in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
Fig.1 Strontium isotope variations in Late Bajocian-Callovian seawater
The time scale of the diagram is based on the assumed equal duration of ammonite subchronozones. Black diamonds=data from
the present study. Grey squares=data from Jones et al. (1994); grey triangles=data from Callomon and Dietl (2000); grey circles=data
from Podlaha et al. (1998); grey crosses=data from Page et al. (2009). The fitted curves are solely based on the data from the present
study
The Middle-Late Jurassic minimum of the
seawater 87 Sr/ 86 Sr ratio might have been linked to the
enhanced hydrothermal activity of the seafloor, which
resulted in the influx of isotopically light strontium to
the oceans (cf. Jones and Jenkyns, 2002). The magni-
tude of the minimum (lowest 87 Sr/ 86 Sr ratio in the
Phanerozoic) and its long time-span (ca. 30 my) are,
however, unusual. The relatively rapid fall in the
seawater 87 Sr/ 86 Sr ratio observed at the Bathonian-
Callovian transition as well as during the Early
Callovian (in contrast to the Late Bajocian-Late
Bathonian interval) may result from the disparate
duration of ammonite subchronozones used for the
construction of the time-scale of the diagram (see Fig.
1). The decrease of the 87 Sr/ 86 Sr ratio at the Bathonian-
Callovian transition correlates, nevertheless, with a
warming and a change in facies distribution across
Europe (Brigaud et al., 2009; Wierzbowski et al., 2009).
This may prove that acceleration of the volcanic
activity of the seafloor and an increase in atmospheric
CO2 level occurred at the Bathonian-Callovian transi-
tion. The plateau of the seawater 87 Sr/ 86 Sr curve in the
Middle-Late Callovian correlates, on the other hand,
with a global transgression (cf. Norris and Hallam,
1995, Wierzbowski et al., 2009). The Middle-Upper
Callovian sediments of European Tethyan and peri-
Tethyan areas show abundant condensations and
omission surfaces. The Middle-Late Callovian is
additionally marked by the waning of the production of
marine carbonates (Bartolini et al., 1996; Morettini et
al., 2002; Dromart et al., 2003; Cecca et al., 2005). All
the data show a link between palaeoceanography and
the tectonic activity of the Earth during the Jurassic.
The presented isotope data from belemnite rostra may
be used to obtain a best fit of the strontium isotope
curve in the Late Bajocian-Callovian.
Studied ammonite shells are characterized by
lower δ 18 O values when compared to coeval belemnites
and bivalves (Wierzbowski and Joachimski, 2007).
This fact together with sedimentologic and faunistic
proxies allowed Wierzbowski and Joachimski (2007) to
assume that the ammonites lived (contrary to belem-
nites) in surface or shallow near-coastal waters of the
basin affected by freshwater influx from neighbouring
land areas. The slight enrichment of ammonite shells in
87 Sr isotope may be hence linked to a change in
strontium isotope composition of the brackish water
owing to the riverine strontium input. Alternative
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