Universität zu Köln

Wombacher, F., Eisenhauer, A., Boehm, F., Gussone, N., Regenberg, M., Dullo, W. -Chr. and Rueggeberg, A. (2011). Magnesium stable isotope fractionation in marine biogenic calcite and aragonite. Geochim. Cosmochim. Acta, 75 (19). S. 5797 - 5819. OXFORD: PERGAMON-ELSEVIER SCIENCE LTD. ISSN 1872-9533

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Abstract

This survey of magnesium stable isotope compositions in marine biogenic aragonite and calcite includes samples from corals, sclerosponges, benthic porcelaneous and planktonic perforate foraminifera, coccolith oozes, red algae, and an echinoid and brachiopod test. The analyses were carried out using MC-ICP-MS with an external repeatability of +/- 0.22 parts per thousand (2SD for delta Mg-26; n = 37), obtained from a coral reference sample (JCp-1). Magnesium isotope fractionation in calcitic corals and sclerosponges agrees with published data for calcitic speleothems with an average Delta Mg-26(calcite-seawater) = -2.6 +/- 0.3 parts per thousand that appears to be weakly related to temperature. With one exception (Vaceletia spp.), aragonitic corals and sclerosponges also display uniform Mg isotope fractionations relative to seawater with delta Mg-26(biogenic) (aragonite-seawater) = -0.9 +/- 0.2. Magnesium isotopes in high-Mg calcites from red algae, echinoids and perhaps some porcelaneous foraminifera as well as in all low-Mg calcites (perforate foraminifera, coccoliths and brachiopods) display significant biological influences. For planktonic foraminifera, the Mg isotope data is consistent with the fixation of Mg by organic material under equilibrium conditions, but appears to be inconsistent with Mg removal from vacuoles. Our preferred model, however, suggests that planktonic foraminifera synthesize biomolecules that increase the energetic barrier for Mg incorporation. In this model, the need to remove large quantities of Mg from vacuole solutions is avoided. For the high-Mg calcites from echinoids, the precipitation of amorphous calcium carbonate may be responsible for their weaker Mg isotope fractionation. Disregarding superimposed biological effects, it appears that cation light isotope enrichments in CaCO3 principally result from a chemical kinetic isotope effect, related to the incorporation of cations at kink sites. In this model, the systematics of cation isotope fractionations in CaCO3 relate to the activation energy required for cation incorporation, which probably reflects the dehydration of the cation and the crystal surface and bond formation at the incorporation site. This kinetic incorporation model predicts (i) no intrinsic dependence on growth rate, unless significant back reaction upon slow growth reduces the isotope fractionation towards that characteristic for equilibrium isotope partitioning (this may be observed for Ca isotopes in calcites), (ii) a small decrease of isotope fractionation with increasing temperature that may be amplified if higher temperatures promote back reaction and (iii) a sensitivity to changes in the activation barrier caused by additives such as anions or biomolecules or by the initial formation of amorphous CaCO3. (C) 2011 Elsevier Ltd. All rights reserved.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Wombacher, F. UNSPECIFIED UNSPECIFIED UNSPECIFIED Eisenhauer, A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Boehm, F. UNSPECIFIED UNSPECIFIED UNSPECIFIED Gussone, N. UNSPECIFIED UNSPECIFIED UNSPECIFIED Regenberg, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED Dullo, W. -Chr. UNSPECIFIED UNSPECIFIED UNSPECIFIED Rueggeberg, A. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-488704
DOI:	10.1016/j.gca.2011.07.017
Journal or Publication Title:	Geochim. Cosmochim. Acta
Volume:	75
Number:	19
Page Range:	S. 5797 - 5819
Date:	2011
Publisher:	PERGAMON-ELSEVIER SCIENCE LTD
Place of Publication:	OXFORD
ISSN:	1872-9533
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language COCCOLITHOPHORES EMILIANIA-HUXLEYI; PLANKTONIC-FORAMINIFERA; SEAWATER-CHEMISTRY; TRACE-ELEMENTS; EQUILIBRIUM FRACTIONATION; TEMPERATURE-DEPENDENCE; STRONTIUM ISOTOPES; MG/CA THERMOMETRY; CRYSTAL-GROWTH; MG ISOTOPES Multiple languages Geochemistry & Geophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/48870