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

Abstract

The Eastern Tropical Pacific (ETP) is the largest oceanic source of carbon dioxide (CO 2 ) 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 CO 2 outgassing was largely suppressed by an enhanced biological pump during the La Niña events in the negative PDO phase. Plain Language SummaryWe 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 CO 2 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 CO 2 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. CO 2 outgassing was largely suppressed by an enhanced biological pump during the La Niña events in the negative PDO phase.

Identifying the biological control of the annual and multi-year variations in South Atlantic air–sea CO2 flux

“…Therefore, the trend in pCO 2 should be ∼ 0 with increasing pCO 2 (atm) . This could suggest a missing component within the SA-FNN to estimate pCO 2 (sw) , such as changes in the biological export efficiency (Kim et al, 2019), which could then suppress upwelling-induced CO 2 outgassing.…”

Identifying the biological control of the interannual and long-term variations in South Atlantic air-sea CO<sub>2</sub> flux

“…An increase in ΔpCO2 is counter intuitive for the Equatorial upwelling where ΔpCO2 would in theory decrease with increasing pCO2 (atm), assuming a constant deep water CO2 concentration. This could suggest a missing mechanism within the SA-FNN to estimate pCO2 (sw), such as changes in the biological export efficiency (Kim et al, 2019), which could suppress upwelling induced CO2…”

Interannual‐to‐Decadal Variations of Particulate Organic Carbon and the Contribution of Phytoplankton in the Tropical Pacific During 1981–2016: A Model Study

“…15 of 24 There have been studies revealing decadal changes in biogeochemical fields in the tropical Pacific resulting from the underlying physical changes due to the PDO phase shifts (Kim et al, 2019;Rodgers et al, 2008;Uz et al, 2017). For example, a modeling study showed a decrease in iron and chlorophyll levels in the upwelling region of the equatorial Pacific following the 1976/1977 PDO regime shift (Rodgers et al, 2008); Chavez et al (2003) reported that the Pacific Ocean changed from a cool "anchovy regime" to a warm "sardine regime" in the mid-1970s, then shifted back to an anchovy regime in the late 1990s.…”