Boreholes drilled deep into the substrate under the heart of Wellington City are revealing ancient geological secrets.
Engineers taking core samples more than 100 metres below the surface to ensure new buildings are earthquake resilient, have unearthed a treasure trove of information dating back past the last great ice age.
Evidence of the world's youngest and biggest supervolcanic eruption has also been clearly identified in a thick layer of volcanic ash at a depth of 52 metres under the construction site.
Layers of shells and fossilised marine life in the samples tell a story of when the sea covered much of the low-lying land after the ice age.
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Further down, large sections of sandy sedimentation reveal a time during the last glacial period when sea levels were around 125m below where they are now, and a landbridge connected what is now the North and South Islands.
These discoveries were coming out of a commercial project designed to give a new building on Whitmore St in the centre of the CBD the best possible foundations.
Environmental consultancy firm ENGEO were analysing samples taken from 10 bores on the site, and specialist Dr Matt Ryan soon realised there were telltale signs of internationally significant geological events.
Ryan has a Ph.D from Victoria University of Wellington in geology focusing on analysing paleo vegetation and saw in one of the samples what appeared to be evidence of supervolcanic ash.
"I opened up this box and saw that and said 'let's take a sample to the university'. I thought the material was from an eruption either from 345,000 years ago or the one 25,000 years ago," he said.
Using technology that fired lasers through a microscopic piece of glass (silica) taken from the samples, Dr Jenni Hopkins compared its chemical composition to other samples taken from around the country.
"Every eruption has its distinctive chemistry," Ryan said.
The results matched the massive Oruanui super eruption around 25,000 years ago that was so big it even had an impact on Antarctica.
It obliterated a previous lake in the central North Island creating what we now know as Lake Taupō.
A thick layer of ash and volcanic material covered much of the country and the Wellington region was not spared.
The bore samples found at the site were of extraordinary quality holding as much as 200mm of material from the eruption.
The discovery also helped date the sample at that depth, showing high rates of sedimentation that transformed the landscape in and around the harbour — around 2m every thousand years.
"Nowhere have I seen a higher sedimentation rate than this," Ryan said.
He knew the scientific knowledge being generated by the investigations was significant and made a presentation to New Zealand Geoscience Society last year, and as part of his research found out more about the city's more recent history.
A lot of land on which the central city stands today was reclaimed from the sea not long after Europeans settled in the area which was visible in the top 3-4m of the core samples.
It is incredible to consider the 100,000s of tonnes of fill material that was shifted with manual labour and hauled down to the waterfront by horse and cart to lay the foundations of the modern city.
Another important factor that shaped the area was the massive 1855 Wairarapa earthquake which lifted the region several metres and caused widespread slips.
A lot of the reclamation fill came from cleaning up those slips as well as roading and construction projects around the hills and valleys in the surrounding area.
Two significant Māori pa were located in the area – Kumutoto Pa to the west and Te Aro Pa to the south.
Further research in the cores may reveal evidence relating to tangata whenua settlement, Ryan said.
The cores could also be used as a tool to map out a local climate history of the area.
Analysis of the microfossils could show what type of plants were part of the ecosystem during different geological and climatological eras, which was one of Ryan's specialities.
Scientists used radiocarbon dating of organic material found in the samples to establish their age, and analysis of microfossils to find out what types of plants they were.
"We have this amazing record now, and we can continue using it to create an age model of the area.
"We might be able to see how the climate impacted the plants of Wellington over key time periods since the last ice age," Ryan said.
ENGEO engineer geologist Connor Chisnall had been analysing the cores from a geotechnical perspective to ensure they had as much knowledge as possible to engineer the required seismic performance of the new building.
"This core is significant because it goes all the way to the bedrock- it's rare to do an investigation this intensive," he said.
ENGEO senior geotechnical engineer Ayoub Riman said they had created a technical excellence group which was responsible for a lot of this groundbreaking work.
He said the additional knowledge added to the wider scientific research and strengthened their internal research and development.
"We go the extra mile which has benefits both for ENGEO and the wider community."
The new building was owned by Newcrest to be leased to the Bank of New Zealand.
The BNZ's previous premises was one of the 15 buildings damaged during the Kaikōura earthquake of 2016 and has been demolished.
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