Gas emission from diffuse degassing structures (DDS) of the Cameroon volcanic line (CVL): Implications for the prevention of CO2-related hazards

Abstract

In the mid-1980s, lakes Nyos and Monoun violently released massive gas, mainly magmatic CO2 killing about 1800 people. Subsequent geochemical surveys and social studies indicate that lakes Nyos and Monoun event is cyclic in nature and may occur anywhere in the about 37 other volcanic lakes located in the corridor of the Cameroon volcanic line (CVL). This potential threat motivated us to check if, alike Nyos and Monoun, the internal dynamic of the other lakes is also controlled by inputs of deep-seated-derived CO2 and attempt to measure and provide comprehensive insights on the passive gas emission along the CVL. This knowledge shall contribute to the prevention of volcanic lake-related hazards in Cameroon and the refinement of the Global Carbon Cycle. We used in situ fixation and dry gas phase sampling methods to determine CO2 origin and the concentration, and the accumulation chamber technique to measure diffuse CO2 emission from nine lakes and on soil at Nyos Valley and Mount Manenguba Caldera. The results suggest that, although in minor concentrations (compared to Nyos and Monoun), ranging from 0.56mmolkg−1 to 8.75mmolkg−1, the bottom waters of some lakes also contain measurable magmatic CO2 with δ13C varies from −4.42‰ to −9.16‰ vs. PDB. That finding implies that, under certain circumstances, e.g. increase to volcanic and/or tectonic activities along the CVL, the concerned lakes could develop a Nyos-type behavioural scheme. The diffuse gas emission results indicate that the nine surveyed lakes release approximately 3.69±0.37ktkm−2yr−1 of CO2 to the atmosphere; extrapolation to the approximately 39 volcanic lakes located on the CVL yields an approximate CO2 output of 27.37±0.5ktkm−2yr−1, representing 0.023% of the global CO2 output from volcanic lakes. In addition to the precedent value, the gas removal operation in lakes Nyos and Monoun released approximately 2.52±0.46×108molkm−2yr−1 CO2 to the atmosphere from January 2001 to March 2013, more than double the per-area CO2 released by the Yellowstone volcanic system. The CO2 emission from soil was estimated to be 4.57±1.30ktkm−2yr−1; the soil gas geochemistry of the Mount Manenguba Caldera also indicates a dominant magma-derived CO2 (mean δ13C=−8.6‰ vs. VPDB).