Reliability of the Strontium Isotope Stratigraphy approach in marginal seas: Insights from the Miocene Paratethys

Abstract

The robustness of the Strontium Isotope Stratigraphy (SIS) as a chemostratigraphic tool was tested in the Miocene epicontinental Central Paratethys using new ⁸⁷Sr/⁸⁶Sr results obtained for 129 foraminiferal tests and various marine invertebrate shells that were converted to numerical ages using the global ⁸⁷Sr/⁸⁶Sr curve for the Phanerozoic open oceans. These ages were compared with those expected from either magnetostratigraphic, radiometric or biostratigraphic studies applied to selected sections from different Central Paratethys basins. The results from different basins showed variations in ⁸⁷Sr/⁸⁶Sr values for specific time intervals; therefore, it is not possible to construct a unique ⁸⁷Sr/⁸⁶Sr curve for the Central Paratethys region that can be used for chemostratigraphic purposes. The ⁸⁷Sr/⁸⁶Sr signal in semi-enclosed basins tends to reflect the lithology of the source areas. For example, marine basins surrounded by areas of Mesozoic carbonate rock cover are associated with a reduced ⁸⁷Sr/⁸⁶Sr signal compared to the global ⁸⁷Sr/⁸⁶Sr signal primarily due to the drainage of these Mesozoic carbonate rocks and transfer of their specific Sr signal. By contrast, areas affected by continental runoff from crystalline terrains in combination with limited connection to surrounding marine domains were found to show an elevated ⁸⁷Sr/⁸⁶Sr signal compared to the global one. Additionally, it was demonstrated that infaunal, eutrophic and hypoxic species (foraminiferal genus Uvigerina and molluscs) show a shift towards a relatively higher ⁸⁷Sr/⁸⁶Sr signal. Therefore, our data show that variable ⁸⁷Sr/⁸⁶Sr patterns occur in semi-enclosed domains, primarily reflecting local settings and thus hampering reliable SIS ages. In particular, the best SIS results in the studied Paratethys area occur during periods of maximum marine connectivity, when other (biostratigraphic) methods can be applied, and then become unreliable as connectivity diminishes.