The Jurassic/Cretaceous transition was accompanied by significant changes in palaeoceanography and palaeoenvironments in the Tethyan Realm, but outside the Tethys such data are very scarce. Here we present results of a study of the most complete section in the Panboreal Superrealm, the Nordvik section. Belemnite δ 18 O data show an irregular decrease from values reaching up to +1.6‰ in the Middle Oxfordian and from +0.8 to −1.7‰ in the basal Ryazanian, indicating a prolonged warming. The biodiversity changes were strongly related to sea-level oscillations, showing a relatively low belemnite and high ammonite diversity during sea-level rise, accompanied by a decrease of the macrobenthos taxonomical richness. The most prominent sea-level rise is marked by the occurrence of open sea ammonites with Pacific affinities. Peak abundances of spores and prasinophytes correlate with a negative excursion in organic carbon δ 13 C near the J/K boundary and could reflect blooms of green algae caused by disturbance of the marine ecosystem. Keywords: Biodiversity; stable isotopes; J/K boundary; Arctic Realm; palaeoceanography (Published: 25 March 2014) Citation: Polar Research 2014, 33 , 19714, http://dx.doi.org/10.3402/polar.v33.19714