in Jurassic and Cretaceous Stratigraphy

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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Middle-Late Jurassic : Evidence from δ 18 O values of marine fish teeth. Paleoceanography, 2003, 18 (3): 1076. Longinelli A., Nuti S. Oxygen isotope measurements of phosphate from fish teeth and bones. Earth and Planetary Science Letters, 1973, 20: 337-340. O’Neil J.R., Roe L.J., Reinhard E., et al. A rapid and precise method of oxygen isotope analysis of biogenic phosphate. Israel Journal of Earth Sciences, 1994, 43: 203. Pucéat E., Lécuyer C., Donnadieu Y., et al. Fish tooth 324 δ 18 O revising Late Cretaceous meridional upper ocean water temperature gradients. Geology, 2007, 35: 107-110. Trotter J.A., Williams I.S., Barnes C.R., et al. Did cooling oceans trigger ordovician biodiversi- fication? Evidence from conodont thermometry. Science, 2008, 321: 550-554. Vennemann T.W., Fricke H.C., Blake R.E., et al. Oxygen isotope analysis of phosphates: a com- parison of techniques for analysis of Ag3PO4. Chem. Geology, 2002, 185: 321.
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. 325
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