RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 9, ES1002, doi:10.2205/2007ES000221, 2007

Discussion

Correlation of 87Sr/86Sr Ratios of the Permian From Pechishchi and Associated Sections With the Global Data

[31]  Age estimations and correlation of sediments using the 87Sr/86Sr ratio are based on its regular variations over geological time. On the one side, the comparison of measured 87Sr/86Sr ratios in marine carbonates with the Phanerozoic 87Sr/86Sr curve allows the study of the position of samples relative to this curve. On the other side, the 87Sr/86Sr ratio can be used for correlating sections.

2007ES000221-fig02
Figure 2
[32]  Let us correlate the acquired 87Sr/86Sr data on the studied section with the Phanerozoic 87Sr/86Sr curve for an interval of 200-350 million years (Figure 2). The 87Sr/86Sr ratio of 0.70815 in the Carboniferous sample (Point 1 at ca. 301 Ma) is in agreement with the curve. Two Asselian samples (upwards) have 87Sr/86Sr ratios of 0.70810 (Point 2 at ca. 299 Ma) and 0.70854. Only the first ratio is in agreement with the curve. The second one substantially deviates upwards. The Sakmarian sample has a strontium ratio of 0.70775 (Point 3 at ca. 279.6 Ma) corresponding to the Artinskian but not to the Sakmarian of the Phanerozoic curve. The Lower Kazanian (or Roadian/Guadalupian) middle Spirifer limestones have a 87Sr/86Sr ratio of 0.70769 (Point 4 at ca. 279.5 Ma) corresponding to the Artinskian. The ratio of 0.70727 (Point 5 at ca. 268 Ma), determined for the Sloisty Kamen member, corresponds to the boundary between the Roadian and Wordian. The ratio of ca. 0.70738 (Point 6 at ca. 252 Ma), determined for the Perekhodnaya member, is close to the 87Sr/86Sr average ratio of 0.70740. The position of this point on the curve cannot reflect real age corresponding to the end of the Kazanian, probably, due to the dominant effect of the considerable isolation of the local sedimentation basin from the ocean.

[33]  The discrepancies observed at some points (Figure 2) can be explained by the following:

[34]  1. local features of the Permian deposition in the eastern part of the Russian Plate (the evaporitic trend and considerable isolation from the ocean),

[35]  2. problems with the chronostratigraphic positioning of the 87Sr/86Sr Phanerozoic evolution curve and local 87Sr/86Sr curves within the Permian.

[36]  The obtained isotope ratios cannot be reliably correlated with the global curve without determining the absolute age of rocks.

[37]  Generally, the resulting 87Sr/86Sr isotopic ratios from the Permian of the Pechishchi section are well correlated with the global curve. It should also be emphasised that the global curve of 87Sr/86Sr variations may contain errors. Some ambiguities relating to the global character of this curve are as follows:

[38]  1. The Permian portion of the curve is controlled stratigraphically without absolute dating.

[39]  2. The character of the curve is in fact not absolutely global. It only contains samples from a number of outcrops, and their age was determined using local stratigraphic scales.

[40]  However, general evolutionary features of the Permian palaeobasins are preserved over geological time.

The Nature of 87Sr/86Sr Variations in Carbonate Deposits of the Pechishchi Section

[41]  It can be assumed that the dispersion of the 87Sr/86Sr isotope ratio in the Permian carbonate samples from the Pechishchi section is mostly due to strontium isotope variations that took place in the seawater of the Permian ocean. In some cases, 87Sr/86Sr changes can be related to local variations of this ratio in the Permian basins in the eastern portion of the Russian Plate that were separate from the ocean due to the fall of the ocean level. In some samples, 87Sr/86Sr changes occur against the background of geochemical variations (Mn/Sr and Fe/Sr deviations in the Asselian, the middle Spirifer limestone and Layer 27 of the Upper Kazanian; see Figure 1) probably caused by the secondary redistribution of strontium and by possible changes in the 87Sr/86Sr isotope ratio.

[42]  The Early Permian sea basins of the eastern portion of the Russian Plate are characterised by high 87Sr/86Sr ratios corresponding to the global 87Sr/86Sr ratios in the ocean at that time (Figure 2).

[43]  A decrease in the 87Sr/86Sr ratio, recorded for the Early Kazanian and early Late Kazanian, generally corresponds to the global curve (Figure 2) and indicates the connection between sedimentation basins and the sea, but local 87Sr/86Sr ratios are relatively high reflecting the role of continental water currents that carried heavy strontium. At later times, the salinity of the basin apparently increased, probably due to the aridisation of the climate. Salinisation of the Kazanian palaeobasins over time is indicated by a considerable increase in the content of dolomite (relative to the volume of carbonate rocks), from 40% in the Lower Kazanian to 85% in the Upper Kazanian, and by a two-fold increase in the content of sulphates in the Upper Kazanian relative to the Lower Kazanian [Sementovsky, 1973]. The generally low background 87Sr/86Sr Sr ratios on the local and global curves indicate the gradual drying of continental currents under arid conditions on the vast Pangaea continent, although their local effect could periodically increase 87Sr/86Sr ratios as demonstrated by a local maximum of 0.70766 in Layer 27 (Figure 1).


RJES

Citation: Nurgalieva, N. G., V. A. Ponomarchuk, and D. K. Nurgaliev (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.

Copyright 2007 by the Russian Journal of Earth Sciences

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