Re–Os geochronology of Arctic black shales to evaluate the Anisian–Ladinian boundary and global faunal correlations

Boreal and Tethyan faunas differ throughout the Triassic, limiting the use of biostratigraphy for global correlation. Rhenium–osmium (Re–Os) dating of organic-rich black shales, calibrated with existing biostratigraphy, provides a new means to establish correlations of global fauna using absolute time. Here we present Re–Os radiometric ages for Middle Triassic organic-rich shales from two biostratigraphically defined sections at Svalbard and the Svalis Dome in the Barents Sea. Mature black shales from Svalbard, inferred to be Middle to Late Anisian, define a 241.2 ± 2.2 Ma isochron with an initial 187Os/188Os ratio of 0.831 ± 0.025, the highest seawater Os isotope ratio yet recorded between the Cambrian and Lower Jurassic. Svalis Dome shales in the uppermost Anisian yield a 239.3 ± 2.7 Ma age and initial 187Os/188Os ratio of 0.679 ± 0.020. The higher initial 187Os/188Os ratio for Svalbard shales most likely reflects global seawater at the time of deposition, whereas the lower initial 187Os/188Os ratio for the Svalis Dome may represent seawater with limited communication to the open ocean, as suggested by the ratio of trace metal abundances and total organic carbon contents. These age data indicate a maximum age of 239.3 ± 2.7 Ma for the Anisian–Ladinian boundary in the Arctic Boreal realm. This age agrees with the proposed boundary in the Tethyan realm, 240 to 243 Ma, and thus confirms the correlations between Boreal and Tethys based on biostratigraphy and magnetostratigraphy. Our study affirms that Re–Os geochronology of black shale is a reliable method for obtaining depositional ages and establishing biostratigraphic correlations between paleogeographically separated regions.