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Equatorial Pacific Sea‐Air CO2 Exchange Modulated by Upper Ocean Circulation During the Last Deglaciation
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Equatorial Pacific Sea‐Air CO2 Exchange Modulated by Upper Ocean Circulation During the Last Deglaciation
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Equatorial Pacific Sea‐Air CO2 Exchange Modulated by Upper Ocean Circulation During the Last Deglaciation
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Journal Article
Equatorial Pacific Sea‐Air CO2 Exchange Modulated by Upper Ocean Circulation During the Last Deglaciation
2023
Overview
The eastern equatorial Pacific (EEP) is a source of atmospheric CO2 during the last deglaciation, but the associated oceanic dynamics in the broader low‐latitude Pacific is not fully understood. Here, we report 30,000‐year‐long surface and subsurface pCO2 records for the western equatorial Pacific (WEP), based on boron isotopes in two planktonic foraminiferal species from core MD10‐3340. Our results show that the WEP surface became a significant atmospheric CO2 sink despite that its subsurface waters were enriched by CO2 during the last deglaciation to early Holocene. Combined with EEP proxy data and model results, we suggest that a deglacial‐early Holocene zonal seesaw of sea‐air CO2 exchange across the equatorial Pacific led to a net CO2 outgassing much greater than the modern situation. This can be ascribed to strengthened Subtropical‐Tropical Circulation, resulting in stronger upper ocean stratification in the WEP concurrent with enhanced upwelling of CO2‐rich subsurface waters in the EEP. Plain Language Summary The modern equatorial Pacific is one of the main sources of CO2 released from the ocean into the atmosphere, with the strongest outgassing in the central‐eastern part and nearly neutral state in the western part. However, according to our boron isotope data of planktonic foraminifera, the western equatorial Pacific was turned into a significant atmospheric CO2 sink rather than a source between 16,000 and 7,000 years ago. At the same time, the eastern equatorial Pacific released more CO2, resulting in a zonal seesaw of sea‐air CO2 exchange across the equatorial Pacific that generally accelerated the CO2 outgassing and favored global warming. We further argue that these deglacial‐early Holocene CO2 changes were ascribed to the strengthened Subtropical‐Tropical Circulation, in which the accelerated eastward Equatorial Undercurrent leads to stronger upper ocean stratification in the WEP, and also converges seawater CO2 into the upper thermocline and finally releases at the surface EEP. This finding helps us understand and quantitatively assess future global carbon budget, particularly how the upper ocean circulation modulates the sea‐air CO2 flux in a manner similar to the past. Key Points Strengthened upper ocean stratification led to significant atmospheric CO2 sink in the western equatorial Pacific during ∼16–7 ka The zonal seesaw of sea‐air CO2 exchange in the equatorial Pacific resulted in a net CO2 outgassing much greater than the modern situation The sea‐air CO2 exchange across the equatorial Pacific was modulated by the Subtropical‐Tropical Circulation on orbital timescales
