Palaeoclimate and palaeoenvironmental changes through the onset of the Valanginian carbon-isotope excursion: Evidence from the Polish Basin

The Valanginian Weissert episode of environmental change is associated with a positive carbon-isotope excursion (CIE) in δ¹³C_carb and δ¹³C_org records and a crisis in pelagic and neritic carbonate production. The CIE was interpreted to represent the first oceanic anoxic event (OAE) of the Cretaceous, linked with the formation of the Paraña-Etendeka large igneous province (LIP). Recent studies suggest though that the extent of oceanic anoxia was limited to high-latitude epicontinental seas and the Pacific, and that the Paraña-Etendeka LIP postdates the Valanginian CIE. With these new interpretations, the palaeoenvironmental changes behind the observed perturbation of the carbon cycle remain to be elucidated. Here we present sedimentological and geochemical results from a drill core near Wąwał, central Poland. The excavated succession is of particular interest due to its near-shore depositional setting within the former Polish Basin and its preservation (up to 17wt.% of aragonite). The core consists of lower to upper Valanginian silty to sandy clays deposited under fully marine conditions on top of an Upper Jurassic karstified limestone. A change in weathering mode from very humid and highly hydrolysing towards less humid and seasonally contrasted conditions is indicated by the abrupt change from a kaolinite- to a smectite-dominated clay-mineral association near the boundary between the early and late Valanginian. Moreover, two phosphate-rich horizons were identified, which correlate in time to northern Tethyan occurrences in the Helvetic Alps. The lower level is associated with the early Valanginian transgression and corresponds to peak humidity recorded in the Wąwał core. The upper layer corresponds to the paroxysmal phase of the Valanginian crisis in carbonate production. In the Wąwał core, evidence for anoxic conditions during the Valanginian CIE is lacking. The climatic conditions and changes identified in the Polish Basin are different from those of the northern Tethyan area, where an increase in humidity is observed near the early-late Valanginian boundary. The contrasting climate conditions are probably an expression of latitudinal shifts in the position of the northern mid-latitude humid belt.