EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions Open Access Open Access Atmospheric Atmospheric Chemistry Chemistry and Physics and Physics Discussions Open Access Open Access Atmospheric Atmospheric Measurement Measurement Techniques Techniques Discussions Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Open Access Biogeosciences Discuss., 10, 9797–9818, 2013 Open Access www.biogeosciences-discuss.net/10/9797/2013/ Biogeosciences Biogeosciences BGD doi:10.5194/bgd-10-9797-2013 Discussions © Author(s) 2013. CC Attribution 3.0 License. 10, 9797–9818, 2013 Open Access Open Access This discussion paper is/has been under review for the journal BiogeosciencesClimate (BG). Climate A new model for Please refer to the correspondingof finalthe Past paper in BG if available. of the Past Discussions biomineralization and trace-element Open Access Open Access signatures A new modelEarth for System biomineralization Earth System and Dynamics Dynamics G. Nehrke et al. trace-element signatures of foraminiferaDiscussions Open Access Open Access tests Geoscientific Geoscientific Title Page Instrumentation Instrumentation G. Nehrke1, N. Keul2, G. LangerMethods3, and L. J. de Nooijer4, J. Bijma1,Methods and A. Meibomand 5 Abstract Introduction Data Systems Data Systems 1 Alfred Wegener Institute, Bremerhaven, Germany Discussions Conclusions References Open Access Open Access 2Lammont–Doherty Earth Observatory, Columbia University, Palisades,Geoscientific New York, USA 3 Geoscientific Tables Figures Departement of Earth Sciences, Cambridge University, Cambridge,Model UKDevelopment 4 Model Development Department of Marine Geology, Royal Netherlands Institute of Sea Research,Discussions Horntje, the Netherlands J I Open Access 5 Open Access Laboratory for Biological Geochemistry,Hydrology and School of Architecture, CivilHydrology and Environmental and Engineering (ENAC), Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, J I Earth System Earth System Switzerland Back Close Sciences Sciences Received: 27 May 2013 – Accepted: 30 May 2013 – Published: 18 June 2013Discussions Full Screen / Esc Open Access Open Access Correspondence to: G. Nehrke ([email protected]) Ocean Science Ocean Science Printer-friendly Version Published by Copernicus Publications on behalf of the European GeosciencesDiscussions Union. Interactive Discussion Open Access Open Access 9797 Solid Earth Solid Earth Discussions Open Access Open Access The Cryosphere The Cryosphere Discussions Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract BGD 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, 10, 9797–9818, 2013 the mechanisms involved in the carbonate biomineralization process are poorly under- 5 stood. The current paradigm holds that calcium ions for the test are supplied primarily A new model for by endocytosis of seawater. Here, we combine confocal-laser scanning-microscopy biomineralization and observations of a membrane-impermeable fluorescent marker in the extant benthic trace-element 44 species Ammonia aomoriensis with dynamic Ca-labeling and NanoSIMS isotopic signatures imaging of its test. We infer that Ca for the test in A. aomoriensis is supplied primar- 10 ily via trans-membrane transport, but that a small component of passively transported G. Nehrke et al. (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 Title Page seawater Mg / Ca ratio. This places foram-based paleoclimatology into a strong con- Abstract Introduction 15 ceptual framework. Conclusions References 1 Introduction Tables Figures Calcium carbonate tests (shells) formed by unicellular foraminifera are present in ma- J I rine sedimentary records since the Ordovician (∼ 290 million yr ago) (Martin, 1995; Schallreuter, 1983). With different species adapted to specific environmental condi- J I 20 tions, their relative taxonomic abundances as well as the elemental (e.g. Mg / Ca) and Back Close isotopic (e.g. δ18O) composition of their tests are frequently used to reconstruct global climate change associated with e.g. glacial-interglacial cycles (Elderfield and Ganssen, Full Screen / Esc 2000) and mass-extinctions (Kiessling et al., 2008). However, the use of foraminifera tests as paleo-environmental archives is complicated by biological processes, which Printer-friendly Version 25 cause their chemical and isotopic compositions to be significantly different from Ca- carbonates precipitated inorganically under the same environmental conditions. These Interactive Discussion 