Vascular plant biomarkers as proxies for palaeoflora and palaeoclimatic changes at the Dogger/Malm transition of the Paris Basin (France)

Abstract

Vascular plant biomarkers are used here as a chemostratigraphic tool for tracing both palaeoflora and palaeoclimatic changes during the Callovian and Oxfordian in the Paris Basin. The retene/cadalene ratio shows low values in claystones from the Middle Callovian to the end of the Lower Oxfordian, then increases drastically from the end of the Lower Oxfordian ( cordatum Zone) synchronously with the progressive development of the Oxfordian carbonate platform. The relative proportion of retene, a conifer biomarker, is strongly correlated with the abundance of some of its diagenetic precursors and with saturated diterpanes. Some of these are very common and abundant in the Pinaceae family while specific markers for non-Pinaceae conifers are not detected. Thus, the increase in the retene/cadalene ratio around the cordatum Zone reflects the rising proportion of Pinaceae on the London-Brabant Massif, which was the nearest land from the locations studied. As Pinaceae show many morphological adaptations to dry climate compared to the other plants reported in the Jurassic taphofloras of the Paris basin, their increasing proportion indicates an increasing aridity. This climatic change is also supported by published geochemical, palaeobiogeographical, palynological, palaeobotanical and mineralogical data. A comparable increase in the relative proportion of retene was also described for the Oxfordian of Western Australia, which could be an argument for a worldwide increase in aridity at the end of the Lower Oxfordian.