Quantification of diffuse CO2 flux emission from the crater lagoon of Vila Franca do Campo Islet (São Miguel, Azores)

Abstract

AbstractTo investigate unexplored areas and apply methodologies suitable for monitoring volcanic activity, a pioneer study was undertaken in the crater lagoon of Vila Franca do Campo Islet to identify possible CO2 degassing zones in the marine environment. This study was carried out using a floating accumulation chamber, which to the best of our knowledge, has only been applied to lacustrine environments.A survey was carried out in the crater lagoon in August 2017, resulting in a total of 143 CO2 flux measurements. Other parameters, such as water temperature, pH, electrical conductivity, and depth, were also determined at each measuring point, covering a study area of 0.02 km2. At one of the deepest sites with higher CO2 flux values, water samples were collected at depth for determination of stable isotopic composition.Water temperatures ranged between 21.0 and 27.0 °C, pH from 8.24 to 8.89, and electrical conductivity from 52.5 to 53.9 mS/cm. As expected, due to the marine water composition, sampled waters are of the Na-Cl type. CO2 flux varied between 0.581 and 1.0 g m-2 d-1 (average = 2.967 g m-2 d-1). A single CO2 population, characterized by low CO2 fluxes, points to a biogenic CO2 origin (δ13C = −9.62‰). The estimated value for the total CO2 emitted from the water surface is 0.05 t d-1. CO2 flux measurements were also compared with the structural features that cross the tuff cone, but no clear relation was observed with fractures/faults.This methodology should be applied to other coastal marine areas, especially where anomalous gas emissions have been reported.