5th Swiss Geoscience Meeting, Geneva 2007
Catastrophe, catastrophes : single or multiple events at
the Domerian-Toarcian transition (Lower Jurassic) ?
Morard, Alain
Institut de Géologie et Paléontologie ; Anthropole ; UNIL-Dorigny ; CH-1015 Lausanne
The major biological crises recorded in Earth history are often considered as
global events of relatively short duration, although biodiversity may drop before
extinction and take some time to recover. Many stage boundaries have been
defined around these catastrophic faunal turnovers. However, our detailed
study of the Domerian-Toarcian transition revealed that during this stressful
episode, several “crises” could be differentiated in time and space. Moreover,
depending on the province and/or type of organisms considered, the main
extinction was not contemporaneous.
After a careful revision of biostratigraphical correlations, the sequence of events
at the Domerian-Toarcian transition can be summarised as follows :
1) cooling event during the Middle-Upper Domerian, associated to a progressive
marine regression, culminating in a stratigraphic gap spanning the Uppermost
Domerian and Lowermost Toarcian in NW-Europe, whereas Tethyan localities
record shallow marine environments with abundant benthic and nektonic faunas
; these first changes resulted in the extinction of ammonoids such as the
Amaltheidae family in NW-Europe, whereas bursts of variability are observed in
W-Tethyan basins first among the Arietitinae and Protogrammoceratinae,
followed by the Dactylioceratidae (their “mass apparition” was used to define
the base of the Toarcian stage).
2) warming and rapid marine transgression ; ammonoid faunas seem to spread
again from their Tethyan refuges, but still with low diversity.
3) further warming and transgression, leading to a global spread of anoxia, most
prominently at the floor of NW-European epicontinental basins, with extension
into adjacent oceanic regions ; benthic faunas are largely affected, whereas
nektonic organisms such as ammonoids radiate anew.
The chain of events recorded at the Domerian-Toarcian transition are all linked
to a “single” perturbation (possibly with increased volcanic activity as the main
trigger), involving complex interactions between environmental parameters
unfolding one after the other. The consequences of these global changes are
further modulated by regional and local settings (epicontinental sea vs. oceanic
basins), and – eventually – by ecological requirements of the organisms living in
different habitats (benthos vs. nekto-plankton). Therefore, when examining the
fossil record with high stratigraphic detail, the so-called Domerian-Toarcian
mass extinction turns out to decompose into “multiple” convulsions.