The calcification process of planktic foraminifers and its implication for the incorporation of paleo-oceanographic signals to their shells
Zusammenfassung der Projektergebnisse
The application of the planktic foraminiferal shell geochemistry in paleo-oceanography is strongly hindered by the lack of knowledge on the ecology and the depth habitat of species. We developed a sophisticated calcification model for foraminiferal shell growth and confirmed the validity of the model by applying it to field observation. Our approach is highly innovative as, for the first fime, the depth-integrated growth pattems of different species of foraminifera are precisely quantified, a phenomenon that cannot be simulated in laboratory culture experiments. We can provide direct information on the specific rate of shell mass increase with depth as well as on the depth at which shell growth ceases. Continuous distribution of apparent calcification depths for these species throughout the water column suggests that a reconstruction of thermocline profiles based on a multispecies approach may yield higher-resolution thermocline reconstructions than have been previously possible in paleo-oceanographic studies. We envision multiple regressions using a matrix inversion approach, in which a multipoint thermocline profile may be reconstructed rather than a simple thermocline depth. A potential perspectives for application of our calcification model might also be the combination of foraminiferal ecosystem-model with our depth-habitat module. Laser ablation and secondary ion mass spectrometry significantly offer advantages compared to conventional bulk analyses through its potential to derive the range and variability of seawater temperatures, in addition to a simple mean value, from a population of fossil tests. However, the trace element/Ca approach poses new questions and problems. Both species show large internal variability of TE/Ca in test chamber wall crosssections and also a large heterogeneity within the shell as a whole, i.e. between successive chambers. This implies that depth migration and ontogeny of foraminiferal species is likely to generate heterogeneity within the test that may lead to fractionation in TE/Ca ratios. Small changes in the degree of encrustation of G. inflata shift whole test TE/Ca ratios significantly and hence overprint environmental signal (this is also valid for the stable oxygen and carbon isotope geochemistry of foraminiferal shells). The magnitude of these intertest variations probably varies from site to site with changing environmental parameters. In literature several hypothesis exist to explain TE/Ca heterogeneity within test of planktic foraminifera. Considering the two non-symbiotic species G. inflata and G. bulloides, mainly two theories opposed each other a) foraminiferal vertical migration during its life cycle and related changes in calcification temperature and other environmental parameter and b) the fact that foraminifera are able to regulate the TE composition of their tests in a number of different ways. The present study provides a basis for critical evaluation of these various hypotheses, especially because we know under which conditions these test have calcified.
Projektbezogene Publikationen (Auswahl)
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(2008), A study of the alkenone, Tex86, and planktonic foraminifera in the Benguela Upwelling System: Implications for past sea surface temperature estimates, Geochemistry Geophysics Geosystems, 9, Q10019
Lee, K. E., J.-H. Kim, I. Wilke, P. Helmke. and S. Schouten
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(2009). Depth habitats and seasonal distributions of recent planktic foraminifers in the Canary Islands region (29°N) based on oxygen isotopes. Deep Sea Research, 56, pp. 89-106
Wilke, I., Meggers, H. and Bickert, T.
