Middle and Late Jurassic radiolarian palaeoecology versus carbon-isotope stratigraphy

Jurassic radiolarians are sensitive to palaeoclimatic-palaeoceanographic ch anges, recorded by the stable carbon-isotope curve. Carbon-isotopic variati ons have been linked to changes in ocean structure, productivity and concen trations of greenhouse gases in the atmosphere. In the Middle-Upper Jurassi c Terminilletto section of the Umbria-Marche-Sabina Apennines (central Ital y), well preserved radiolarians are present in a sequence of cherty limesto nes interbedded with platform-derived carbonates. The delta(13)C curve in t he Aalenian-Kimmeridgian interval indicates three main positive delta(13)C 'events' in the lower Bajocian, late Bathonian-lower Callovian and middle O xfordian. The positive delta(13)C shifts are correlated to increases in vis ible chert in the outcrop and may represent periods of increased biological productivity of surface sea-water. Spumellaria/Nassellaria ratios and the fluctuations of relative abundance of selected genera tend to correlate wit h delta(13)C and chert abundance values, and seem to be related to episodes of high nutrient mobilisation. From the base of the Bajocian, the radiolar ian associations show a slight tendency towards diversification, coincident with increasing delta(13)C values. In the upper part of the middle Bajocia n, last appearances prevail over first appearances associated with the peak of the delta(13)C event. This may indicate maximum eutrophic conditions. C oincident with the late Bathonian-early Callovian positive delta(13)C shift , the radiolarian associations record a different trend with respect to the Bajocian event. When the delta(13)C values are increasing to a maximum, ra diolarian associations tend to be reduced in species diversity, followed by a small diversification trend when the delta(13)C values are decreasing. W e speculate that during the Callovian a more 'stressful' palaeoenvironment, due to high level of eutrophication, is indicated by the combination of hi gh productivity of biogenic silica and low carbonate input. Although varyin g preservational conditions may have influenced the observed trends in the radiolarian associations, the good correlation between changes of sedimenta tion patterns, radiolarian assemblages and delta(13)C positive anomalies, o pen interesting avenues for further investigations concerning the palaeoeco logy of Mesozoic radiolarians. (C) 1999 Elsevier Science B.V. All rights re served.