Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments

Abstract

Magnesite forms a series of 1-to 15-m-thick beds within the »2á0 Ga (Palaeoproterozoic) Tulomozerskaya Formation, NW Fennoscandian Shield, Russia. Drillcore material together with natural exposures reveal that the 680-m-thick formation is composed of a stromatolite±dolomite±`red bed' sequence formed in a complex combination of shallow-marine and non-marine, evaporitic environments. Dolomite-collapse breccia, stromatolitic and micritic dolostones and sparry allochemical dolostones are the principal rocks hosting the magnesite beds. All dolomite lithologies are marked by d13 C values from +7á1& to +11á6& (V-PDB) and d18 O ranging from 17á4& to 26á3& (V-SMOW). Magnesite occurs in different forms: ®nely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coarsely crystalline magnesite. All varieties exhibit anomalously high d13 C values ranging from +9á0& to +11á6& and d 18O values of 20á0±25á7&. Laminated and structureless micritic magnesite forms as a secondary phase replacing dolomite during early diagenesis, and replaced dolomite before the major phase of burial. Crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from sea water-derived brine, diluted by meteoric uids. Magnesitization was accomplished under evaporitic conditions (sabkha to playa lake environment) proposed to be similar to the Coorong or Lake Walyungup coastal playa magnesite. Magnesite and host dolostones formed in evaporative and partly restricted environments; consequently, extremely high d13 C values re¯ect a combined contribution from both global and local carbon reservoirs. A 13 C-rich global carbon reservoir (d 13 C at around +5&) is related to the perturbation of the carbon cycle at 2á0 Ga, whereas the local enhancement in 13 C (up to +12&) is associated with evaporative and restricted environments with high bioproductivity.

Genesis of magnesite deposits in the view of isotope geochemistry

“…Extremely heavy δ 13 C +7 to +12‰ is reported from Paleoproterozic (~2 Ga) magnesite beds within the Tulomozerskaya Formation, NW Fennoscandian shield. The extremely high δ 13 C values reflect a combined contribution of global reservoir (δ 13 C +5‰) and local carbon sources (δ 13 C up +12‰), The δ 13 C-rich global reservoir is related to the perturbation of the carbon cycle at 2,0 Ga, and the local enhancement δ 13 C is caused by evaporation and high bioactivity ( Melezhik et al 2001). …”

Molecular biogeochemistry of modern and ancient marine microbes

“…(e) 2800-1800 Ma The next billion years begin with the apparent onset of oxygenic photosynthesis (Hayes 1983(Hayes , 1994, which includes a period during which the abundance of 13 C in sedimentary carbonates was sometimes markedly enriched Melezhik et al 1999), and ends with the likely onset of sulphidic conditions in the deep sea .…”

“…Contents of 13 C in carbonates are frequently elevated, occasionally exceeding C10‰ (figure 8). Accepting these compositions as representative of the global pool of oceanic DIC would imply a near-trebling of f. Writing three years after published 'Carbon isotopes and the rise of atmospheric oxygen', Melezhik et al (1999) drew on additional data to define three apparently separate pulses of isotopic enrichment. That dissection of the record yields the three spikes in f that are represented by broken lines in figure 9a (alternative stratigraphic correlations yield only two spikes; Bekker et al 2003).…”

“…searched for a signal in the organic-carbon record and found its absence 'rather odd'. If the record were not so fragmentary and noisy (preservation is a much greater problem for organic carbon than for carbonate minerals) that the problem could be set aside (Melezhik et al 1999), this alone would be fatal to the interpretation. (ii) Evidence is lacking for the large deposits of organic carbon that should have formed (Melezhik et al 1999;Aharon 2005).…”

“…Particularly near oxygen-producing communities (many of the isotopically enriched dolostones are associated with stromatolites; Melezhik et al 1999), methanogens will eventually have been pushed to deeper levels in the sediment. When this occurred, apparently beginning at about 2400 Ma, isotopically enriched pools of porewater DIC will have developed.…”