Carbon dioxide emissions and fate from Vailulu’u seamount mapped using SAGE, a new in situ optical sensor

Vailulu’u seamount, located at the eastern extent of the Samoan hotspot chain, is an active deep-sea volcano with strong hydrothermalism and a recent history of eruptive episodes. Since 1999, a cone more than 500 m in diameter and with height greater than 300 m has formed within the caldera, while hydrothermal vent fields are known throughout the basin. These include regions of diffuse venting, substantial microbial mat formation, and chimneys emitting buoyant plumes of gas bubbles that are dominated by carbon dioxide. Measurements of turbidity and chemical enrichment in the caldera, temperature anomalies, and tracer dye studies have all previously been used to estimate and model the hydrothermal exports from this active volcano. Biological observations across the caldera – including the prevalence of carcasses in the so-called “Moat of Death” surrounding the Nafanua cone – have been used to infer the impact and fate of these volcanic emissions. However, it has previously been challenging to acquire high resolution in situ measurements of the discharged carbon dioxide at Vailulu’u, with the gas composition and distribution instead being determined primarily from discrete samples.

We present here a comprehensive spatiotemporal study of the carbon dioxide discharge from across the Vailulu’u seamount, acquired using SAGE, a new in situ carbon dioxide sensor. SAGE – the Sensor for Aqueous Gases in the Environment – was developed at Woods Hole Oceanographic Institution to quantify the concentration of dissolved gases in the deep sea. Dissolved gases are extracted across a gas-permeable membrane and into a hollow core optic fiber, which acts as an absorption cell for infrared absorption spectroscopy. SAGE has both a low detection limit (ca. 10 ppm) and a fast time response time (1-5 minutes). During a cruise on EV Nautilus in September 2024, SAGE was deployed on ROV Hercules and AUV Sentry to acquire high-resolution spatiotemporal maps of the carbon dioxide discharges from Vailulu’u. Comprehensive analysis of the hydrothermal exports from the vent fields on the rim allow estimation of the fate and flux of these discharges. Surveys from the caldera provide chemical context for the so-called “Moat of Death” and evidence for a significant diffuse venting across the Nafanua cone. To the best of our knowledge, this is the first study using in situ sensing of carbon dioxide at Vailulu’u.