A comparative study of the northwest Africa and eastern equatorial Pacific upwelling zones as sources of CO₂ during glacial periods based on boron isotope paleo‐pH estimation

Abstract

Paleo‐pH reconstructions based on boron isotopic composition of foraminifera have been used to estimate glacial‐interglacial changes in surface ocean p CO 2 of the northwest African upwelling zone. On comparison with a similar study for the eastern equatorial Pacific upwelling zone, it can be concluded that the two major upwelling zones acted quite differently during the glacial periods as compared to today. While the pH of the surface ocean off northwest Africa was 0.2±0.07 units higher during the glacial period compared to that during Holocene, there was no significant glacial‐interglacial change in the surface ocean pH in the eastern equatorial Pacific. Carbonate chemistry reconstructions based on the estimated pH changes suggest that the ocean‐atmosphere p CO 2 gradient off northwest Africa was lower by at least 70±40 µatm during glacial periods compared to during the Holocene. In contrast, the ocean‐atmosphere p CO 2 gradient in the eastern equatorial Pacific was higher by at least 80±40 µatm during glacial periods as compared to during the Holocene. Hence the eastern equatorial Pacific upwelling system was a significantly larger source of CO 2 to the atmosphere, while the one off northwest Africa was a significantly smaller source of CO 2 during the last glacial period. The p CO 2 reconstructions further indicate that in spite of higher glacial productivity compared to during the Holocene, neither of the two areas became a sink of CO 2 .