B\uf6hm, F.

Bosellini, Francesca

Brachert, T.

Dullo, W. C.

Eisenhauer, A.

Gussone, N.

Paytan, A.

Reitner, J.

Reynaud, S.

W\uf6rheide, G.

English

BÖHM F.

GUSSONE N.

EISENHAUER A.

REYNAUD S.

PAYTAN A.

BRACHERT T.

REITNER J.

WÖRHEIDE G.

DULLO W. C.

BOSELLINI, Francesca

Archivio istituzionale della ricerca - Università di Modena e Reggio Emilia

Ca isotope fractionation of inorganic, biologically induced and biologically controlled calcium carbonates.

Böhm, Florian

Gussone, Nikolaus

Eisenhauer, Anton

Brachert, Th.

Wörheide, G.

Dullo, Wolf-Christian

OceanRep

Geophysical Research Abstracts, Vol. 8, 09686, 2006SRef-ID: 1607-7962/gra/EGU06-A-09686© European Geosciences Union 2006Ca isotope fractionation of inorganic, biologicallyinduced and biologically controlled calcium carbonatesF. Böhm (1), N. Gussone(2), A. Eisenhauer (1), S. Reynaud (3), A. Paytan (4), F.Bosellini (7) Th. Brachert (5), J. Reitner (6), G. Wörheide (6), W.-Chr. Dullo (1)(1) IFM-GEOMAR, Kiel, Germany, (2) DFG-FZ Ozeanränder, Univ. Bremen, Germany, (3)Centre Scientifique de Monaco, (4) Stanford Univ., Stanford CA, USA, (5) Inst. f.Geowissenschaften, Universität Mainz, Germany, (6) Geobiologie, Universität Göttingen,Germany, (7) Dipt. del Museo di Paleobiologia e dell’Orto Botanico, Università di Modena eReggio Emilia, ItalyIt has been shown recently (Gussone et al., 2005) that calcium isotope fractionationin inorganic and simple biogenic precipitates depends on mineralogy. Aragonite is de-pleted by about 0.7 permill with respect to calcite, i.e. fractionation with respect toseawater increases from calcite to aragonite. This fractionation has been observed inexperimental inorganic precipitates, in early marine cements and in simple biogeniccarbonates (e.g. sclerosponges). The weak temperature dependence of isotope frac-tionation is similar in calcites and aragonites (0.015 permill/K) and can be explainedby the temperature-control on the CO32- chemistry (Lemarchand et al. 2004).The Ca isotope fractionation of calcite (Lemarchand et al., 2004) and aragonite (Gus-sone et al., 2005) depends on precipitation rates. At higher precipitation rates increas-ingly more unfractionated Ca is incorporated into the crystal and consequently Caisotopes are less fractionated. Following theoretical considerations, the Ca isotopefractionation can be modeled as an exponential function of growth rate. Fitting thismodel to measured data we find an equilibration time of about 100 s and equilib-rium fractionation values for aragonite and calcite of -1.1 permill and -1.7 permill,respectively. At high precipitation rates the fractionation between aragonite and cal-cite decreases and becomes negligible (<0.1 permill) above 10ˆ3.5 µmol/m2/h. Thisis within the typical range of reef coral calcification rates (10ˆ4 µmol/m2/h). In fact,measured Ca isotope values of reef corals are significantly higher than low-rate arag-onite values (e.g. from sclerosponges). On the other hand the coral d44/40Ca valuesare still much lower than predicted by the high calcification rates.We found several other organisms producing carbonates that deviate from the "low-rate" isotope fractionation values: A benthic gastropod shell and the aragonitic partof a Mytilus bivalve are about 0.4 permill enriched in 44Ca with respect to "low-rate" aragonite. Temperature dependencies of these "anomalous" carbonates, however,show the common slope (0.015 permill/K).In the fast growing skeletons of these organisms the observed fractionation is muchstronger than predicted by the precipitation rate model. Therefore, the precipitationrate model cannot explain the “anomalous” fractionation observed in fast growingaragonitic skeletons. We can also exclude closed-system calcification as an expla-nation,because this would only reduce fractionation, which is contrary to our obser-vations. We therefore propose that a biological effect controls the d44/40Ca of fastgrowing calcifiers like corals and bivalves.References: Gussone et al. (2005) Geochim. Cosmochim. Acta, 69, 4485-4494;Lemarchand et al. (2004) Geochim. Cosmochim. Acta, 68, 4665-4678.

Ca isotope fractionation of inorganic, biologically induced and biologically controlled calcium carbonates

Ca isotope fractionation of inorganic, biologically induced and biologically controlled calcium carbonates

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