4.1 Article

Using Mg Isotopes to Trace Cyanobacterially Mediated Magnesium Carbonate Precipitation in Alkaline Lakes

Journal

AQUATIC GEOCHEMISTRY
Volume 19, Issue 1, Pages 1-24

Publisher

SPRINGER
DOI: 10.1007/s10498-012-9174-3

Keywords

Magnesium isotope fractionation; Hydromagnesite precipitation; Alkaline lakes

Funding

  1. ANR CO2-FIX
  2. MC ITN DELTA-MIN [ITN-2008-215360]
  3. MC RTN GRASP-CO2 [MRTN-CT-2006-035868]
  4. MC MIN-GRO [MRTN-CT-2006-035488]
  5. Associated European Laboratory LEAGE

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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 delta Mg-26 offset of 0.8-1.4 aEuro degrees is observed between Salda Lake water and it is the incoming streams and groundwaters, and precipitated hydromagnesite has a delta Mg-26 0.9-1.1 aEuro degrees 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, 161-174). Batch reactor experiments performed in the presence of Salda Lake cyanobacteria and stromatolites resulted in the precipitation of dypingite (Mg-5(CO3)(4)(OH)(2)center dot 5(H2O)) and hydromagnesite (Mg-5(CO3)(4)(OH)(2)center dot 4H(2)O) 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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