Séminaire du Mercredi 21 mars 2012 à 14H

Coupled molecular and 14C studies of microbial carbonate laminae formation and growth rates in dolomite stromatolites from Lagoa Salgada, Brazil

Anelize BAHNIUK : (Geological Institute ETHZ Switzerland)

IMPMC, Université P. et M. Curie, 4, Place Jussieu, 75005 Paris

Salle de Conférence (401), 4ème étage, Tour 23,  Couloir 22-23

Résumé :

This study investigates the application of radiocarbon (14C) measurements for age determination of the growth rate of modern dolomite stromalites from Lagoa Salgada, Rio de Janeiro, Brazil. Improved estimation of growth rates related with microbial processes during stromatolite formation may provide insights into ancient microbial carbonate sedimentation processes. Radiocarbon dating is frequently used to determine the age of carbonate materials, however, lagoonal carbonates often contain significant 14C content anomalies due to the “hard water effect.” Lagoonal dissolved inorganic carbon (DIC) may sometimes achieve isotopic equilibrium with atmospheric CO2, and, in such cases, a coherent 14C chronology for paleo-carbonate precipitates may be established. However, more frequently, DIC does not show complete equilibrium with atmospheric CO2 due to major inputs of groundwater carbon. As a consequence, paleo-lagoonal carbonates often yield 14C ages which, after isotopic normalization, are older than those obtained, for instance, for time-equivalent terrestrial organic matter. Located on the Rio de Janeiro coastline, Lagoa Salgada is a unique environment, which has been studied for its particular carbonate sedimentation and development of dolomite stromatolites. This milieu provides an opportunity to test how Earth surface processes can change from normal carbonate sedimentation to the formation of microbialite structures (e.g., stromatolites, oncolites). In Lagoa Salgada, the formation of laminated structures involves diverse groups of microorganisms, which leads to the trapping of distinctive biomarkers or organic molecules as intracrystalline organic matter within the stromatolite laminae. We report on our research into the characterization of this trapped organic matter and the potential to utilize it to radiometrically determine the growth rate of these modern stromatolites. We also report 14C measurements of DIC from Lagoa Salgada.