С, О, S, and Sr Isotopes in Rocks of the Lower Eifelian Osveya Horizon in Belarus

• Published in: Lithology and Mineral Resources Vol. 58; no. 4; pp. 338 - 355
• Main Authors: Makhnach, A. A., Pokrovsky, B. G., Murashko, O. V.
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 01.08.2023; Springer Nature B.V
• Subjects: Atmospheric precipitations; Calcite; Carbonates; Chemical analysis; Devonian; Diagenesis; Disintegration; Earth and Environmental Science; Earth Sciences; Evaporation; Gypsum; Horizon; Isotope composition; Isotopes; Marine invertebrates; Mineral Resources; Mineralogy; Oxygen isotopes; Precipitation; Rock; Rocks; Runoff; Seawater; Sedimentation; Sedimentation & deposition; Sedimentology; Soil water; Strontium 87; Strontium isotopes; Sulfates; Water analysis; Water heating; Belarus; Belarus; isotope geochemistry; Lower Eifelian
• ISSN: 0024-4902; 1608-3229; 1573-8892
• DOI: 10.1134/S0024490223700165

Complex interpretation of the distribution of δ 13 C and δ 18 О values in carbonates, as well as δ 34 S and 87 Sr/ 86 Sr values, in gypsum from the lower Eifelian Osveya horizon (Belarus) showed that significance of the diagenetic isotope signals is low and the isotope variations can be explained consistently within a sedimentation model. Along with the section intervals with standard marine δ 13 C values, there are negative excursions of the parameter (up to –7…–11‰) marking segments with carbonate–sulfate rocks and smaller sulfate manifestations. The excursions correspond to evaporitic episodes when the basin became shallower and, probably, disintegrated into separate depressions, leading to increase in the role of continental water enriched with soil carbon in the geochemical sedimentation system due to the continental runoff activation and/or shallowing. The waters could enter both from the prolonged exposed land areas where the Eifelian rocks are absent until now and from the periodically existing islands. The oxygen isotope composition does not respond to negative δ 13 C excursions by similar shifts due to the possible absence of significant difference between the δ 18 О values of the atmospheric precipitation generating the land water and the seawater at low latitudes where the Belarus region was situated in the Devonian. In 40% of the studied samples, the δ 18 О values range from –4 to –2‰, corresponding to the Eifelian “plateau” (approximately –3‰) in the generalized chemostratigraphic curve based on calcite of brachiopod shells from several regions of the world. The δ 18 О values higher than –2‰ (44% of samples), probably, are related to water evaporation, whereas values lower than –2‰ (16% of samples), which can decrease to –7.0…–9.5‰ at the peaks of evaporation episodes, are caused by the water heating (sometimes very significant) during the basin shallowing. Activation of the role of continental water accompanying the evaporation episodes is supported by the following fact: the 87 Sr/ 86 Sr ratio in the Osveya gypsum (0.708402–0.708742) is much higher than in the Eifelian seawater according to a global estimate (0.70772), whereas the δ 34 S value in gypsum (15.5–21.2‰) in more than one-half of the analyzed samples is lower than typical values in the Middle Devonian seawater sulfate (19–20‰).