Volcanic soil gas 4He/CO2 ratio: a useful geochemical tool for real-time eruption forecasting.

At many dormant volcanoes, magmatic gases are not channeled through preferential degassing routes as fumaroles and only percolate through the flanks of the volcano in a diffuse way. This type of volcanic gas emission provides valuable information, even though the soil matrix contains an important atmospheric component. This study aimed to demonstrate that chemical ratios such as He/CO2 in soil gases provide excellent information on the evolution of volcanic unrest episodes and help forecast the volcanic eruption onset. Before and during the occurrence of the October 2011-March 2012 submarine of El Hierro, Canary Islands, more than 8500 soil He analyses and diffuse CO2 emission measurements were performed. The results show that the soil He/CO2 emission ratio began increasing drastically one month before eruption onset, reaching the maximum value 10 days before. During the eruptive period, this ratio also showed a maximum value several days before the period with the highest magma emission rate. The He/CO2 ratio was also helpful in forecasting the eruption onset. We demonstrate that this tool can be applied in real-time during volcanic emergencies. Our results also encourage a reevaluation of the global He emission from the subaerial volcanism.

Diffuse CO2 degassing precursors of the January 2020 eruption of Taal volcano, Philippines.

On January 12, 2020, Taal volcano in Philippines erupted, 43 years after its previous eruption in 1977. This eruption was preceded by diffuse CO2 degassing precursory signals. Significant temporal variations in diffuse CO2 emission from Taal Main Crater Lake (TMLC) were observed across the ~ 12 years reaching high CO2 degassing rates in 2011 and 2017, with values typical of plume degassing volcanoes. In addition to these CO2 surveys at the TCML, soil CO2 efflux continuous monitoring was implemented at Taal volcano since 2016 and a clear increasing trend of the soil CO2 efflux in 2017 was observed. These geochemical observations are most simply explained by magma recharge to the system, and represent the earliest warning precursor signals to the January 2020 eruptive activity.

Dietary changes across time: Studying the indigenous period of La Gomera using δ13 C and δ15 N stable isotope analysis and radiocarbon dating.

OBJECTIVES: This article presents new radiocarbon and isotopic data to provide further information about the diet of the indigenous population of La Gomera and its possible changes across time. MATERIALS AND METHODS: δ13 C and δ15 N of 53 human and 19 faunal samples from different sites on the island have been obtained and analyzed. Of these, 52 have been radiocarbon dated to provide insight on chronological changes. RESULTS: Human dates range from the 3rd to 15th centuries AD, while faunal dates range from the 1st to 17th centuries AD. Stable carbon and nitrogen values are significantly different between the human and goat samples and have also a trophic increase of 3.4‰. Although male and female δ15 N data are not significantly different. Both δ13 C and δ15 N values of both human and animal samples tend to discretely decrease over time. DISCUSSION: Radiocarbon dates from humans correlate with other dates obtained in the rest of the archipelago. Animal radiocarbon dates generally coincide except for one date, which requires further study. Isotopic δ15 N data suggest a mix of marine and terrestrial protein consumption in humans, the latter being more abundant given the seasonality of the first. δ13 C data also suggest a possible mixed diet in humans, with a predominance of C3 plants, like Hordeum vulgare, the only grain found in archaeological sites so far. Variations of both δ13 C and δ15 N over time suggest a slight modification on the diet, which could be related to environmental changes.