Carbon isotopic disequilibrium in polar planktonic foraminifera and its impact on modern and Last Glacial Maximum reconstructions

Publication Type  Journal Article
Year of Publication  2000
Authors  Kohfeld, K. E.; Anderson, R. F.; Lynch-Stieglitz, J.
Journal Title  Paleoceanography
Volume  15
Issue  1
Pages  53-64
Journal Date  Feb
ISBN Number  0883-8305
Accession Number  ISI:000084956800006
Key Words  southern-ocean; atmospheric co2; atlantic sector; drake passage; deep; paleoceanography; fractionation; oxygen; water; equilibrium
Abstract  

Calibration studies of Neogloboquadrina pachyderma (sinistral: coiling, hereinafter referred to as N. pachyderma (s.)) in the South Atlantic raise questions about the interpretation of this species' carbon isotope composition as a paleoceanographic tracer of Southern Ocean surface nutrient contents. Carbon isotope disequilibrium between delta(13)C of total CO2 (TCO2) of seawater and delta(13)C of N. pachyderma (s.) increases systematically between 70 degrees and 40 degrees S. Several effects could cause the observed carbon isotope disequilibrium, including a combination of foraminiferal diet, calcification temperature, and carbonate ion chemistry. Combining these corrections to the delta(13)C of N. pachyderma (s.) improves the reconstruction of the delta(13)C of seawater TCO2 between 0 and 200 m in the modern South Atlantic. With these corrections applied, the delta(13)C for equilibrium calcite reconstructed from N. pachyderma (s.) at the Last Glacial Maximum is very similar to preindustrial values, suggesting that one cannot rule out the possibility that surface nutrient concentrations in the South Atlantic Ocean were comparable to today. However, the magnitude of the uncertainties associated with these corrections make it difficult to assess absolute paleonutrient concentrations with much confidence.

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