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‰).

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

The behavior of stable carbon and oxygen isotopes in carbonates during the deposition and diagenesis of sediments in the bioproductive Upper Famennian Pripyat Trough (southern Belarus) is discussed. Limestones and clayey limestones (Corg 0.92 ± 0.11%) are characterized by very low δ13C values (–9.6 ± 0.3‰). Parental sediments of these rocks were deposited in the shallow-water zone during slow downwarping episodes of the seafloor. Lithification of the sediments took place in oxidative conditions of the diagenesis zone. Organic matter was actively oxidized by free oxygen. Carbon dioxide with isotopically light organic carbon formed in this process was used during the crystallization of diagenetic carbonates that are visually indiscernible from the sedimentational variety. Marls, clayey marls, and carbonate-bearing clays (Corg 6.02 ± 0.80%) are characterized by δ13C values as high as –3.5 ± 0.6‰. In combustible shales (Corg >10%), δ13C value is ‒1.2 ± 0.6‰). The clayey rocks mark the episodes of sedimentation in relatively deep-water conditions that appear during the uncompensated sagging of the basin floor. The diagenetic zone with free oxygen was significantly decreased or absent at all. Here, oxygen of marine sulfates was the main or single oxidizer of organic matter (sulfate reduction). The sulfate oxygen is a weaker oxidizing agent than free oxygen. Therefore, much more organic matter was retained and fossilized in clayey rocks than in carbonate rocks. Organic carbon released during the sulfate reduction and mobilized later for the diagenetic carbonate formation was insufficient for the significant decrease of δ13C values relative to the marine carbonate standard. Isotopic composition of carbonate oxygen in the studied rocks is invariable and does not depend on the content of clay and organic matter. In these rocks, δ18O values are at the level (approximately –5‰) shown for the Famennian in the Global Chemostratigraphic Chart. This fact is consistent with the supposition that oxygen isotope composition of atmospheric precipitates, which influenced the rock formation via continental flow, is close to that of sea water in low latitudes where the territory of Belarus was located in the Late Devonian.

The paper describes the results of study of the Silurian clayey–carbonate rocks ranging from the Telychian Stage (Llandovery) to the Gorstian Stage (Ludlow) recovered by the Borehole Davtyuny 3k in northwestern Belarus. Rocks of the Sheinwoodian Stage demonstrate a positive excursion of δ 13 C with amplitude of 4.7‰, marking the Ireviken biotic event recorded in the global chemostratigraphic curve. Values of δ 18 O for the carbonate material in the studied section (25.5–29.2‰ SMOW) are close to those for Silurian rocks from the Baltic region, Scandinavia, Ukraine, Poland, and Canada. The whole section contains postsedimentary gypsum as nodules and the infilling of fissures and fenestrae. Values of δ 34 S in gypsum (21.3–26.7‰ CDT) are close to those for the Silurian rocks on the Phanerozoic isotope plot. The formation of gypsum was related to a partial development of the supralittoral environment over the sublittoral and littoral clayey–carbonate substrate. The seawater accumulated in lowlands of the supralittoral plain after storms was intensely concentrated during arid conditions and accumulated in the clayey–carbonate sediment. The subsequent underground evaporation promoted the formation of gypsum as nodules in the unlithified sediments and the infilling of fissures and fenestrae in the lithified rocks.

Gypsum was found in the Silurian deposits of Belarus for the first time. It composes nodules and fills cracks and fenestras in clayey-carbonate rocks revealed by the Davtiuny 3k borehole (Grodno area, Ostrovets region). With the use of the sulphur isotope data, it is shown that gypsum was formed at the diagenetic stage in the Silurian seawater concentrated under interground evaporation. The process was the result of a repeated superimposition of supratidal environment onto clayey- carbonate deposits formed in shallow subtidal and intratidal conditions.

Peculiar features of evaporitic process at the stage of potassium accumulation are considered on the basis of carbon and oxygen isotope data on carbonate rocks and sulfur isotope data on anhydrite from the Famennian potassium-bearing subformation of the Starobin potassic salt deposit in the Pripyat trough. It was found that potassium accumulation was accompanied by the influx of continental waters and highly concentrated brines, while the formation of thick salt-free units was related to the replenishment of fresh seawater to the basin.

