Middle and Late Jurassic radiolarian palaeoecology versus carbon-isotope stratigraphy

Abstract

Jurassic radiolarians are sensitive to palaeoclimatic–palaeoceanographic changes, recorded by the stable carbon-isotope curve. Carbon-isotopic variations have been linked to changes in ocean structure, productivity and concentrations of greenhouse gases in the atmosphere. In the Middle–Upper Jurassic Terminilletto section of the Umbria–Marche–Sabina Apennines (central Italy), well preserved radiolarians are present in a sequence of cherty limestones interbedded with platform-derived carbonates. The δ 13C curve in the Aalenian–Kimmeridgian interval indicates three main positive δ 13C `events' in the lower Bajocian, late Bathonian–lower Callovian and middle Oxfordian. The positive δ 13C shifts are correlated to increases in visible 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 with δ 13C and chert abundance values, and seem to be related to episodes of high nutrient mobilisation. From the base of the Bajocian, the radiolarian associations show a slight tendency towards diversification, coincident with increasing δ 13C values. In the upper part of the middle Bajocian, last appearances prevail over first appearances associated with the peak of the δ 13C event. This may indicate maximum eutrophic conditions. Coincident with the late Bathonian–early Callovian positive δ 13C shift, the radiolarian associations record a different trend with respect to the Bajocian event. When the δ 13C values are increasing to a maximum, radiolarian associations tend to be reduced in species diversity, followed by a small diversification trend when the δ 13C values are decreasing. We speculate that during the Callovian a more `stressful' palaeoenvironment, due to high level of eutrophication, is indicated by the combination of high productivity of biogenic silica and low carbonate input. Although varying preservational conditions may have influenced the observed trends in the radiolarian associations, the good correlation between changes of sedimentation patterns, radiolarian assemblages and δ 13C positive anomalies, open interesting avenues for further investigations concerning the palaeoecology of Mesozoic radiolarians.