The latest Jurassic–earliest Cretaceous climate and oceanographic changes in the Western Tethys: The Transdanubian Range (Hungary) perspective

Abstract The Jurassic/Cretaceous transition was characterized by several distinct palaeoenvironmental processes and events, amongst which some of the best known are the late Tithonian–early Berriasian aridization and the late Berriasian tectonic reactivation in the Neotethyan Collision Belt. This study aims to reconstruct the latest Jurassic–earliest Cretaceous palaeoenvironment and its evolution in the area of the Transdanubian Range (Hárskút and Lókút successions, Hungary), and provide new data on the relation between climate changes, palaeoceanography and marine ecosystems. Herein, calcareous nannofossil and geochemical data are presented and put into a geochronological framework in order to evaluate and compare palaeoenvironmental signals recorded in the two studied successions; the resultant scenario is compared with published literature data from both the western Tethyan region and northern Europe. Accordingly, in the Transdanubian Range, the relatively humid climate of the early Tithonian was followed by the late Tithonian–early Berriasian arid phase and the late Berriasian humidification. Besides, two intervals of palaeoceanographic perturbations were documented in the upper Tithonian–lowermost Berriasian (OD I), and the lower/upper Berriasian transition beds (OD II); these are manifested by the record of seafloor hypoxia and elevated accumulations of micronutrients, as well as changes in calcareous nannofossil assemblages. In the view of this study, the most probable trigger of the late Tithonian–early Berriasian aridization was a restriction in the atmospheric circulation (including monsoons), which was forced by climate cooling and lowering thermal gradient between landmasses and the ocean. Under such conditions, the mechanism of a wind‐induced water mixing might become less efficient, driving seawater stratification, seafloor hypoxia and increased burial of nutrients. Although the humid climate of the late Berriasian likely resumed the monsoon‐type circulation, the tectonic reactivation and uplift in the NeoTethyan Collision Belt might have effectively separated the Bakony Basin from the ‘open’ NeoTethys, which hampered the marine circulation and efficient water mixing in the former.