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. Plain Language Summary: We examined the composition and flux of particles sinking to the deep Eastern Tropical Pacific from 2003 to 2013. This region is known as the largest oceanic source of CO2 to the atmosphere. We observed that the flux of particulate organic and inorganic carbon to a depth of 4,950 m increased as the climate variability called the "Pacific decadal oscillation" shifted from a positive to negative phase (i.e., more La Niña events) in 2008. The biological pump efficiency (i.e., how efficiently biological production of organic matter is transported to the deep ocean) also increased during La Niña years. To investigate how biological production affects the CO2 exchange with the atmosphere, we estimated the export of carbon from the surface layer based on our flux data at 4,950 m and the current understanding of carbon flux attenuation with depth. CO2 outgassing was largely suppressed by an enhanced biological pump during the La Niña events in the negative PDO phase. Key Points: Fluxes of biogenic particles were much higher during La Niña years in the deep Eastern Tropical PacificBiogenic carbon export was estimated based on the observed particle fluxes and empirical equations for the vertical attenuation of carbon fluxCO2 outgassing was considerably suppressed by an enhanced biological pump during La Niña years [ABSTRACT FROM AUTHOR]