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

Abstract

Complex interpretation of the distribution of δ¹³C and δ¹⁸О values in carbonates, as well as δ³⁴S and ⁸⁷Sr/⁸⁶Sr 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 δ¹³C 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 δ¹³C excursions by similar shifts due to the possible absence of significant difference between the δ¹⁸О 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 δ¹⁸О 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 δ¹⁸О 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 ⁸⁷Sr/⁸⁶Sr 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 δ³⁴S 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‰).