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

The upper 500 or 1000 in 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 delta(18)O and delta(13)C values of the species N. pachyderma (sin.) are compared to water delta(18)O values and delta(13)C values of dissolved inorganic carbon (DIC). The strong gradient in delta(18)O composition and temperature in the upper water column is reflected in the delta(18)O 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 I parts per thousand in delta(18)O. The delta(13)C composition of N. pachyderma (sin.) is about constant with water depth and the reflection of delta(13)C(DIC) 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 in of the water column. The gradient of delta(13)C(DIC) extends well below this depth, therefore the lack of correlation can partly be attributed to this fact. The remaining discrepancy between delta(13)C of N. pachyderma (sin.) and delta(13)C(DIC) 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 delta(13)C of N. pachyderma (sin.) is a direct reflection of delta(13)C(DIC) in the water column with a constant offset of 1.2parts per thousand. (C) 2002 Elsevier Science B.V. All rights reserved.