In the Middle Urals the Carboniferous sequence composed of carbonate and terrigenous facies. The oxygen isotopic composition for the Lower and Middle Carboniferous limestones of the Western and Eastern Urals is around +25 - +28 and +19 per mil SMOW, respectively. Diagenetic siderite from the coal-bearing series of the Western Urals enriched in oxygen-18 (with an average of about 8-10 per mil) relatively to those of the Eastern Urals. The oxygen isotopic value in calcite concretions of the Eastern Urals ranging from +17.1 to +25 per mil SMOW.
Depletion of carbonates in oxygen-18 with age is well known and its interpretation is a controversial subject. Three alternative explanation have been offered. It is resulted from: 1) diagenetic alteration of rocks; 2) evolution of oceanic waters oxygen-isotopic composition; 3) decreasing of oceanic waters temperature. Recently it was found that mentioned trend has some abrupt shifts in Ordovician, Mississippian, Cretaceous and Oligocene (Hudson and Anderson, 1989). This variation is thought related to changes in climate and oceanic circulation (Railsback, 1990), accompanied by storing saline waters at depth and leaving in surface brackish ones, which are, consequently, responsible for precipitation depleted in oxygen-18 carbonates. All of these assumptions unsuitable for explanation the light-oxygen carbonate origin in the Eastern Urals.
Oxygen-isotope variation in diagenetic carbonates is more large than in the sedimentary ones, and is independent of their age. It could be caused not entirely by temperature changes of their precipitation media, placed in the subsurface levels of uncompacted fresh sediments, where today temperature is in much the same as in bottom waters. Probably
this relationship between bottom and initial waters existed
in the Phanerozoic, during which deep waters temperature fluctuation was in range from +2 to +14 °C (Schopf, 1980).
Taking into account this fact we accept average temperature for the initial waters of the Phanerozoic sedimentary basins about + 7°C. Using latter and oxygen isotopic value of diagenetic carbonates by equations expressing oxygen isotopic fractionation with temperature in carbonate-water systems (Carathers, 1988; Friedman and O'Neil, 1977) have been calculated bottom waters oxygen-isotopic compositions for the Carboniferous basins of the Western and Eastern Urals as about -5 and -10 per mil SMOW, respectively.
These values are related to basin waters, where occurred deposition of terrigenous rocks, including diagenetic
carbonates. But according to geological date deposition of carbonate and terrigenous facies during the Carboniferous as in the Western, as well as Eastern Urals was synchronous and took place in the same basins. Our investigation shows that oxygen-isotopic composition of their waters was different, which could be responsible for oxygen-isotopic differences in precipitated here carbonates.
Carathers, W.W., Adami, L.H. & Rosenbauer, R.J., Geochim. Cosmochim Acta 52, 2445-2450 (1988).
Erickson, A.L., Deep Sea Drilling Project 19, 643-656 (1973).
Friedman, I.O. & O'Neil, J.R., US Geol. Surv. Prof. Paper 440 KK (1977).
Hudson, J.D. & Anderson, T.F., Roy. Soc. Sci. Edinburgh Trans. Earth Sci. 80, 183-192 (1989).
Railsback, L.B., Geochim. Cosmochim Acta 54, 1501-1509 (1990).
Schopf, T., Paleoceanography, Cambridge (1980).