Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments   总被引：2，自引：0，他引：2  

Victor A. Melezhik  Anthony E. Fallick  Pavel V. Medvedev  & Vladimir V. Makarikhin 《Sedimentology》2001,48(2):379-397

Magnesite forms a series of 1‐ to 15‐m‐thick beds within the ≈2·0 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 δ¹³C values from +7·1‰ to +11·6‰ (V‐PDB) and δ¹⁸O ranging from 17·4‰ to 26·3‰ (V‐SMOW). Magnesite occurs in different forms: finely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coarsely crystalline magnesite. All varieties exhibit anomalously high δ¹³C values ranging from +9·0‰ to +11·6‰ and δ¹⁸O values of 20·0–25·7‰. 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 fluids. 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 δ¹³C values reflect a combined contribution from both global and local carbon reservoirs. A ¹³C‐rich global carbon reservoir (δ¹³C at around +5‰) is related to the perturbation of the carbon cycle at 2·0 Ga, whereas the local enhancement in ¹³C (up to +12‰) is associated with evaporative and restricted environments with high bioproductivity.  相似文献   

Possible reasons for the δ13C anomaly of Lower Proterozoic sedimentary carbonates     

M. Tikhomirova  V.V. Makarikhin 《地学学报》1993,5(3):244-248

Detailed isotopic study (δ¹³C, δ¹⁸O) of Lower Proterozoic sedimentary carbonates from biostratigraphically dated successions in Karelia (Russia) revealed a large positive δ¹³C anomaly (average δ¹³C-value constitutes approximately +10‰ PDB) at 2.3 Byr ago and a huge drop of δ¹³C-values in the subsequent ‘black shale period’(2.2 Byr ago). These data are interpreted to be a result of large-scale changes in the oxygen content of the atmosphere. According to this model the positive carbon isotopic anomaly of sedimentary carbonates at about 2.3 Byr ago reflects a very high O₂-content of the atmosphere at this time; the subsequent drop in δ¹³C-values equates with a sharp decrease of the O₂-content of the atmosphere.  相似文献