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A New Global Paleocene–Eocene Apparent Polar Wandering Path Loop

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A New Global Paleocene–Eocene Apparent Polar Wandering Path Loop Earth and Planetary Science Letters 260 (2007) 152–165 www.elsevier.com/locate/epsl A new global Paleocene–Eocene apparent polar wandering path loop by “stacking” magnetostratigraphies: Correlations with high latitude climatic data ⁎ Marie-Gabrielle Moreau , Jean Besse, Frédéric Fluteau, Marianne Greff-Lefftz Institut de Physique du Globe de Paris, France Received 22 December 2006; received in revised form 10 May 2007; accepted 10 May 2007 Available online 23 May 2007 Editor: T. Spohn Abstract A new apparent polar wander path (APWP) from the beginning of the Paleocene (65 Ma) to the middle of the mid-Eocene (42 Ma) is shown to be correlated with polar climatic data of the same time period. Rather than applying the classical method based on analysis of site-based poles, we “stacked” the APWPs obtained from magnetostratigraphies. Magnetostratigraphies have the advantage of displaying an unbroken record of local APWPs through time and, for a magnetozone (defined as the a combination of normal and reversed polarity intervals), the instantaneous poles are synchronous. Seven magnetostratigraphies located on 4 different plates covered sufficient time to be used in the analysis. An average APWP was then determined with respect to age at the magnetozone level for the African plate, which was arbitrarily chosen as a reference frame; virtual geomagnetic poles were transferred onto the African plate using ocean kinematic Euler rotations. The calculated APWP is characterized by a loop with two main changes of direction at magnetozones 26–25 (61.5–56.5 Ma) and 24–22 (56.5–48.6 Ma) distinct at a 95% level of probability, and indistinct poles related to magnetozones 29–27 (65.5–61.5 Ma) and 21–19 (48.6–40.6 Ma). We also show that the implied rapid shift of the lithosphere with respect to the geographic pole, possibly an episode of true polar wander, was coeval with the time evolution of vertebrate occurrence on Ellesmere Island (Canadian Arctic) and with the tree ring growth rate in Western Antarctica. © 2007 Published by Elsevier B.V. Keywords: Paleocene; Eocene; magnetostratigraphy; APWP (apparent polar wander path); polar-regions; climatic changes 1. Introduction 55 Ma (Zachos et al., 1993). This event, known as the Paleocene–Eocene Thermal Maximum (PETM), has been The Early Tertiary era was marked by the development studied intensively and there is a considerable amount of of a globally warm climate, as indicated by fossils of data available regarding climate change and associated tropical flora and fauna observed in rocks north of the faunal changes, including radiation of mammalian species Arctic Circle (see Hickey et al. (1983) for an inventory). (Maas et al., 1995; Bowen et al., 2002) and extinction of This globally warm climate was marked by an abrupt deep-sea benthic species (Thomas and Shackleton, 1996). episode of warmth at the end of the Paleocene, about In the early 1980s (Estes and Hutchison, 1980; Wolfe, 1980; McKenna, 1980), the idea that the presence of ⁎ Corresponding author. tropical flora and fauna at very high latitudes was caused E-mail address: [email protected] (M.-G. Moreau). by the near-disappearance of the polar night due to a 0012-821X/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.epsl.2007.05.025 M.-G. Moreau et al. / Earth and Planetary Science Letters 260 (2007) 152–165 153 change in the obliquity of the Earth was considered. windows. However, in a large number of paleomagnetic However, calculations using celestial mechanics show that studies in the literature, uncertainty related to age (or the this scenario is highly improbable (Laskar et al., 1993). exact time window) is very often larger than 10 myr; the Moreover, climate models show that very small obliquity result is that small amplitude (b∼10°) or rapid leads to a cooling of high latitudes (Barron, 1984)whereas (b∼10 myr) APW features are often unresolvable. the climate was warm during this period. In the present study, which covers the period from 65 One possible cause of the PETM may have been the to 40 Ma, we therefore used paleomagnetic directions sudden release of large quantities of methane due to obtained from magnetostratigraphic sections already dissociation of sedimentary methane hydrate along published in the literature. Magnetostratigraphic data continental slopes (Dickens et al., 1995; Katz et al., have the advantage of displaying a natural unbroken 1999); indeed, the presence of a negative δ13C anomaly record of directions with globally correlative time markers can only be explained by an influx of