Water mass exchange and variations in seawater temperature in the NW Tethys during the Early Jurassic: Evidence from neodymium and oxygen isotopes of fish teeth and belemnites

Oxygen and neodymium isotope analyses performed on biostratigraphically well-dated fish remains recovered from the Hettangian to Toarcian of the Paris Basin were used to reconstruct variations of Early Jurassic seawater temperature and to track oceanographic changes in the NW Tethys. Our results indicate a strong correlation between delta O-18 trends recorded by fish remains and belemnites, confirming the paleoenvironmental origin of oxygen isotope variations. Interestingly, temperatures recorded by pelagic fishes and nektobenthic belemnites and bottom dwelling fishes are comparable during the Late Pliensbachian sea-level lowstand but gradually differ during the Early Toarcian transgressive episode, recording a difference in water temperatures of similar to 6 degrees C during the Bifrons Zone. This could suggest that the surface-bottom water temperature difference was not large enough during regressive phases to be recorded by organisms living near the lower and upper part of the water column. The globally unradiogenic Nd budget of Euro-boreal waters through the Early Jurassic suggests that these waters were strongly affected by continental neodymium input from surrounding emerged areas and that exchange with more radiogenic waters from the Tethys and Panthalassa oceans remained limited. This supports the existence of a southward directed current in the Euro-boreal area for most of the Early Jurassic. The only exception is observed at the Early-Late Pliensbachian transition where a positive epsilon(Nd) excursion is recorded, suggesting northward influx of low-latitude Tethyan or Panthalassan waters which may have contributed to the warming of NW Tethyan seawater recorded at this time. The absence of a marked negative excursion in epsilon(Nd) concomitant with a negative delta O-18 shift recorded during the Falciferum Zone (Exaratum Subzone) argues against the influence of less radiogenic Arctic water influxes with low delta O-18 values during this interval. Instead, we suggest that enhanced freshwater inputs related to increasing weathering rates could have contributed to the large 6180 shift recorded by marine organisms, especially in Euro-boreal contexts. (C) 2009 Elsevier B.V. All rights reserved.