Photosynthetic fractionation of 13C and concentrations of dissolved CO2 in the central equatorial Pacific during the last 255,000 years

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Abstract
Carbon isotopically based estimates of CO2 levels have been generated from a record of the photosynthetic fractionation of 13C (≡ εp) in a central equatorial Pacific sediment core that spans the last ∼255 ka. Contents of 13C in phytoplanktonic biomass were determined by analysis of C37 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 εp, derived by comparison of the organic and inorganic δ values, were transformed to yield concentrations of dissolved CO2 (≡ ce) based on a new, site-specific calibration of the relationship between εp and ce. The calibration was based on reassessment of existing εp versus ce data, which support a physiologically based model in which εp is inversely related to ce. 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 U37K′. Uncertainties in these values arise mainly from uncertainties about the appropriateness (particularly over time) of the site-specific relationship between εp and 1/ce. These are discussed in detail and it is concluded that the observed record of εp most probably reflects significant variations in ΔpCO2, the ocean-atmosphere disequilibrium, which appears to have ranged from ∼110 µatm during glacial intervals (ocean > atmosphere) to ∼60 µatm 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 pCO2 differences and other parameters revealed significant (p < 0.01) inverse correlations of ΔpCO2 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.

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