Articles | Volume 9, issue 11
Biogeosciences, 9, 4851–4860, 2012
Biogeosciences, 9, 4851–4860, 2012

Research article 26 Nov 2012

Research article | 26 Nov 2012

Encrustation and trace element composition of Neogloboquadrina dutertrei assessed from single chamber analyses – implications for paleotemperature estimates

L. Jonkers1,*, L. J. de Nooijer2,**, G.-J. Reichart2,3,**, R. Zahn4, and G.-J. A. Brummer5 L. Jonkers et al.
  • 1Institut de Ciència i Tecnologia Ambientals (ICTA), Universitat Autònoma de Barcelona. Campus UAB, Edifici Cn-C7/054, 08193 Bellaterra (Cerdanyola del Vallès), Spain
  • 2Utrecht University, Department of Geochemistry, Budapestlaan 4, 3584 CD Utrecht, The Netherlands
  • 3Alfred Wegener Institute, Bremerhaven, Germany
  • 4Institució Catalana de Recerca i Estudis Avançats (ICREA), Institut de Ciència i Tecnologia Ambientals (ICTA), Departament de Física, Universitat Autònoma de Barcelona, Bellaterra 08193, Spain
  • 5Royal Netherlands Institute for Sea Research, Dept. Marine Geology, P.O. Box 59, 1790 AB Den Burg, The Netherlands
  • *now at: University of Cardiff, School of Earth and Ocean Sciences. Park Place Cardiff CF10 3AT Wales, UK
  • **now at: Royal Netherlands Institute for Sea Research, Dept. Marine Geology, P.O. Box 59, 1790 AB Den Burg, The Netherlands

Abstract. Crust formation is a common phenomenon in planktonic foraminifera. Because of their different formation mechanism and hence composition, crusts affect the overall test composition and therefore complicate the use of crust-bearing foraminifera in paleoceanography. Such species are often used to estimate subsurface paleotemperatures, and although the influence of encrustation on the trace element/Ca ratio is recognised, it has not been systematically explored between and within tests. Here we use laser ablation ICP-MS to assess the variability in trace element composition of the crust of Neogloboquadrina dutertrei within individual chambers, as well as the effect of compositional heterogeneity of the crust on whole test chemistry. Compositionally, the outer crust differs from inner layer by lower Mg/Ca and Mn/Ca, but is indistinguishable in Sr/Ca. Crust thickness decreases towards the younger chambers, and it may be entirely absent from the last chamber. In contrast to Mn/Ca and Sr/Ca, crustal Mg/Ca ratios show a tendency towards higher values on the younger chambers. These patterns in crust thickness and in crust Mg/Ca indicate that temperature is not the dominant factor controlling crust composition. Temperature estimates based on N. dutertrei, and presumably other crust-forming species too, are therefore biased towards too low values. Through comparison of modern and glacial tests, we show that this bias is not constant and that changes in crust thickness and/or in Mg/Ca values can spuriously suggest temperature changes.

Final-revised paper