Distribution of clay minerals in Early Jurassic Peritethyan seas: palaeoclimatic significance inferred from multiproxy comparisons.
Résumé
A set of published, unpublished, and new clay mineral data from 60 European and Mediterranean localities allows us to test the reliability of clay minerals as palaeoclimatic proxies for the Pliensbachian–Toarcian period (Early Jurassic) by reconstructing spatial and temporal variations of detrital fluxes at the ammonite biochronozone resolution. In order to discuss their palaeoclimatic meaning, a compilation of low-latitude belemnite δ18O, δ13C, Mg/Ca, and 87Sr/86Sr values is presented for the first time for the whole Pliensbachian– Toarcian period. Once diagenetic and authigenic biases have been identified and ruled out, kaolinite content variation is considered as a reliable palaeoclimatic proxy for the Early Jurassic. Major kaolinite enrichments occur during times of low δ18O, high Mg/Ca, and increasing 87Sr/86Sr, implying warm climates and efficient runoffs during the Davoei, Falciferum and Bifrons Zones. Conversely, cooler and drier times such as the Late Pliensbachian or the Late Toarcian are characterized by low hydrolysis of landmasses, and correspond to kaolinite depleted intervals. Secondary factors as modifications of sources or hydrothermalism may sporadically disturb the palaeoclimatic signal (e.g., in the Bakony area during the Late Pliensbachian). In addition, a spatial comparison of clay assemblages displays significant kaolinite enrichments towards northern parts of the Peritethyan Realm, probably related to the latitudinal zonation of hydrolyzing conditions. This implies enhanced runoffs on northern continental landmasses that reworked kaolinite-rich sediments from subtropical soils and/or Palaeozoic substrata.