PHOTOSYNTHETIC FRACTIONATION OF C-13 AND CONCENTRATIONS OF DISSOLVED CO2 IN THE CENTRAL EQUATORIAL PACIFIC DURING THE LAST 255,000 YEARS

Carbon isotopically based estimates of CO2 levels have been generated from a record of the photosynthetic fractionation of C-13 (=epsilon(p)) in a central equatorial Pacific sediment core that spans the last similar to 255 ka. Contents of C-13 in phytoplanktonic biomass were determined by analysis of C-37 alkadienones. These compounds are exclusive products of Prymnesiophyte algae which at present grow most abundantly at depths of 70-90 m in the central equatorial Pacific. A record of the isotopic compostion of dissolved CO2 was constructed from isotopic analyses of the planktonic foraminifera Neogloboquadrina dutertrei, which calcifies at 70-90 m in the same region. Values of epsilon(p), derived by comparison of the organic and inorganic delta values, were transformed to yield concentrations of dissolved CO2 (=c(e)) based on a new, site-specific calibration of the relationship between epsilon(p) and c(e). The calibration was based on reassessment of existing epsilon(p) versus c(e) data, which support a physiologically based model in which epsilon(p) is inversely related to c(e). Values of PCO2, the partial pressure of CO2 that would be in equilibrium with the estimated concentrations of dissolved CO2, were calculated using Henry's law and the temperature determined from the alkenone-unsaturation index U-37(K'). Uncertainties in these values arise mainly from uncertainties about the appropriateness (particularly over time) of the site-specific relationship between epsilon(p) and 1/c(e). These are discussed in detail and it is concluded that the observed record of epsilon(p) most probably reflects significant variations in Delta pCO(2), the ocean-atmosphere disequilibrium, which appears to have ranged from similar to 110 mu atm during glacial intervals (ocean > atmosphere) to similar to 60 mu atm during interglacials. Fluxes of CO2 to the atmosphere would thus have been significantly larger during glacial intervals. If this were characteristic of large areas of the equatorial Pacific, then greater glacial sinks for the equatorially evaded CO2 must have existed elsewhere. Statistical analysis of air-sea pCO(2) differences and other parameters revealed significant (p < 0.01) inverse correlations of Delta pCO(2) with sea surface temperature and with the mass accumulation rate of opal. The former suggests response to the strength of upwelling, the latter may indicate either drawdown of CO2 by siliceous phytoplankton or variation of [CO2]/[Si(OH)(4)] ratios in upwelling waters.