Carbon isotopes and habitat of polar planktic foraminifera in the Okhotsk Sea: the ‘carbonate ion effect’ under natural conditions

The upper 500 or 1000 m of the water column in the Okhotsk Sea was sampled for living planktic foraminifera. The polar species Neogloboquadrina pachyderma (sinistral) strongly dominates the foraminiferal assemblage; the subpolar to temperate species Globigerina bulloides accounts for 10–25%. Other species account for up to 3% only. The shell δ18O and δ13C values of the species N. pachyderma (sin.) are compared to water δ18O values and δ13C values of dissolved inorganic carbon (DIC). The strong gradient in δ18O composition and temperature in the upper water column is reflected in the δ18O of N. pachyderma (sin.). Relative to the values expected for inorganic calcite precipitated under equilibrium conditions N. pachyderma (sin.) displays a vital effect of about 1‰ in δ18O. The δ13C composition of N. pachyderma (sin.) is about constant with water depth and the reflection of δ13CDIC in the foraminiferal shell seems to be masked by other effects. Most foraminifera are found above or slightly below the thermocline and can be assumed to calcify in the upper 200 m of the water column. The gradient of δ13CDIC extends well below this depth, therefore the lack of correlation can partly be attributed to this fact. The remaining discrepancy between δ13C of N. pachyderma (sin.) and δ13CDIC correlates with the carbonate ion concentration in the water column. This leads to the conclusion that the ‘carbonate ion effect’ (CIE), which has been derived from culturing experiments for other species [Spero et al. (1997) Nature 390, 497–500], is found here under natural conditions. When the magnitude of the CIE derived for G. bulloides is applied to N. pachyderma (sin.), CIE-corrected δ13C of N. pachyderma (sin.) is a direct reflection of δ13CDIC in the water column with a constant offset of 1.2‰.