Paleoenvironmental controls on the morphology and abundance of the coccolith Watznaueria Britannica (Late Jurassic, southern Germany)
Résumé
The coccolithophore species Watznaueria britannica is dominant in Middle-Upper Jurassic calcareous nannofossil assemblages and presents morphological variation, including different coccolith size, shape and length of the central area and of the bridge. Six morphotypes can be recognized in the polarizing light microscope. The aim of this work is to better understand the morphological variability of W. britannica and determine if this variability is controlled by paleoecological factors. In order to investigate the potential paleoecological controls on W. britannica morphology and abundance, we carried out a biometric study on a restricted temporal interval: the Late Oxfordian, in the Swabian Alb (southern Germany), characterized by increasing carbonate production linked to climatic changes. The Balingen–Tieringen section, where previous works on sedimentology, nannofossil assemblage composition, and δ18O and δ13C analyses were performed,was selected for this study. The variations in morphology and abundances ofW. britannicawere studied on 40 samples of the Balingen–Tieringen section, presenting variable lithologies and calcium carbonate contents. For each level, seven biometric parameters (coccolith length, width and ellipticity, central area length, width and ellipticity and central area proportion with respect to the coccolith) were measured or calculated on digitally captured images of the first 100 W. britannica coccoliths observed in the light microscope. The relationships between the different biometric variables were described using bivariate and Principal Component Analyses. Biometric parameters and Principal Component factors extracted from nannofossil assemblages as well as other paleoenvironmental proxies, were investigated using regression, and their stratigraphic trends were compared. Principal component analysis of the six biometric variables (3938 measurements) on W. britannica coccoliths shows a reduced morphological variability compared to a significant size gradient. An allometric trend recognized on the total placolith and on the central area within theW. britannica assemblages suggests that the different morphotypesmay represent intra-specific variability rather than different species.The general trend through Late Oxfordian shows an increase in size ofW. britannica coccoliths,mainly driven by an increase in the contribution of the largemorphotypes. Increasing placolith size is associated with drier and warmer climatic conditions during the latest Oxfordian.