in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
Significant Climate Disturbances During the Jurassic:
A Statistical Approach
G. Dera 1* , B. Brigaud 2
1. Biogéosciences, UMR CNRS 5561, Université de Bourgogne, 6 bd Gabriel, 21000 Dijon, France
(E-mail: Guillaume.dera@u-bourgogne.fr )
2. Chrono-environment, UMR CNRS 6249, Université de Franche-Comté, 16 route de Gray, 25030 Besançon, France
(E-mail: benjamin.brigaud@univ-fcomte.fr)
During the last two decades, works applying
oxygen and carbon isotope analyses (respectively 18 O
and 13 C) to marine fossils have highlighted major
changes in seawater temperature, polar ice volume, and
carbon cycle during the Phanerozoic (e.g. Zachos et al.,
2001). On this score, recent 18 O data measured on
well-dated bivalves, belemnites, brachiopods, am-
monites, or fish teeth from European domains have
importantly contrasted the former ideas about the
Jurassic climate (200 to 145 Ma) (Brigaud et al., 2008;
Dera et al., 2009; Dromart et al., 2003; Lécuyer et al.,
2003; Price, 2010; Suan et al., 2010; van de Schoot-
brugge et al., 2005; Wierzbowski and Joachimski,
2007), which was for a long time suggested as warm,
with low latitudinal gradients (Frakes et al., 1992).
Though the effectiveness of Jurassic climatic events
may be seriously questioned, because: (1) in spite of
noisy 18 O time series, the geochemical trends are
never statistically tested on exhaustive data sets; (2) the
past evolution of the oxygen isotope composition of
seawater remains unknown; and (3) signals based on
organisms with different ecologies or isotopic frac-
tionation mechanisms may be discordant. In addition,
studies embracing the whole Jurassic interval are scarce,
although they are necessary to point out the processes
responsible to the long-term 18 O variations during this
period marked by major continental break-ups and
volcanic events.
In order to address these questions, we present an
extensive 18 O database covering the whole Jurassic
period, which includes 2741 published and 127 new
18 O values measured on well-dated belemnites and
bivalves from the NW Tethyan domain. In order to
compare the patterns of both organisms and to detect
significant climatic trends standing out from the
background noise, the SiZer approach (SIgnificant
ZERo crossings of the derivatives) was used (Marron
and Chaudhuri, 1998). This multiscale method is based
on the construction of curves fitting time series using
different levels of smoothing (h). The first derivatives
of each curve are simultaneously computed with their
95% confidence intervals, allowing the signs of
derivative estimates to be statistically tested. The
results of multiple tests are then reported under the form
of slope SiZer maps with different colours, which
enables us to identify significant features at different
time scales simultaneously.
Whatever the smoothing resolution, the SiZer
maps based on both fossil organisms depict numerous
significant features, in spite of differences in the
absolute 18 O values and in the amplitude of variations,
which are concordant during the overlap intervals. This
similarity in SiZer results relative to organisms with
distinct ecologies (i.e. life depth or temperature)
validates the robustness and the paleoenvironmental
(not biotic) origin of geochemical trends over the whole
Jurassic interval. Both fossil organisms confounded, 14
significant rapid 18 O changes (0.5 to 1 My) with
different amplitudes are statistically validated by the
SiZer method and related to paleoclimatic events. In
addition, our analysis depicts long-term 18 O trends
characterized by two low 18 O plateaus during the
Toarcian and the Kimmeridgian, and a progressive
increase during the Middle Jurassic. Once a clear
paleoclimatic framework is statistically validated for
the whole Jurassic period, the 18 O changes are
ultimately compared to a synthesis of carbon cycle
disturbances, volcanic and palaeoceanographic events
potentially responsible to short- and long-term
palaeoclimatic variations.
Key words: Jurassic; Palaeoclimate; Oxygen
isotopes; SiZer approach
References:
Brigaud B., Pucéat E., Pellenard P., et al. Climatic
fluctuations and seasonality during the Late
Jurassic (Oxfordian-Early Kimmeridgian) inferred
from 18 O of Paris Basin oyster shells. Earth and
Planetary Science Letters, 2008, 273: 58-67.
Dera G., Puceat E., Pellenard P., et al. Water mass
exchange and variations in seawater temperature
in the NW Tethys during the Early Jurassic: Evi-
dence from neodymium and oxygen isotopes of
fish teeth and belemnites. Earth and Planetary
Science Letters, 2009, 286: 198-207.
Dromart G., Garcia J.P., Picard S., et al. Ice age at the
Middle-Late Jurassic transition? Earth and Plane-
tary Science Letters, 2003, 213: 205-220.
Frakes L.A., Francis J.E., Syktus J.I. Climate Modes of
the Phanerozoic. New York, NY (United States):
Cambridge University Press, 1992.
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