Estimating mixed gas fluxes with geochemically informed broadband acoustic methods: What the flux?

Abstract

Over the past few years, improvements in broadband split-beam echosounders have made it possible to obtain precise measurements of bubble-size distribution and density of gas seeps. Here, we model acoustic volume backscattering of a group of bubbles with a multi-modal size distribution that we matched to real-time acoustical measurement over a gas seep. The results are profiles of bubble size distribution descriptors (e.g., distribution type and parameters), density and ratio of the various components (i.e., gas composition). To test the ability of this methodology to differentiate gas flux composition and estimate gas flux magnitudes, we applied it in a semi-automated fashion on broadband data (12 to 250 kHz) acquired in the Bay of Plenty (NZ) over mixed gas seeps (e.g., CO2, CH4). Model outputs were compared with real-time dissolved gas measurements to constrain gas composition, magnitude, and ground-truth methodologies. The method can be applied to a variety of marine seeps to produce regional flux estimations, improving oceanic carbon budgets and our understanding of downstream feedbacks, such as localized deoxygenation or ocean acidification.