Secular variations of the carbon isotopic composition recorded in sedimentary carbonates (d13Ccarb) can be summarized as stable at around 0‰ for most of the geological times, with some so-called excursions towards more positive or more negative values. These excursions are generally interpreted as signatures of global changes in the biogeochemical carbon cycle at the planetary scale, but remain under-constrained in the Precambrian. The aim of DZIANI project is to test an alternative hypothesis for the two main positive excursions documented on Earth history (2.2-2.0 Ga and 0.78-0.58 Ga): according to which they would have been produced by a local or regional scale accumulation of 13C-enriched dissolved inorganic carbon (DIC) as a result of microbially mediated methanogenesis.

The saline lake Dziani Dzaha (Mayotte, Indian Ocean) has recently been identified by the consortium of this project as a potential analog for Precambrian oceans at the time of positive d13Ccarb excursions. The analogy is currently based on results from preliminary studies, which have identified permanent bottom water anoxia below 1.5 m from the surface; overwhelming prokaryote dominance; methane production; microbialite precipitation and high d13C of +13‰ in both DIC and solid carbonates; a combination of analogies to the conjectural primitive oceans never reported yet to our knowledge.

DZIANI proposal intends to perform a geochemical and microbiological integrated study of this lake to fully characterize this unique ecosystem and its sedimentary record. The first objectives will thus be to characterize (i) the biogeochemical C cycle in the lake; (ii) the biogeochemical cycles of N, S and Fe and their interactions with the C cycle; (iii) the biodiversity of microorganisms (virus, Cyanobacteria and other Bacteria, Archaea and eukaryotes) in the different lake compartments as a preliminary step towards addressing their role in the C, N, S and Fe biogeochemical cycles, (iv) the main metabolic activities operating within the lake and (v) to determine their isotope fractionation factors by laboratory experiments. The second objective is to identify the chemical, mineralogical and isotopic (d13Ccarb, d34S, d56Fe, d15N) signatures that are recorded in the lake sediments and microbialites, and may be used in the future to identify similar ecosystems in the Precambrian rock record.

       Bubbles at the surface of the lake 
photo: S. Turay / Sur une île; (c) DCO 2014