Blanc SIMI 6 - Blanc - SIMI 6 - Système Terre, environnement, risques

Insight into the Precambrian oceans from a biogeochemical study of a present-day analog: the lake Dzinai Dzaha, Mayotte. – Dziani

Insights into the primitive oceans fron a modern lake: the Dziani Dzaha.

Sedimentary rocks consitute the only record of the primitive oceans. To understand them and hence to characterize ancient oceans, we need to look at the sediments from a variety of well characterized modern aquatic systems. The Dziani Dzaha is one of them.

Characterizing the Dziani Dzaha and its sedimentary record.

The overarching objective of Dziani project is to better interpret ancient sedimentary rocks in order to characterize ancient oceans. Looking at modern systems chosen for their possible analogies with anciant oceans is one way to do this. The lake chosen for this study is the Lake Dziani Dzaha, located on the island of Petite Terre, Mayotte. Its main interest is its unique combination of analogies with ancient oceans, including an unusually positive carbon isotopic signature. This signature is also found in ancient sedimentary rocks, and several hypotheses have been advanced to explain it. Understanding the mechanisms that are at the origin of this analogy in the Dziani Dzaha will bring new arguments into the debate.

Four field expeditions were conducted to equip the lake with continuous measurement systems (basic weather station, water level recorder and thermal and oxygen probes) to make one-site measurements and to sample water and sediment. The samples are being analyzed for their chemical composition, biological content (photosynthetic and respiratory activity, metagenomics, optical microscopy and flow cytometry) and isotopic compositions of carbon (dissolved inorganic carbon and organic matter) of nitrogen( organic matter) and sulfur (sulphates and sulphides). The mineralogical composition of sediments is also being studied and the structural links between microorganisms and carbonates in carbonated constructions (called stromatholites) found in the lake.

The prelimary results confirm the stability over time of this unusual system from the perspective of: ecosystem, biomass, chemistry and isotopic composition of dissolved inorganic carbon (+ 13 ‰). As a result a research synergy is gradually building up, with a project funded by the Total Foundation (Dzaha project, 95 k €) to study the biodversity and another one with Total R&D focussing on the sediments (200 k €).

The characteristics of this lake and their stabiilty over time are very atypical. Undersantding their origins can have several impacts: 1- to allow us to grow similar ecosystems industrially to obtain high production of biomass and biogas; 2- to characterize the primitive oceans 3- to understand in which conditions lakes may give rise to oil fields, which would improve our ability to identify new petroleum resources.

Two short videos have been produced by Sébastien Turay (Sur une île) at the occasion of our first two field expeditions. Available at: vimeo.com/suruneile/dziani
vimeo.com/suruneile/dco2

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.


Project coordinator

Madame Magali Ader (Institut de Physique du Globe de Paris)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

IPGP Institut de Physique du Globe de Paris
IRD - UMR ECOSYM INSTITUT DE RECHERCHE POUR LE DEVELOPPEMENT
MCAM Molécules de Communication et Adaptation des Micro-organismes

Help of the ANR 394,987 euros
Beginning and duration of the scientific project: December 2013 - 48 Months

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