Stable isotope records (O, C) of Jurassic aragonitic shells from England and NW Poland: Palaeoecologic and environmental implications

Shells of fully marine Middle to Upper Jurassic molluscs from England and north-western Poland were analysed with respect to their stable isotope (δ18O, δ13C) compositions, and palaeoecological and environmental life conditions of these molluscs were inferred from them. Light microscopical and SEM inspection and an analysis of the minor element content (Fe, Mn, Mg, Sr) suggest rather unaltered isotope signals. The δ18O and δ13C values show a characteristics distribution among three groups of co-occurring organisms. Benthic (adult) bivalves generally preserved higher δ18O and δ13C values than ammonites, whereas planktic bivalve larvae tend to possess the lowest δ18O but higher δ13C than adult bivalves. As this distribution pattern is found in numerous horizons and sections of Bathonian to Kimmeridgian age in NW Poland and England, it is thought to reflect real palaeoenvironmental parameters. All observations can be incorporated in a single model that assumes (i) seasonally induced temperature stratification of the water column, (ii) a correlation between phytoplankton blooms and reproduction season of planktic-planktotrophic bivalves, and (iii) insignificant vital effects with respect to the δ13C in bivalves, but strong biological control in ammonites. In addition, the δ18O evolution suggests that the Late Bajocian to Middle/Late Bathonian and Early Oxfordian to Late Kimmeridgian were considerably warmer than the latest Bathonian to Late Callovian time interval. The oxygen isotopic records from other European regions indicate a similar pattern of long-term palaeotemperature evolution. The comparatively high water temperatures during the Callovian to Oxfordian of the Isle of Skye (NW Scotland) are enigmatic, however. In the Early Oxfordian, sea surface and bottom temperatures began to rise in continental Europe and England. These changes coincide with a south-westward drift of the West European crustal plate, but a causal relationship remains to be demonstrated. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.