Carbon and oxygen isotopic composition of planktonic foraminifera from laboratory culture, plankton tows and Recent sediment; implications for the reconstruction of paleoclimatic conditions and of the global carbon cycle

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Abstract
Oxygen and carbon isotopic analyses were performed on three planktonic foraminiferal species, the spinose Orbulina universa and the non-spinose Globorotalia menardii and Neogloboquadrina dutertrei as well as on O. universa cultured in laboratory under different temperature regimes. In tropical and subtropical waters (22-30.degree. C), the .delta.18O of these three species varies with surface temperature, and the slope of the isotope-temperature regression line is similar to that of Epstein and others (1953) for marine molluscs and to that of O''Neil and others (1969) for carbonates deposited in isotopic equilibrium. However, foraminiferal .delta.18O values are slightly lighter than those predicted by assuming that equilibrium conditions are realized during shell formation. This result is confirmed by the analysis of the laboratory culture O. universa, of which the .delta.18O values do not differ significantly from the plankton tow specimens. Comparisons between plankton tows taken within and below the mixed layer show that O. universa has a similar .delta.18O in both sets, the low value of which suggests that it secretes its shell only within the mixed layer. By contrast, G. menardii and N. dutertrei exhibit higher .delta.18O values below than within the mixed layer because they continue to secrete calcite below the thermocline. Comparison of living specimens with Recent ones from the surface sediment shows that the fossils are generally enriched in 18O as compared to the living foraminifera. This isotopic enrichment is explained by an important deposition of calcite in deep cold waters, below the thermocline, during the terminal stages of the foraminiferal life cycle. For O. universa, our results and laboratory observations suggest that this additional calcite is secreted only during gametogenesis, when the organisms settle freely from the euphotic zone. The .delta.13C values are strongly size-dependent in living G. menardii and N. dutertrei specimens collected within the mixed layer; that is, the smaller the size, the lighter the .delta.13C. When strictly limited and narrow size fractions are analyzed, .delta.13C variations reflect those of the surficial waters. The amplitude of the size effect decreases during ontogeny so that it is much smaller in the fossil shells of N. dutertrei from the sea bed than in the living specimens and it even disappears in fossil G. menardii. In specimens collected below the mixed layer or found on the sea bed, the .delta.13C is lower than that of surficial water samples. This improverishment is correlated with a .delta.18O increase suggesting that these species calcify at depth and that fossil samples mainly reflect conditions prevailing below 100-meter depth.

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