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Have prehistoric eruptions being lying about their age?

Volcanic lies: Have prehistoric eruptions being lying about their age?

Some volcanic eruptions, including the most recent large Taupo eruption in New Zealand, may be lying about their age, according to new University of Canterbury research.

A study by researchers from the University of Canterbury (UC) and a UC graduate lecturing in geoscience at the University of Melbourne, Australia, shows that carbon dioxide gas emitted from deep below volcanoes can be redistributed by groundwater systems and bias radiocarbon dates of prehistoric eruptions.

The study presents evidence that the Taupo eruption occurred between 40 years and 200 years later than the present date of 232 CE. Dates for eruptions of volcanoes elsewhere may also be too old. In exposing these explosive volcanic lies, the UC researchers explore how volcanoes can lie about their age, and what it means for the world.

Accurate dating of prehistoric eruptions is important as it allows them to be correlated with other records, such as historical and climatic events, allowing a better understanding of the linkages between volcanism and the natural and cultural environment.

Wiggle match dating
The current preferred method for eruption dating, “wiggle match dating” relates sequences of dates from tree rings to a record of natural variation in the amount of carbon-14 in the atmosphere through time. In principle this can be used to pinpoint when an eruption killed the tree. However, it is not valid if gas from the volcano is affecting the tree’s version of the wiggle.

The researchers re-analysed more than 40 radiocarbon dates on plant remains from beneath and above the volcanic ash layer from the Taupo eruption in the First Millennium. They found that the oldest dates were closest to the volcano vent, and dates became progressively younger the farther away they were. The geographic pattern suggests that carbon in plants and plant remains such as peat within 60km from the vent give spuriously old dates.

Discovering the real timescale
Integrating information from biology and volcanology, the team found that the ratio of carbon-13 to carbon-12 (the two stable isotopes of carbon) in the radiocarbon date samples and in the water of Lake Taupo and the Waikato River are consistent with volcanic carbon dioxide injected into the groundwater from a developing magma body. This carbon dioxide leaks from the groundwater into the atmosphere and is taken up by photosynthesizing plants. The carbon in that volcanic carbon dioxide lacks carbon-14 and so dilutes the carbon-14 obtained from the atmosphere, making the radiocarbon date measured on the wood appear older than it should be.

The tree wood records where the carbon was coming from over decades and centuries, and provides a real timescale for the changes in the rings, according to one of the researchers, UC volcanologist Associate Professor Ben Kennedy.

“The crucial link between what is happening beneath the volcano and the record in the trees is the groundwater, which conveys the carbon dioxide across the landscape,” Dr Kennedy says.

Forecasting future eruptions
Published in the prestigious British journal Nature Communications, the study also suggests the possibility of forecasting future eruptions. The researchers showed that the radiocarbon dates and isotope data, which underpin the presently accepted wiggle match age for the eruption, plateaued. Effectively, the age of the outer ring of the dated tree, which was knocked down and killed by the eruption, had ceased to follow the tree’s growth several decades before the eruption.

UC alumnus Dr Brendan Duffy, a lecturer in Applied Geoscience at the University of Melbourne who earned his BSc, MSc and PhD at UC, says that “radiocarbon wiggle match date series for other major eruptions, including at Rabaul in Papua New Guinea and Baitoushan on the North Korean border with China, showed similar patterns”.

Apart from supporting the Taupo study, and indicating that there may also be problems with the dates of these volcanic eruptions, the similarities imply that measurements of carbon isotopes in 200-300 annual rings can track changes in the carbon source used by trees growing near a volcano, providing a potential method of forecasting future large eruptions.

UC Adjunct Professor Richard Holdaway identified the relationships between radiocarbon ages, carbon isotopic values and eruptions.

“We found that long-term monitoring of the carbon isotopes in trees growing near a volcano can show trends which reveal the state of the magma body beneath. These may potentially allow long-term forecasting of major eruptions. What people will do with a forecast for an event that may occur several decades later is anyone’s guess,” he says.

“The situation is not unlike that involved with forecasts of future movement on major faults such as the San Andreas and New Zealand alpine faults. The difference is that for an eruption, continued monitoring can refine the forecast whereas predictions of fault movement are statistical, based on estimates of how far fault movements are apart in time and when the fault last moved. And, of course, the eruption of super-volcanos like Yellowstone and Taupo would have global effects that would dwarf the effects of almost any earthquake.”

Dr Kennedy adds: “If the Taupo date is wrong, then our current understanding of any association between large eruptions and changes in Earth’s climate may need to be re-examined.”

Holdaway, Richard N., Duffy, Brendan, and Kennedy, Ben. Evidence for magmatic carbon bias in 14C dating of the Taupo and other major eruptions (2018)Nature Communications.

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