A detailed carbon-isotope stratigraphic study for the uppermost
Pliensbachian lowermost Aalenian interval in the Median Subbetic
palaeogeographic domain (External zones of the Betic Cordillera,
southern Spain) has been carried out. During the Early Jurassic, the
Median Subbetic, which represents a typical basin of the Hispanic
Corridor connecting the Tethys and the Eastern Pacific, was located in
the westernmost Tethys. The analyzed sections encompass the entire
Toarcian stage as represented in the southern Iberian palaeomargin.
Rocks are mainly rhythmic sequences of grey marls and marly limestones
containing a rich ammonite fauna, nannofossils, and benthic
foraminifers-all these provide an accurate biostratigraphic control. The
lower and upper Toarcian boundaries are well represented in some of
these sections and therefore represent optimal sites to link the
carbon-isotope curves to ammonite zones, and to nannofossil events.
delta C-13 values of bulk carbonates from the different localities of
the Subbetic basin have similar variations from the uppermost
Pliensbachian to the lowermost Aalenian, suggesting changes in the
original DIC carbon isotope composition along the Hispanic corridor. The
transition from Pliensbachian to Toarcian is marked by increasing delta
C-13 values from similar to 12 to 2.0 parts per thousand, interrupted in
the Serpentinum Zone by a negative shift concomitant with the Toarcian
oceanic anoxic event (T-OAE), with the major ammonite extinction event
of the Toarcian, and an important turnover of calcareous nannoplankton.
The negative shift observed in the Serpentinum Zone confirms the global
perturbation of the carbon cycling documented along the Tethys and the
palaeo-Pacific in organic material and in marine carbonates. However,
the amplitude of the negative excursion (similar to - 1.5 parts per
thousand) is not compatible with an isotopic homogeneous seawater DIC
and/or CO2 atmospheric reservoirs. The interval from the middle to the
top of the Toarcian delta C-13 shows relatively constant values, minor
ammonite turnovers, and is associated with increasing diversity of
calcareous nannoplankton. (c) 2012 Elsevier B.V. All rights reserved.