Combined radiocarbon and stable carbon isotopes in tree rings to quantify past volcanic CO2 emissions

Volcanoes are a major hazard to vulnerable communities and fragile ecosystems but understanding their behaviour can be difficult. The behaviour and hazards of a particular volcano can sometimes be forecast based on its eruptive history. However, recorded volcanic histories are typically short and contain gaps. Not all eruptions leave geological deposits behind, and many pre-eruptive phases include gas emissions which are infrequently measured and are difficult to trace over decadal or centennial timescales.

Trees record environmental change as variations in the isotopic composition of their wood. Studies have shown that anthropogenic SO₂ pollution can impair leaf stomata function, causing a stable carbon fractionation of ~ 3‰. In a recent study by D’Arcy et al (in review), concentrated volcanic SO₂ emissions were inferred to have a similar effect, but tree uptake of volcanic CO2 complicates this interpretation. Volcanic CO₂ has an isotopic signature that is enriched in ¹³C relative to atmospheric CO₂, which will also affect the isotopic composition of the tree rings, making quantitative estimates at the scale of past degassing difficult, based on ¹³C measurements alone. In this study, we propose to combine radiocarbon analysis with stable carbon isotope analysis, which will allow us to distinguish between (a) carbon fractionation due to the SO₂ damage to the leaves and (b) the added volcanic CO₂ signature from the volcano.

Volcanic CO₂ contains no ¹⁴C, while atmospheric CO₂ has relatively high amounts of ¹⁴C derived from natural cosmogenic nuclide production and atomic weapons testing, and therefore ¹⁴C is a very sensitive measure of plant uptake of volcanic CO₂. Radiocarbon in tree rings has been used to infer magmatic CO₂ emissions in geothermally active regions, and we aim to use these methods to similarly calculate magmatic CO₂ uptake in tree rings two volcanic regions. Then, we will apply a stable carbon isotopic approach to estimate past volcanic SO₂ degassing, with any influence of volcanic CO₂ accounted for. In this way, we can reconstruct the degassing history of a volcano.