Evolution of the Carbon Cycle During Major Turning Point in Oceanogaphy in the Late Jurassic.

The Late Jurassic was a time of accelerated fragmentation of Pangea resulting in a reorganisation of climate and oceanography. Some of these changes culminated in the Oxfordian as reflected in the Sr-isotope record. The 87/86Sr ratio reach the lowest values of the Mesozoic, indicative for high seafloor spreading and/or low continental weathering rates. Oxygen isotope values suggest that the Late Callovian and the Early Oxfordian were an exceptionally cool time in the Jurassic. The opening Tethyan ocean was chosen for an investigation of the evolution of the global carbon cycle and of changing current patterns along the east-west trending seaway. The carbon isotope curve shows a large-amplitude shift from low values around 1.5‰ (VPDB) in the Early Oxfordian towards values of around 3‰ in the Middle Oxfordian (Transversarium ammonite zone). After these high values, the ratio decreases towards values of 2‰. We used bio- and carbon isotope stratigraphy to correlate the evolution of current intensity based on an expanded reference carbon isotope section from the Vocontian trough (France). Stable carbon isotope records provide a very good tool for correlation between different paleoceanographic settings. Strong oceanic currents sweeping the Tethyan seafloor caused the formation of hardgrounds on sedimentary highs, whereas protected basins act as sedimentary sinks, with high sediment accumulation rates. These hardgrounds are of Late Callovian to Early Oxfordian age. The decrease of current activity corresponds to the top of the condensed sediments and the onset of normal accumulation rates. It coincides with the time of most positive carbon isotope values in the Transversarium ammonite zone. The carbon isotope shift corresponds exactly to the change in current pattern along the northern part of the Tethyan seaway. The synchronous change in carbon cycling and in physical oceanography suggests that there are feedbacks between physical oceanography and carbon cycling. The variations of the carbon isotopic signal reflect the complicated interaction of biological and chemical factors associated with a global oceanic and climate reorganisation.