В статье на основе интерпретации данных об изотопном составе углерода и кислорода в карбонатном материале и серы в ангидрите фаменской калиеносной субформации Старобинского месторождения калийных солей в Припятском прогибе рассматриваются особенности развития эвапоритового процесса на стадии калиенакопления. Установлен приток континентальных вод и высококонцентрированных рассолов в бассейн калиенакопления, а также поступление новых порций морской воды при формировании мощных бессолевых пачек.

The isotopic composition of oxygen and carbon was studied in accessory carbonates and quartz separated from salts in Upper Devonian halogenous formations of the Pripyat Trough (Belorus). It is established that isotopic characteristics vary in a wide range. Values of δ18O vary in the following range (SMOW): from 18.2 to 29.2‰ in calcites, from 15.7 to 32.5‰ in dolomites, and from 17.4 to 27.2‰ in quartz. Values of δ13C range from –13.4 to 1.4‰ in calcites and from –11.1 to 1.7‰ in dolomites (PDB). Results obtained indicate highly variable isotope-geochemical conditions of sedimentation and early diagenesis during the formation of evaporitic sediments. Accessory minerals were repeatedly formed in a wide temperature range and probably at various stages of the lithogenesis.

It is shown that strontium and sulfur isotope ratio distribution observed during replacement of anhydrite by gypsum is evidence of metasomatic nature of the process. It involves simultaneous dissolution of anhydrite and gypsum formation.

It has been shown that patterns of sulfur isotope distribution in chloride and sulfate type evaporite basins are essentially different. Sulfur isotopic composition of sulfate minerals in sulfate type basins becomes lighter from the gypsum stage to the chloride one. An opposite trend occurs in chloride type basins. Sulfur isotope ratio of pyrite formed at the chloride stage in sulfate type basins is much lower with respect to the initial sulfate than in chloride type basins.The differences are explained in the terms of chemical systems that are open or closed with respect to sulfur during the sulfate reduction process.

Variations of carbon isotopic composition of modern mollusk shells within the area of Belarus were determined. Interspecific isotopic variations within biotopes were revealed to be less significant than among them. The C-13 content of lacustrine shells decreases with the eutrophication process development. The delta(13)C values of shells from river sites were significantly lower than those of lacustrine shells.

The diagenetic sulphide formation in the Carboniferous coal-bearing sequence of Belarus and the epigenetic nature of sulphides from the Jurassic one are revealed on the basis of the sulphur isotope investigation.

It has been established that the oxygen isotopic composition of water from the Devonian sea of normal salinity within the territory of Belarus didn't differ considerably from the composition of modern ocean water. The Devonian water evaporation was accompanied by the growth of the delta O-18 value up to +5,5-+6,6 parts per thousand at the sulphate stage and by posterior inversion. At the halite stage, the delta(18)O value depletion was as much as 6-11 parts per thousand compared to the maximum value. The subsequent brine evaporation didn't cause the isotope ratio change.

The carbon and oxygen isotopic composition of accessory calcite and dolomite from salt rocks and intrasalt carbonate layers in the Upper Frasnian and Upper Famenian evaporite formations of the Pripyat' trough was studied. A wide range of delta(13)C, delta(18)O variations were established, (-17.5 divided by +1.4 parts per thousand) and (+15.7 divided by +36.4 parts per thousand) correspondingly,The carbonate matter with the revealed combination of delta(13)C and delta(18)O could be formed during the sedimentation and diagenetic stages on sabkhas, subaerally exposed islands, on evaporite depression slopes washed by sea and fresh water and in stagnant zones with hot brines.

Concentration of brines from the halite to carnallite stage in the Pripyat Late Famennian basin was revealed to be accompanied by monotonous increase in C-13 of disseminated carbonates. The accessory dolomites from the rock salt comprise stoichiometric to calcian types depending on the degree of brine metamorphization.

It was revealed on the basis of the carbon isotope data that carbonate bodies of nodular limestones in the Upper Devonian strata of the Pripyat Trough had been formed as a result of the depositional bed deformation but not of concretionary process.

The isotopic composition of carbon and oxygen of carbonate concretions from inter-salt deposits of the Pripyat Trough is descussed. On the basis of the carbon isotopic data two groups of concretions representing analogy of domanic and bazhenov types of diagenesis have been established. The oxygen isotope composition of concretions and rocks differs unconsiderably.