| The million-year fluctuations of the Mesozoic climate are explored through spectral analyses
performed on an exhaustive compilation of δ18O and δ13C data measured on belemnite
rostra. The data include more than 3500 data points, all coming from Western Tethys and
Euro-boreal domains, and covering a time interval spanning 76 Myr from the Sinemurian
(~197 Ma; Early Jurassic) to the Aptian (~123 Ma; Early Cretaceous) with an average
sample step of ~0.04 Myr. Spectral analyses are performed using the multi-taper method and
the evolutive Fast Fourier Transform in order to get an accurate estimate of significant
periods and their evolution during geological times. The age uncertainties of the
Geological Time Scale 2012 are taken into account to assess the impact of these
uncertainties on the identification of the significant periods. After implementing an error
model that simulates the uncertainties of the Geological Time Scale, two periods
remains significant: the δ13C displays a high-amplitude period at 9.1 Myr, while the
δ18O displays a high-amplitude period at 16.4 Myr. The 16.4-Myr period is only
expressed in the Early and Middle Jurassic, with maximum amplitudes reached
during the “Toarcian Plateau” (Dera et al., 2011). It is probably a consequence of the
activity of the Karoo-Ferrar Large Igneous Province and is an event in the δ18O
rather than a true cycle. The 9.1-Myr period displays a spectacular continuity from
the Toarcian to the Aptian, and could be related to this intriguing 9.1-Myr cycle
observed in the δ13C from the Cenozoic, related to a Myr-amplitude modulation
of the eccentricity cycles (Boulila et al., 2012). The δ13C in the Western Tethys
thus appears to have a very rhythmic behaviour, interpreted here as a long-term
orbital modulation of moisture and heat transfer from equatorial to higher latitudes,
modulating in return continental weathering, nutrient and detrital exports to basins, neritic
vs. pelagic productivity and finally preservation of organic matter in the oceanic
basins.
References:
Dera, G., Brigaud, B., Monna, F., Laffont, R., Pucéat, E., Deconinck, J.-F., Pellenard, P.,
Joachimski, M.M. and Durlet, C., 2011. Climatic ups and downs in a disturbed Jurassic
world. Geology 39(3), 215-218.
Boulila, S., Galbrun, B., Laskar, J. and Pälike, H., 2012. A ~9 myr cycle in Cenozoic
δ13C record and long-term orbital eccentricity modulation: Is there a link? Earth and
Planetary Science Letters 317-318, 273-281. |