Re-Os geochronology of the Middle to Upper Jurassic marine black shales in the Agardhfjellet Formation, Central Spitsbergen, Svalbard: A cornerstone for global faunal correlation and Os isotopic change

Abstract

Ammonite provincialism makes it difficult to link the Boreal and Tethyan realms during the Late Jurassic to Early Cretaceous time. Absolute ages through radiometric dating offer new age information that is independent and complementary to relative ages based on biostratigraphy. In this study, we report seven new Re-Os ages for black shales from the Agardhfjellet Formation, Svalbard. We also report pyrolysis results, carbon and sulfur stable isotopic data, and initial 187Os/188Os ratios to evaluate the paleoenvironment during organic-rich shale deposition. Re-Os ages are derived from three Boreal ammonite zones: (1) 159.2 and 160.1 Ma from the shale intervals containing Cadoceras sp.; (2) 149.6, 149.9, 150.8, and 151.9 Ma from the Rasenia cymodoce Zone in the Early Kimmeridgian, which is the equivalent of the Sutneria platynota and Ataxioceras hypselocyclum zones in the Tethyan realm; (3) 146.8 Ma from an upper Volgian interval above the occurrence of Laugeites sp.. Within this 14.5 Myr interval, the initial Os profile shows a gradually increasing trend from 0.335 (one of the lowest shale values throughout the Phanerozoic) to 0.529, consistent with previous initial Os studies and Sr isotopic ratio chemostratigraphic studies. The trend of increasing initial Os ratios indicates an increase in continental runoff relative to unradiogenic Os input from mantle sources, such as seafloor hydrothermal activity or ultramafic source rocks. We provide new Re-Os geochronological anchors in the Late Jurassic with new Re-Os ages and propose correlations of specific Late Jurassic ammonite zones between the Tethyan and Boreal realms. In turn, the gradual increase in seawater Os isotopic ratios may signal climate change in the Late Jurassic, indicating the increase of radiogenic Os or decrease of unradiogenic Os, induced by high continental runoff or less hydrothermal/volcanic activity.