С, О, S, Sr-Isotopes in the Osveya Horizon Deposits of the Lower Eifelian on the Territory of Belarus

Abstract

The complex interpretation of distribution of δ13C and δ18О in carbonates, δ34S and 87Sr/86Sr in gypsum from the Osveya horizon (Lower Eifelian) on the territory of Belarus showed that significance of diagenetic isotope signals is low and variations of the values can be consistently explained within a sedimentation model. Along with intervals of sections with standard marine δ13C values, there are negative excursions of the parameter (up to –7…–11‰) marking segments of sequence with carbonate-sulphate rocks and smaller scale sulphate manifestations. The excursions correspond to evaporite episodes when the basin shallowed and, probably, dismembered into separate ponds. This led to increasing the role of continental water enriched with soil carbon in the geochemical system of sedimentation due to the land runoff activation and/or shallowing. Continental water could come from for a long time exposed land areas where the Eifelian deposits are absent until now as well as from periodically existing islands. Oxygen isotopic composition does not react to δ13C negative excursions by similar shifts due to the possible absence of significant difference between the δ18О values of atmospheric precipitation, giving rise to land water, and seawater in low latitudes where the territory of Belarus was situated in the Devonian. In 40% of the studied samples, the δ18О values range from –4 to –2‰, corresponding to the Eifelian “plateau” (~–3‰) in the generalized chemostratigraphic curve in the generalized chemostratigraphic curve constructed using calcite of brachiopod shells from several regions of the world. The δ18О values higher than –2‰ (44% of cases), probably, are the result of water evaporation, while figures –2‰ (16%) which may decrease up to –7.0…–9.5‰ at the peaks of evaporite episodes, are caused by water heating (sometimes very significant) during the basin shallowing. The activation of continental water role accompanying evaporite episodes is supported by the fact that the 87Sr/86Sr ratio in the Osveya horizon gypsum (0.708402–0.708742) is much higher than that of the Eifelian seawater according to a global estimate (0.70772), while δ34S of gypsum (15.5–21.2‰) in more than a half of the analyzed samples is lower than figures characteristic of the Middle Devonian seawater sulphate (19–20‰).