ISOTOPIC EVIDENCE FOR LATE JURASSIC-EARLY CRETACEOUS CLIMATE CHANGE

Abstract

Strontium-, oxygen- and carbon-isotope ratios have been determined from Late Jurassic–Early Cretaceous belemnites from the Volga Basin, Russia, and Kawhia Harbour, New Zealand. 87Sr/86Sr ratios derived from well-preserved belemnites from the Volga Basin support a Middle Tithonian age derived from the analysis of the endemic ammonite fauna. The Kawhia Harbour section records a gradual rise in 87Sr/86Sr values and in comparison with the published 87Sr/86Sr curve suggests that the lower part of the section is latest Oxfordian in age, whilst the upper part of the section correlates well with the biostratigraphic correlation suggestion of an Early–Middle Tithonian age. Although the published strontium calibration curve shows a degree of scatter, our new data confirm the uniform rise in 87Sr/86Sr values from the Late Jurassic into the Early Cretaceous. Such an increase may result from either a decrease in mid-oceanic ridge spreading and/or an increase in weathering rates and flux of radiogenic strontium, although a eustatic drop in sea level and concurrent Western Cordillera uplift suggests that weathering may have been the controlling factor of Late Jurassic seawater strontium-isotope composition. Palaeotemperatures derived from the well-preserved belemnite δ18Ocarb values from the Volga Basin indicate that the Middle Volgian (Late Kimmeridgian) was warm (∼14–20°C), followed by a slight cooling and a subsequent gradual increase to the Jurassic–Cretaceous boundary. The δ18Ocarb values from New Zealand (located at a palaeolatitude of ∼80°S), if interpreted in terms of palaeotemperature, indicate a high degree of variability. Such variability may not be related to palaeotemperature, but to changes in oceanic chemistry resulting from the formation and dissolution of an ice-sheet and/or snow during the Oxfordian–Kimmeridgian. Carbon-isotope trends for the Late Jurassic show a fall in values from the Oxfordian with lowest values occurring in the Early–Middle Tithonian, before rising but without reaching values obtained in the Oxfordian. The overall low δ13Ccarb may be related to a global increase in continental weathering and/or upwelling of cooler oceanic water enriched in oxidised organic carbon (12C-enriched).