black carbon at different times over the last 12,000 years,” Brugger said.
Because there was almost no charcoal preserved in the South Island lakes before human arrival, Brugger said the black carbon was most likely aerially transported from fires burning far away in Tasmania or the Australian mainland.
“The possibility of such long-range transport is also supported by sophisticated atmospheric models that were run by our colleagues at University of Vienna in Austria for our study.”
Following early human settlement, sediment records from the South Island indicated an increase in black carbon - along with charcoal - and it was this period of burning that likely contributed black carbon to the deposits recorded in Antarctic ice cores dating from this time.
In another interesting finding, the team found no marked increase of black carbon in any of the New Zealand lake sediment records during the industrial period, from 1850 onward - meaning that black carbon was mainly coming from biomass burning, such as through bush clearance, rather than fossil fuels.
Brugger said the latest New Zealand data helped fill in the Southern Hemisphere’s wider picture of black carbon deposition, with her team also having measured black carbon in lakes in Patagonia, Tasmania and South Africa.
“Identifying the sources of black carbon during the industrial era and disentangling fossil fuel emissions from biomass burning emissions in New Zealand adds to knowledge about the atmospheric composition over the Southern Ocean - and helps to interpret the sources of black carbon found in Antarctic ice records,” she said.
“Furthermore, these records of fire-created aerosols from mid-latitude lake sediments are key to helping to constrain regional fire and climate models.”
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
