The causal link between changes in calcareous nannofossil abundances, their size and the palaeoenvironmental conditions in the Tethyan Realm across the Valanginian Weissert episode was examined through high-resolution quantitative analyses of the genera Nannoconus and Micrantholithus from the Angles section (Vocontian Basin, SE France). A set of published mineralogical, micropalaeontological and geochemical data from numerous European localities (including nannofossil data from Italy, France and Romania) and from the Atlantic and Pacific Oceans, allows us to propose a coherent palaeoecological and palaeoceanographic model, and to discuss the possible forcing factors behind the palaeoenvironmental changes that occurred during the Valanginian. In the Vocontian Basin, the genus Nannoconus appears to be particularly sensitive to global fertility conditions and local continental input while Micrantholithus is clearly associated with salinity levels. During the first phase of the Weissert episode, global increase in nutrients, together with a breakdown of the upper water column stratification explain the nannoconid decline. Elevated freshwater fluxes into the Vocontian Basin under warmer and more humid conditions account for pentalith decreases. Low-resolution data on pCO2 and ocean Ca2 + concentrations cannot highlight large-magnitude rapid fluctuations in pCO2 and oceanic Ca2 + if they exist. However, relatively low pCO2 levels and high oceanic Ca2 + concentrations probably helped the recovery of Nannoconus and Micrantholithus from the aftermath of the Weissert episode to the early Hauterivian. The long-term changes observed in the Valanginian nannolith distribution are consistent with increased hydrothermal activity that influenced fertility levels. Low- to mid-latitudinal monsoonal circulation probably contributed to their short-term changes by the way of rapid changes in the chemical weathering pattern that triggered significant fluctuations in turbidity, fertility, and salinity conditions.