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Using Mg Isotopes to Trace Cyanobacterially Mediated Magnesium Carbonate Precipitation in Alkaline Lakes |
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| Authors: | Liudmila S Shirokova Vasileios Mavromatis Irina A Bundeleva Oleg S Pokrovsky Pascale B??n??zeth Emmanuelle G??rard Christopher R Pearce Eric H Oelkers |
| Institution: | 1. Geoscience and Environment Toulouse (GET), CNRS, UMR 5563, Observatoire Midi-Pyr??n??es, 14 Avenue Edouard Belin, 31400, Toulouse, France 2. Institute of Ecological Problems of the North, 23 Nab. Sev. Dviny, Russian Academy of Science, Arkhangelsk, Russia 4. Equipe g??obiosph??re actuelle et primitive, UMR CNRS 7154, Institut de Physique du Globe de Paris, 1 rue Jussieu, 75238, Paris, France 5. Department of Earth and Environmental Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK 3. University of Iceland, Sturlugata 7, 101, Reykjavik, Iceland |
| Abstract: | This study assesses the potential use of Mg isotopes to trace Mg carbonate precipitation in natural waters. Salda Lake (SW Turkey) was chosen for this study because it is one of the few modern environments where hydrous Mg carbonates are the dominant precipitating minerals. Stromatolites, consisting mainly of hydromagnesite, are abundant in this lake. The Mg isotope composition of incoming streams, groundwaters, lake waters, stromatolites, and hydromagnesite-rich sediments were measured. Because Salda Lake is located in a closed basin, mass balance requires that the Mg isotopic offset between Lake Salda water and precipitated hydromagnesite be comparable to the corresponding offset between Salda Lake and its water inputs. This is consistent with observations; a ??26Mg offset of 0.8?C1.4??? is observed between Salda Lake water and it is the incoming streams and groundwaters, and precipitated hydromagnesite has a ??26Mg 0.9?C1.1??? more negative than its corresponding fluid phase. This isotopic offset also matches closely that measured in the laboratory during both biotic and abiotic hydrous Mg carbonate precipitation by cyanobacteria (Mavromatis, V., Pearce, C., Shirokova, L. S., Bundeleva, I. A., Pokrovsky, O. S., Benezeth, P. and Oelkers, E.H.: Magnesium isotope fractionation during inorganic and cyanobacteria-induced hydrous magnesium carbonate precipitation, Geochim. Cosmochim. Acta, 2012a. 76, 161?C174). Batch reactor experiments performed in the presence of Salda Lake cyanobacteria and stromatolites resulted in the precipitation of dypingite (Mg5(CO3)4(OH)2·5(H2O)) and hydromagnesite (Mg5(CO3)4(OH)2·4H2O) with morphological features similar to those of natural samples. Concurrent abiotic control experiments did not exhibit carbonate precipitation demonstrating the critical role of cyanobacteria in the precipitation process. |
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