Upwelling enhances cycling and emissions of very short-lived halocarbons: Biogeochemical mechanisms and climate feedbacks

Abstract

Very short-lived (VSL) halocarbons have significant impacts on global atmospheric chemistry and climate change. However, the complex effects of different marine environments on the budgets of VSL halocarbons with primarily marine origin such as dibromomethane (CH₂Br₂), tribromomethane (CHBr₃), and methyl iodide (CH₃I) have led to significant uncertainty in estimating their oceanic emissions. To elucidate the key processes and mechanisms controlling VSL halocarbons in seawater and atmosphere in upwelling regions where are typically hot spots for VSL halocarbons emissions, field investigation and ship-based incubation experiments were conducted in the Southeastern Atlantic Ocean (SAO), including the Benguela Upwelling System, from February to March 2023. Higher VSL halocarbons concentrations in seawater and atmosphere were observed in the upwelling region compared to the non-upwelling region. Upwelling transported abundant nutrients into the euphotic layer, promoting primary production and stimulating rapid VSL halocarbons biological production. Furthermore, by evaluating the source-sink rates of VSL halocarbons within the mixed layer, we identified the vertical exchange is a potential source in most cases which transporting substantial amounts of these gases from thermocline to the mixed layer—a process enhanced by upwelling. These combined effects led to rapid accumulation of VSL halocarbons in surface waters, ultimately driving 38.8–215.7 % greater sea-air fluxes of CH₂Br₂, CHBr₃ and CH₃I relative to non-upwelling regions, indicating that upwelling drives greater emissions of marine VSL halocarbons to the atmosphere. Our research advances the understanding of the VSL halocarbons source-sink processes and emission in upwelling systems.