During the Early Toarcian, major paleoenvironnemental and
paleoceanographical changes occurred, leading to an oceanic anoxic event
(OAE) and to a perturbation of the carbon isotope cycle. Although the
standard biochronology of the Lower Jurassic is essentially based upon
ammonites, in recent years biostratigraphy based on calcareous
nannofossils and dinoflagellate cysts is increasingly used to date
Jurassic rocks. However, the precise dating and correlation of the Early
Toarcian OAE, and of the associated delta C-13 anomaly in different
settings of the western Tethys, are still partly problematic, and it is
still unclear whether these events are synchronous or not. In order to
allow more accurate correlations of the organic rich levels recorded in
the Lower Toarcian OAE, this account proposes a new biozonation based on
a quantitative biochronology approach, the Unitary Associations (UA),
applied to calcareous nannofossils. This study represents the first
attempt to apply the UA method to Jurassic nannofossils. The study
incorporates eighteen sections distributed across western Tethys and
ranging from the Pliensbachian to Aalenian, comprising 1220 samples and
72 calcareous nannofossil taxa. The BioGraph [Savary, J., Guex, J.,
1999. Discrete biochronological scales and unitary associations:
description of the Biograph Computer program. Memoires de Geologie de
Lausanne 34, 282 pp] and UA-Graph (Copyright Hammer O., Guex and Savary,
2002) softwares provide a discrete biochronological framework based upon
multi-taxa concurrent range zones in the different sections. The
optimized dataset generates nine UAs using the co-occurrences of 56
taxa. These UAs are grouped into six Unitary Association Zones (UA-Z),
which constitute a robust biostratigraphic synthesis of all the observed
or deduced biostratigraphic relationships between the analysed taxa. The
UA zonation proposed here is compared to ``classic'' calcareous
nannofossil biozonations, which are commonly used for the southern and
the northern sides of Tethys. The biostratigraphic resolution of the
UA-Zones varies from one nannofossil subzone or part of it to several
subzones, and can be related to the pattern of calcareous nannoplankton
originations and extinctions during the studied time interval. The Late
Pliensbachian - Early Toarcian interval (corresponding to the UA-Z II)
represents a major step in the Jurassic nannoplankton radiation. The
recognized UA-Zones are also compared to the carbon isotopic negative
excursion and TOC maximum in five sections of central Italy, Germany and
England, with the aim of providing a more reliable correlation tool for
the Early Toarcian OAE, and of the associated isotopic anomaly, between
the southern and northern part of western Tethys. The results of this
work show that the TOC maximum and delta C-13 negative excursion
correspond to the upper part of the UA-Z II (i.e., UA 3) in the sections
analysed. This suggests that the Early Toarcian OAE was a synchronous
event within the western Tethys. (c) 2006 Elsevier B.V. All rights
reserved.