Impact of diagenetic alteration on brachiopod shell magnesium isotope (δ26Mg) signatures: Experimental versus field data

Sylvia Riechelmann, Vasileios Mavromatis, Dieter Buhl, Martin Dietzel, Anton Eisenhauer, Adrian Immenhauser

Research output: Contribution to journalArticleResearchpeer-review

Abstract

Due to their thermodynamically stable low-Mg calcite mineralogy, the shells of brachiopods are often counted among the most reliable archives of the physicochemical conditions that occurred during the Phanerozoic in marine waters. Consequently, traditional and non-traditional isotope and elemental proxy data from brachiopod valves have been analyzed in numerous studies and results obtained have been placed in context with ancient seawater properties. This paper tests the sensitivity of brachiopod shell magnesium isotope (δ26Mg) data to diagenetic alteration. We apply a dual approach by: (i) performing hydrothermal alteration experiments using meteoric, marine, and burial reactive fluids; and (ii) comparing these data to naturally altered, ancient brachiopod shells. The degree of alteration of individual shells is assessed by a combination of fluorescence and cathodoluminescence microscopy. The absence of luminescence might indicate both well-preserved shell material, but also the secondary enrichment of quenching elements such as iron along diagenetic pathways. Complementary oxygen isotope data provide insight into the question of open versus closed system behavior of brachiopod shells. Brachiopod shell magnesium isotope values respond to differential fluid temperature, chemistry, and experiment durations. The patterns observed are complicated by the interplay of kinetic and thermodynamic patterns and the presence of variable amounts of water soluble and water insoluble organic matter within these biominerals. Generally, the range in bulk δ26Mg from experimentally altered (1.52‰) and that of bulk samples from ancient, diagenetically altered brachiopod valves (1.53‰) exceed the geochemical variability of δ26Mgbrachiopod bulk values of most recent specimens (1.26‰) in the lower and upper range. More 26Mg enriched (0.8‰) and more 26Mg depleted (0.7‰) values, respectively, are found in altered shells in comparison to unaltered ones. The data shown here are considered significant for those aiming to reconstruct palaeoenvironmental parameters based on brachiopod archives. Consequently, we propose tentative guidelines for magnesium isotope research applied to ancient carbonates.

Original languageEnglish
Pages (from-to)191-206
Number of pages16
JournalChemical Geology
Volume440
DOIs
Publication statusPublished - 15 Nov 2016

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brachiopod
Isotopes
Magnesium
magnesium
isotope
shell
Water
Oxygen Isotopes
Fluids
Cathodoluminescence
Mineralogy
Calcium Carbonate
Carbonates
Seawater
Biological materials
Luminescence
Quenching
Microscopic examination
Iron
Experiments

Keywords

  • Brachiopods
  • Carbon and oxygen isotopes
  • Diagenesis
  • Magnesium isotopes

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

Cite this

Impact of diagenetic alteration on brachiopod shell magnesium isotope (δ26Mg) signatures : Experimental versus field data. / Riechelmann, Sylvia; Mavromatis, Vasileios; Buhl, Dieter; Dietzel, Martin; Eisenhauer, Anton; Immenhauser, Adrian.

In: Chemical Geology, Vol. 440, 15.11.2016, p. 191-206.

Research output: Contribution to journalArticleResearchpeer-review

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