Suppressed CO₂ Outgassing by an Enhanced Biological Pump in the Eastern Tropical Pacific

The Eastern Tropical Pacific (ETP) is the largest oceanic source of carbon dioxide (CO₂) to the atmosphere. Sinking particle fluxes at a depth of 4,950 m (50 m above the seafloor) in the ETP were monitored from 2003 to 2013, during which the Pacific decadal oscillation (PDO) shifted from a positive to negative phase. We show a disproportionate increase in the efficiency of the biological pump in this region relative to the increase in primary production that occurred during La Niña years following the shift of the PDO in 2008. Biogenic carbon export from the surface mixed layer was estimated from the observed particulate organic carbon and inorganic carbon fluxes at a depth of 4,950 m and from empirical equations of the vertical attenuation of carbon flux. Enhanced biological carbon export accounted for 2.3–5.5 mol C m^–2 year^–1 during the La Niña events. Despite a large uncertainty associated with these estimates, we propose that CO₂ outgassing was largely suppressed by an enhanced biological pump during the La Niña events in the negative PDO phase.