RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 9, ES1002, doi:10.2205/2007ES000221, 2007
[2] The Permian system is a stratigraphically complex formation (particularly, in its upper portion) characterised by numerous regional stratigraphic patterns and, as a result, by ongoing arguments on the international stratigraphic scale. The causes of the existing diversity of viewpoints are well known. During the Permian period, a gigantic continental agglomerate - supercontinent Pangaea-2 - has been formed. By the end of the Permian, the sea level was extremely low. At the end of the Permian period, the greatest biotic termination occurred. Permian sediments accumulated near the land in isolated, endemic, shallow basins with specific faunas. Under the conditions of the global aridisation of the climate and the alternation of marine and continental environments, sediment erosions and depositional breaks became increasingly frequent and the stratigraphic record more complex. It is obvious that the differentiation and correlation of Permian and similar sequences require special tools to identify parameters reflecting the most universal, global changes in the development of the Earth, oceans and seas. Isotope stratigraphy, particularly strontium isotope stratigraphy, is becoming more and more popular as a reliable method to be used for the above purposes.
[3] Strontium isotope stratigraphy is increasingly frequently used for estimating the duration of stratigraphic breaks and intervals [McArthur et al., 2001) and for identifying marine and non-marine depositional environments [Poyato-Ariza et al., 1998; Schmitz et al., 1991]. The 87Sr/86Sr ratio changes regularly over geological time and, therefore, allows age estimation and correlation of sediments. Firstly, the comparison of measured 87Sr/86Sr ratios in marine carbonates with the Phanerozoic 87Sr/86Sr curve allows time correlation of the studied samples. Secondly, the 87Sr/86Sr ratio can be used for correlating sections formed at the same geological time. The latter problem can be solved without possessing detailed data on the strontium ratio's global trend, but this trend should nevertheless be taken into consideration to avoid ambiguities at its turning points.
[4] As a scientific school, strontium isotope stratigraphy was announced in 1948 with a publication [Wickman, 1948] showing that the decay of 87Rb and its transformation into 87Sr in the Earth's crust and its further penetration into the hydrosphere govern the strontium isotopic composition over geological time. The 87Sr/86Sr ratio was measured in organic carbonates [Peterman et al., 1970] to show that the strontium ratio in seawater decreased during the Palaeozoic, reached a minimum in the Mesozoic and again was increasing up until the present time.
[5] The present strontium content in seawater has been estimated at a sustained level of 7.75 ppm and the isotope ratio at 0.70920. The 87Sr/86Sr ratio is governed by the interaction of two major sources of strontium coming into the ocean: weathering zones of silicate minerals producing "river strontium" and hydrothermal sources in the mid-oceanic ridges producing "juvenile strontium" [Faure, 1989; Spooner, 1976]. River strontium is heavier than juvenile strontium. The mixing of strontium from these two sources forms the final strontium ratio.
[6] The general evolutionary trend of 87Sr/86Sr variations in seawater during the Phanerozoic has been demonstrated in the pioneering works on the subject [Burke et al., 1982; Peterman et al., 1970; Veizer and Compston, 1974]. The curve published by these authors served as the basis for further studies and conclusions [McArthur et al., 1994; Veizer et al., 1999].
[7] At the end of the Permian period, the global recession of the ocean level, the enlargement of the continent area and the lowering of the base level of erosion increased the 87Sr/86Sr ratio, but this phenomenon was preceded by the ratio's global minimum of 0.70685 dated ca. 260 Ma [Harland et al., 1990], i.e. as the Capitanian [Gradstein et al., 2004].
[8] The average rate of the 87Sr/86Sr ratio decreased to its minimum during 10 million years was 0.000062 Ma-1, and after passing the minimum it was 0.000097 Ma-1. Model calculations [Martin and Macdougall, 1995] showed that these changes were due to varying rates of sediment supply from rivers and juvenile oceanic sources. Weathering in the Late Permian became more intensive in combination with an increase in the content of carbon dioxide.
[9] The
87Sr/86Sr decrease at the end of the Sakmarian is associated with a sharply continental
arid climate, with a small number of waterways from the vast Pangaean land and, accordingly,
with a decrease in the river strontium content
[Harland et al., 1990].
A continuous decrease in
87Sr/86Sr until the Capitanian with its Permian minimum can
be only explained by a more significant phenomenon, such as the activation of light strontium sources.
These sources could be represented by Siberian Traps but these are dated as younger rocks: 248
4 Ma
by the U/Pb and zircon methods and 249
1.6 Ma by the Ar and biotite methods
[Canaghan et al., 1994].
A better explanation is provided by the Eimeshan basalts of Southwest China dated 230-280 Ma,
the major portion of which effused during the Late Permian
[Yin et al., 1992].
However, the most powerful source could lie in the oceanic spreading zones. Such zones, related
to Neotethys, can indeed be modelled
[Korte et al., 2006].
The Guadalupian Neotethyan sequences in Oman contain widely occurring pillow lavas overlain by
Wordian/Capitanian radiolarites or pelagic limestones
[Bechennec, 1988;
Pillevuit et al., 1997].
Thus, Neotethyan hydrothermal circulation supplied light strontium to the ocean and reduced
the
87Sr/86Sr ratio. A gradual increase in the
87Sr/86Sr ratio during
Lopingian time can be explained by the attenuation of basaltic volcanism in
Palaeotethys and by an intensified runoff of river water with radiogenic strontium into
the ocean. These rivers flowed down from vast continental humid areas. During the Permian,
these were mostly in a regressive phase
[Korte et al., 2006].
[10] This paper reviews the 87Sr/86Sr data acquired from one of the best known Permian open sections, Pechishchi, located on the right bank of the Volga river near the town of Kazan.
Citation: 2007), Strontium isotope stratigraphy: Possible applications for age estimation and global correlation of Late Permian carbonates of the Pechishchi type section, Volga River, Russ. J. Earth Sci., 9, ES1002, doi:10.2205/2007ES000221.
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