9798 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | differences are referred to as the “vital effect” (Urey et al., 1951). A better understand- ing of the foraminifera biomineralization process is essential to the identification of the BGD mechanism(s) responsible for such chemical and isotopic fractionations. One funda- 10, 9797–9818, 2013 mental question in this regard is how the constituents of the tests, especially the dom- 2+ 5 inant cation Ca and the minor/trace-elements Mg and Sr are transported from the ambient seawater to the site of calcification. A new model for A widely accepted model holds that endocytosis of seawater is the principal mecha- biomineralization and nism involved in test formation; i.e. special vesicles transport seawater to the site of cal- trace-element cification (Erez, 2003; Bentov et al., 2009). This model is primarily based on the obser- signatures 10 vation that labeling of seawater with the membrane-impermeable fluorescent marker FITC (fluorescin isothiocyanate-dextran) results in staining of newly formed chambers G. Nehrke et al. in previously decalcified benthic foraminifera Amphistegina lobifera. This observation led to the conclusion that endocytosed seawater is the primary source of ions for calci- Title Page fication. However, the endocytosis model suffers from two problems. 15 First, a mechanism for the modification of the elemental composition of the vac- Abstract Introduction uolized seawater, in particular Mg-removal, during transport to the site of calcification has to be postulated in order to explain the chemical composition of the tests (Erez, Conclusions References 2003). Tables Figures Secondly, the endocytosis-model is challenged by a calculation of the volume of sea- 20 water needed to supply the required Ca to the site of calcification. For the ubiquitous J I benthic species Ammonia tepida this is about 75 times the volume of the foraminifera itself (de Nooijer et al., 2009), assuming initial seawater composition in the vesicles. J I De Nooijer and co-workers (de Nooijer et al., 2009) did observe seawater-endocytosis Back Close in A. tepida using the FITC marker, but the low vesicle activity observed during nor- 25 mal calcification renders endocytosis insufficient as the major transport mechanism for Full Screen / Esc Ca from seawater to the site of calcification. To overcome this problem, an internal Ca-pool was hypothesized for A. tepida (de Nooijer et al., 2009), but never experimen- Printer-friendly Version tally demonstrated. (It should be noted that in some studies we cite (de Nooijer et al., 2009;Dissard et al., 2010), the authors have misidentified the species investigated as Interactive Discussion 9799 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Ammonia tepida, which actually was Ammonia aomoriensis T6 (the species also used in the present study) (Schweizer et al., 2011; Hayward et al., 2004).) BGD Nonetheless, it is important to emphasize that endocytosis, or vacuolization of sea- 10, 9797–9818, 2013 water in foraminifera is an observed fact, which allows FITC to be transported to the 5 site of calcification and, as will become apparent below, does play an important role for the trace element composition of the test. A new model for Here we use the fluorescent marker FITC together with confocal-laser scanning- biomineralization and microscopy, and 44Ca pulse-chase experiments in combination with NanoSIMS isotopic trace-element imaging to investigate the hypothesis of an intracellular calcium pool in A. aomoriensis. signatures 10 (For details on sample collection, culturing, and incubation conditions see SI.) Incuba- tion experiments with FITC were performed prior to, during, and after chamber forma- G. Nehrke et al. tion to visualize and relate seawater vacuolization to calcification, i.e. formation of new chambers. Formation of a new chamber in A. aomoriensis starts with an organic sheet, Title Page onto which calcitic CaCO3 nucleates. In our incubated specimens, chamber formation 15 takes 3–5 h, after which the animal resumes movement and extends its pseudopodial Abstract Introduction network. Conclusions References 2 Results Tables Figures We did not observe an increased abundance of vesicles prior to chamber formation J I (Fig. 1a). Moreover, vesicle activity was very low during chamber formation (Fig. 1c). J I 20 Our results (Fig. 1) confirm the vacuolization of seawater as described by de Nooijer et al., 2009), but a systematic