Victoria University of Wellington PhD student Hana Ishii is working through a 100,000-year section of sediment cores from beneath the Whanganui and Rangitīkei riverbeds to uncover what ocean warmth and land conditions were like before humans inhabited Aotearoa.
Sediment cores from the Whanganui basin, which provide the “world’s best record of sea level change”, have been retrieved from storage and put into the lab again.
The cores will be studied by Victoria University of Wellington PhD student Hana Ishii to uncover what ocean warmth and land conditions were like before humans inhabited New Zealand.
Ishii said Whanganui’s geological record was “such a miracle” because of its high sedimentation and continuity.
“Whanganui is such a special place for scientists,” Ishii said.
“Before I came to New Zealand, the only thing I noticed was sheep, rugby, and Whanganui sediment cores.”
Geologist Dr Georgia Grant, who is co-supervising Ishii’s PhD, first worked on the cores during her PhD after they were drilled in 2014.
Grant processed as much as she could and sent samples to other scientists until her Marsden funding ran out and the cores were put into storage indefinitely.
She said they were nearly thrown out several times and accumulated mould and biological growth.
But that didn’t prevent them from containing what Grant describes as the world’s “best record of sea level change for the last five million years”.
Ishii’s other co-supervisor, Professor Robert McKay, director of the Victoria University Antarctic Research Centre, got the cogs turning again three years ago after meeting her at a conference.
She inquired about the future of the Siberia core, samples extracted from a site in Taihape in 2014.
McKay secured a Marsden grant for Ishii, who has a background in chemistry, to continue research into the cores.
“I was aware of [Grant’s] sea level record, and was wondering what other datasets would come from that sediment core,” Ishii said.
Ishii is now completing her three-year PhD, working through the 100,000-year section of sediment cores from Antarctica and Whanganui to uncover what ocean warmth and land conditions were like before humans inhabited Aotearoa.
Grant said the Whanganui cores were extracted from Whanganui and Rangitīkei riverbeds within the Whanganui Basin.
“Nobody else has demonstrated another site that gives us as much as Whanganui does,” Grant said.
The Whanganui Basin stretches 100km wide and 180km long, in an oval shape.
Half of it is onshore, and the other half is underwater.
At the eastern edge of the basin are the North Island’s axial ranges - the Ruahine, Kaweka and Kaimanawa.
It’s less than five million years old, which is young in geological terms.
For millions of years, the entire basin was underwater, accumulating layers of sediment and preserving a clear and gradual timeline from Taupō and the mountains to the west coast of Whanganui.
The shoreline essentially reached the mountains, Grant said.
When tectonic plate movements raised the land, those layers were first exposed along the riverbanks.
“You’re able to go down the Whanganui and Rangitīkei rivers and literally walk back through time,” Grant said.
Her broader research marries the Whanganui data with analyses of sediment cores drilled from land deep beneath Antarctica and Greenland’s polar layers to understand these ice sheets’ contribution to past sea-level change, and therefore better sketch out future rise.
The cores were drilled from carefully identified underground packages, or distinct, cohesive clumps, where they were closest to the surface.
Cores from Antarctica and Greenland present information from opposite sides of the Earth, and the Whanganui Basin cores supply a “combined signal”.
The polar cores revealed whether the Antarctic and Greenland ice sheets were there or not at certain time periods; however, they couldn’t reveal the volumes of those sheets, Grant said.
The Whanganui Basin was in a good position to record the combined influence of both ice sheets.
Grant said its global latitude, continuous records, and “incredibly” high-resolution evidence made it a particularly good spot to approximate what global sea levels were.
Facing the West Coast, fanned by huge westerlies, means the waves were stirring up deeper sediment.
“We call Whanganui the ‘Goldilocks zone.’ You need it to be enough that when you have sea level rise and fall, you don’t wipe out the record.”
Typical sediment cores extracted from the deep sea contained about 1cm of sediment for every 1000 years. Whanganui basin sediment contained about 2m of sediment for every 1000 years, Grant said.
“We’re able to look at a centennial scale ... fairly unheard of across the world.
“Having it all in one spot enables us to ask very different questions,” she said.
Theoretically, this same process has been applied before, but Grant’s PhD research quantified water depth based on how sandy or muddy sediments were, which enabled more confined research.
The significance of records previously found in Whanganui coastline cliffs was first recognised by Sir Charles Fleming in the 1940s, and described in his 1953 book The Geology of Wanganui Subdivision.
“He identified those, and then that work has continued. In academia, and maybe geology, you kind of inherit from what your supervisor and your supervisor’s supervisor had worked on previously.
“It’s like our own little whakapapa of academic research.
“I was supervised by Tim Naish, and Tim Naish was supervised by Peter Kamp.”
“Science is being defunded at the moment ... science takes a long time. It’s been over 100 years since Charles Fleming was born.
“It takes us a long time to work through these things,” Grant said
Earth Sciences New Zealand said the work would eventually contribute to New Zealand’s coastal planning and sea level projections.
Communities like Whanganui will use these to make decisions about land and infrastructure under a changing climate, they said.
Hana Ishii is looking to publish her PhD work in the coming months.
Dr Georgia Grant is a finalist for the early career researcher category of the 2026 Science New Zealand Awards and is preparing for a June workshop where the Antarctic core will be analysed.
Noam Mānuka Lazarus (Ngāti Whātua o Kaipara) is a multimedia journalist at the Whanganui Chronicle.