OCEAN-ATMOSPHERE CARBON EXCHANGE - IMPACT OF THE BIOLOGICAL PUMP IN THE ATLANTIC EQUATORIAL UPWELLING BELT OVER THE LAST 330,000 YEARS

Past atmospheric pCO2 variations largely resulted from a global integration of the changing CO2 input and output from the various parts of the ocean. These changes of the local CO2 Pressure in surface water, which are documented in carbon isotopic records of marine TOC, resulted from two opposing processes: (1) CO2 extraction by phytoplankton production, i.e. the ''biological pump'', and (2) the thermodynamic ps exchange linked to upwelling subsurface water. In the East Atlantic equatorial upwelling belt the local pCO2 record shows strong 100 ka excentricity and 40 ka obliquity signals in the spectral domain which generally paralleled the atmospheric CO2 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 pCO2 clearly exceeded the atmospheric CO2 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 CO2 solubility and upwelling related outgassing as dominant processes which, however, were inversely related to local seawater pCO2. In the precessional frequency band (19/23 ka), the atmospheric pCO2 was governed by low-latitude climate components, varying in phase with the local pCO2. In contrast, in the obliquity band (40 ka) the atmospheric CO2 led the local seawater pCO2 by more than 4000 years and thus was controlled by climate components outside the equator, i.e., in high latitudes as expected by models.