1Alfred Wegener Institute, Bremerhaven, Germany
2Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA
3Department of Earth Sciences, Cambridge University, Cambridge, UK
4Department of Marine Geology, Royal Netherlands Institute of Sea Research, Horntje, the Netherlands
5Laboratory for Biological Geochemistry, School of Architecture, Civil and Environmental Engineering (ENAC), Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
Received: 27 May 2013 – Discussion started: 18 Jun 2013
Abstract. The Mg/Ca ratio of Foraminifera calcium carbonate tests is used as proxy for seawater temperature and widely applied to reconstruct global paleo-climatic changes. However, the mechanisms involved in the carbonate biomineralization process are poorly understood. The current paradigm holds that calcium ions for the test are supplied primarily by endocytosis of seawater. Here, we combine confocal-laser scanning-microscopy observations of a membrane-impermeable fluorescent marker in the extant benthic species Ammonia aomoriensis with dynamic 44Ca-labeling and NanoSIMS isotopic imaging of its test. We infer that Ca for the test in A. aomoriensis is supplied primarily via trans-membrane transport, but that a small component of passively transported (e.g., by endocytosis) seawater to the site of calcification plays a key role in defining the trace-element composition of the test. Our model accounts for the full range of Mg/Ca and Sr/Ca observed for benthic Foraminifera tests and predicts the effect of changing seawater Mg/Ca ratio. This places foram-based paleoclimatology into a strong conceptual framework.
Revised: 24 Sep 2013 – Accepted: 24 Sep 2013 – Published: 29 Oct 2013
Nehrke, G., Keul, N., Langer, G., de Nooijer, L. J., Bijma, J., and Meibom, A.: A new model for biomineralization and trace-element signatures of Foraminifera tests, Biogeosciences, 10, 6759-6767, doi:10.5194/bg-10-6759-2013, 2013.