Ocean-atmosphere carbon exchange: impact of the “biological pump” in the Atlantic equatorial upwelling belt over the last 330,000 years

Abstract

Past atmospheric pCO 2 variations largely resulted from a global integration of the changing CO 2 input and output from the various parts of the ocean. These changes of the local CO 2 pressure in surface water, which are documented in carbon isotopic records of marine TOC, resulted from two opposing processes: (1) CO 2 extraction by phytoplankton production, i.e. the “biological pump”, and (2) the thermodynamic gas exchange linked to upwelling subsurface water. In the East Atlantic equatorial upwelling belt the local pCO 2 record shows strong 100 ka excentricity and 40 ka obliquity signals in the spectral domain which generally paralleled the atmospheric CO 2 variations over the last 220,000 years as recorded in icecores (Jouzel et al., 1993). Since the amplitudes of the glacial-to-interglacial shifts in local pCO 2 clearly exceeded the atmospheric CO 2 changes and were closely linked to changes in local nutrient budgets and primary production ( r = 0.8), we infer that the “biological pump” strongly controlled the local carbon transfer between the atmosphere and the ocean in equatorial upwelling zones. This contrasts to modelling results that suggest physical CO 2 solubility and upwelling related outgassing as dominant processes which, however, were inversely related to local seawater pCO 2. In the precessional frequency band (19/23 ka), the atmospheric pCO 2 was governed by low-latitude climate components, varying in phase with local pCO 2. In constrast, in the obliquity band (40 ka) the atmospheric CO 2 led the local seawater pCO 2 by more than 4000 years and thus was controlled by climate components outside the equator, i.e., in high latitudes as expected by models.