Solving the mystery of NZ's volcanic CO2 emissions

The geothermal field that gives Rotorua its steaming, bubbling, tourist-pulling wonders pumps about 1000 tonnes of carbon dioxide (CO2) into the atmosphere each day. Photo / File

Every day, the huge geothermal field that gives Rotorua its steaming, bubbling, tourist-pulling wonders pumps about 1000 tonnes of carbon dioxide (CO2) into the atmosphere.

Farther south, the Rotokawa and Ohaaki fields together put out a daily average of more than 700 tonnes.

By comparison, the average car produces several tonnes - over the space of a year.

Despite this huge volume of natural greenhouse gases constantly being belched up from the Central North Island, just how it physically travels from earth to air remains far from clear.

The current theory is that intrusions of magma, less than 5km below the surface, send the gases up through the ground.

But Dr Isabelle Chambefort said this notion has been coming under increasing scrutiny in New Zealand, for striking reasons.

An obvious one is that there are no tell-tale signatures of such an effect happening within our rock.

As it happens, the GNS Science geothermal geologist has her own hypothesis.

Chambefort suspected these gases actually originated much deeper, from eight or more kilometres below the ground, and found their way to the surface through weaknesses in the Earth's crust.

Chambefort's theory could up-end a 30-year-old model, but confirming it wouldn't be simple.

In a new study, she and a team of Kiwi and international scientists would measure CO2, along with inert "noble" gases such as helium and argon, to build a picture of gas flow up from the Earth's mantle and lower crust.

By drawing on a combination of measurements at the surface and isotope analysis of gases, they'd be able to trace not just their point of origin, but also how they travelled upward and interacted with each other.

In a separate part of the project, scientists will take measurements over three to four months of each year to build a detailed profile of emissions stemming from New Zealand's volcanic environment.

Chambefort said the success of the study, supported with a $958,000 grant from the Marsden Fund, strongly relied upon field work – and it was here that much could go wrong.

Another challenge was the fact it would involve looking at gas isotopes trapped in magmatic minerals - something that hadn't ever been attempted with rhyolite, the dominant rock of the Central North Island.

In fact, Chambefort said, no one had attempted such a gas survey at that scale and in a populated area, except likely in Yellowstone National Park in the US.

While answering a fundamental question facing science, her project stood to improve current volcanic monitoring, and potentially, also renewable energy technologies.

"We have a 20- to 30-year vision in which deep geothermal will be driving our energy needs – and this research is a step toward understanding of this resource."