HR: 13:40h
AN: B43G-01    [Abstracts]
TI: BORON ISOTOPIC COMPOSITION CORRELATES WITH ULTRA-STRUCTURE IN A DEEP- SEA CORAL LOPHELIA PERTUSA: IMPLICATIONS FOR BIOMINERALIZATION AND PALEO- PH
AU: * Blamart, D
EM: blamart@lsce.cnrs-gif.fr
AF: Laboratoire des Sciences du Climat et de l Environnement, CEA-CNRS-UVSQ, Domaine du CNRS, Bat 12, 4 Avenue de la Terrasse, Gif sur Yvette, 91198, France
AU: Rollion-Bard, C
EM: rollion@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, 15 rue Notre-Dame des Pauvres, BP 20, Vandoeuvre-lès-Nancy, 54500, France
AU: Meibom, A
EM: meibom@mnhn.fr
AF: Museum National d'Histoire Naturelle, Laboratoire d'Etude de la Matière Extraterrestre, USM 0205 (LEME), Case Postale 52, 61 rue Buffon, Paris, 75005, France
AU: Cuif, J
EM: jean-pierre.cuif@u-psud.fr
AF: Université Paris XI, UMR IDES 8148, Bat. 504, Orsay Cedex, 91405, France
AU: Juillet-Leclerc, A
EM: Anne.Juillet@lsce.cnrs-gif.fr
AF: Laboratoire des Sciences du Climat et de l Environnement, CEA-CNRS-UVSQ, Domaine du CNRS, Bat 12, 4 Avenue de la Terrasse, Gif sur Yvette, 91198, France
AU: Dauphin, Y
EM: yannicke.dauphin@u-psud.fr
AF: Université Paris XI, UMR IDES 8148, Bat. 504, Orsay Cedex, 91405, France
AU: Douarin, M
EM: Melanie.Douarin@lsce.cnrs-gif.fr
AF: UMR CNRS 5805 EPOC - OASU - Université Bordeaux 1, Avenue des Facultés, TALENCE CEDEX, 33405, France
AB: The geochemistry (stable isotopes and trace elements) of biogenic carbonates has been widely used for more than fifty years to reconstruct past climatic variability. During this time, the studies were mainly based on bulk sampling limiting sometimes the interpretations of the geochemical data as paleoclimatic proxies. Recently, high spatial resolution sampling techniques, such as micro-mill and SIMS, have been employed in the study of C, O and B isotopic compositions and trace elements (Mg, Sr) in the skeletons of a variety of (deep-sea) coral species. These studies have documented dramatic ‘vital effects' and uncovered a systematic relationship between skeletal ultra-structure and stable isotopic composition. The formation of skeleton corals follows a universal two-step growth process. At the tips of the skeletal structures, the mineralizing cell layer produces centers of calcification (COC) or, equivalently, Early Mineralization Zone (EMZ). These EMZ are subsequently overgrown by fibrous aragonite(FA) consisting of cyclically added layers. The EMZ are characterized by systematically lighter C and O isotopic compositions compared with the adjacent FA. A number of geochemical models have been proposed, in which this systematic stable isotopic difference between EMZ and FA is ascribed to a biologically induced variation in the pH of a proposed Extra-cytoplasmic Calcifying Fluid (ECF) reservoir. In these models, relatively high pH conditions during the formation of EMZ result in relatively light C and O isotopic compositions compared with FA, which form under generally lower pH conditions. A direct test of such models would be possible if the Boron isotopic composition, which is pH sensitive, of EMZ and FA could be measured. We performed ion microprobe d11B measurements for EMZ and FA in Lophelia pertusa, a deep-sea coral common in the North-East Atlantic Ocean. We observe a systematic difference in B isotopic composition between the EMZ and FA skeleton. In EMZ, the measured δ¹¹B values are consistently low. Fibrous aragonite is characterized by systematically higher d11B values, but also display B isotopic heterogeneity associated with specific growth bands in the calyx wall. The magnitude of the observed B isotopic variations cannot be explained by changes in environmental conditions and are likely caused by biological processes involved in the biomineralization of new skeleton; i.e. ‘vital' effects. The observed B isotopic variations are opposite to the predictions of geochemical models for vital effects. Our data indicate that pH variations are not responsible for the observed stable isotopic fractionations. Geochemical models therefore do not provide an adequate framework within which to understand coral skeletal formation. Without a better understanding of these processes, which require experiments, the use of B isotopic composition to reconstruct paleo-pH variations in the oceans must be considered problematic - at least as far as Lophelia pertusa is concerned.
DE: 0419 Biomineralization
DE: 0429 Climate dynamics (1620)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
DE: 4916 Corals (4220)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting