| Summary: | Analysis of planktonic and benthic foraminifers’ accumulation rates from the Iberian margin reveal a substantial change in the biogenic ocean-atmosphere CO2 exchange during the Mid-Pleistocene Transition (MPT; ~800–650 ka from present). Such changes resulted from the major reorganisations in both surface and deep-water circulation that occurred in the North Atlantic at the time, and affected the behaviour of this upwelling region as a CO2 uptake/release area during climate cycles before and after the MPT. During Marine Isotope Stages (MIS) 21-MIS 20 (860–780 ka), this margin acted mostly as an uptake area during interglacials and early glacials. During glacial maxima and terminations it would be neutral because, although surface production and export were very low, carbon storage occurred at the seafloor. During MIS 15-MIS 14 (630–520 ka), the pattern was the opposite, and the Iberian margin worked as a neutral, or as a source area during most interglacials, while during glacials it acted as an important uptake area. Present findings support the idea that glacial/interglacial atmospheric pCO2 oscillations are partly driven by alterations in the meridional overturning circulation that results in substantial variations of the biological pump, and carbon sequestration rate, in some high-productivity regions.
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