Arctic reappraisal of global carbon-cycle dynamics across the Jurassic–Cretaceous boundary and Valanginian Weissert Event

Abstract

Late Jurassic – Early Cretaceous global carbon-cycle dynamics have mainly been inferred from Tethyan, Atlantic and Pacific carbon-isotope (δ13C) records. These records indicate a steady deceleration of the carbon cycle in Late Jurassic and Jurassic–Cretaceous (J–K) boundary times followed by a major mid-Valanginian carbon-cycle perturbation demarcated by the well-documented positive excursion named the Weissert Event. However, this model remains to be confirmed in Boreal records, which are rare and typically difficult to correlate with the global δ13C signal. In this study, the first continuous and biostratigraphically calibrated organic carbon-isotope (δ13Corg) stratigraphy is established for the Bathonian – lower Barremian (representing >30 Myr) of the Boreal Realm, based on high-resolution sampling of bulk-rock marine shale from four time-equivalent sections in Arctic Svalbard, supplemented by existing δ13Corg data. The δ13Corg stratigraphy displays several distinctive trends, notably: (i) a Kimmeridgian positive excursion, followed by a large negative excursion (≤6.4‰) in the upper Kimmeridgian – middle Volgian, consistent with existing Boreal δ13C records; (ii) an upper Volgian – Ryazanian overall positive recovery across the J–K boundary; and (iii) a prominent positive excursion (≤5.5‰) in the Valanginian, interpreted to represent the Weissert Event, followed by steady, high values into the lower Barremian with no apparent excursion decay. Time-calibrated correlation of the observed trends with existing δ13C records representative of the entire exchangeable carbon reservoir reflects major global carbon-cycle adjustments, including: (i) Boreal decoupling from the global carbon reservoir during Late Jurassic and J–K boundary times due to basinal isolation; and (ii) global carbon-cycle recoupling at the onset of the Weissert Event coinciding with incipient oceanographic connectivity. Boreal carbon cycling was inherently controlled by the basin configuration. The new δ13Corg stratigraphy provides a well-calibrated tool for Boreal and Boreal–Tethyan correlations, and confirms that the Weissert Event represents an important perturbation of the global carbon cycle.