material highly every few hundred thousand to millions of years. depleted in δ13C, such as natural gas hydrate. However, Moreover, virtual geomagnetic poles (VGPs) derived the process by which this catastrophic release of methane from the same magnetozone level (a magnetozone is was triggered is still under debate (Thomas et al., 2002; defined as a combination of normal and reversed polarity Dickens et al., 2003; Svenssen et al., 2004). Other intervals) are synchronous for all sites if the duration of mechanisms such as oxidation of large amounts of the magnetozone is sufficiently short. In our case, the sedimentary organic carbon have also been suggested as duration of the magnetozone ranged from 1.1 to 4.0 myr. potential causes of the negative δ13C excursion (Higgins Due to climatic and sedimentation changes or tectonic and Schrag, 2006). disturbance within a site, recording and preservation of At the same time, and unconnected to the climatic magnetic signal vary during and after deposition. As a problem stated above, many paleomagnetic studies have result, the measured APWP defined from a single had difficulties interpreting Paleogene planetary data magnetostratigraphy is deformed with respect to the within the widely accepted geocentric axial dipole (GAD) true path. Stacking data from several distinct records has hypothesis. A deviation of the dipolar magnetic field from proved to be a valuable method to minimize noise in the Earth's rotation axis (Westphal, 1993), a significant many examples of signal processing. We therefore contribution made by non-dipolar terms (Wilson, 1970; derived a global APWP by “stacking” the APWPs Van der Voo and Torsvik, 2001), problems in the from magnetostratigraphies from different lithospheric recording of the magnetic field (Kodama and Dekkers, plates (North America, Africa, Australia, Europe) onto a 2004; Hankard et al., 2006), and a combination of the two common single plate. We have computed a mean pole for last (Tauxe and Kent, 2004) have been proposed as each successive magnetozone for each magnetostrati- solutions to this problem. graphy. These poles were then averaged at the For the period extending from the beginning of the magnetozone level after transfer onto the African plate Paleocene to the middle Eocene (65.5–40.4 Ma (Grad- using interpolated kinematic parameters; as previously stein et al., 2004)), the present study tests another (Besse and Courtillot, 2002), we arbitrarily chose Africa possible cause of the PETM: rapid movement of the and used the same kinematic parameters (Müller et al., lithosphere, perhaps in connection with a true polar 1993). Uncertainty regarding the position of the wander wander event. path of the magnetic pole as a whole was estimated using classical Fisherian confidence cones attached to each 2. Methods mean pole. Paleomagnetic noise such as shallowing, local rotations, erroneous tectonic corrections and others Accurate determination of Apparent Polar Wandering can be approximated by a small rotation on the sphere. Paths (APWPs) is required for meaningful paleogeo- The boundary conditions of this method are defined in graphic reconstruction. Many factors affect the determi- the Appendix I (supplemental file on-line) where we nation of APWPs, but the primary limitation is the partial show that a mean APWP derived from stacking APWPs or total lack of paleomagnetic data for certain periods in rotated by angles up to ∼20° is not significantly any given region. To overcome this, synthetic APWPs deformed, but rather is actually preserved. have been proposed, based on transfer of the best data for major continents onto a common single plate using high- 2.1. Construction of the database quality plate kinematic models (see for example (Besse and Courtillot, 2002)). The overall transferred data set is For the Paleocene and the Middle Eocene (65.5– then averaged, for example in 10 or 20-Ma time 40.4 Ma, magnetozones 29 to 19), a large number (more 154 M.-G. Moreau et al. / Earth and Planetary Science Letters 260 (2007) 152–165 M.-G. Moreau et al. / Earth and Planetary Science Letters 260 (2007) 152–165 155 than 100) magnetostratigraphic studies have been Each chron from each section is regarded as a “site” published; however, most cover only a short duration, for calculation of the mean magnetozone direction. The are discontinuous or are concerned only with polarity paleomagnetic declinations and inclinations are shown zones, without available corresponding directions. Very in Fig. 2. In the Bottaccione section for the 29 and 28 often, the goal of magnetostratigraphic studies is not to magnetozones, we used the data of (Rocchia et al., provide reliable directions. The ideal way of applying 1990), since they were obtained by demagnetisation and our method would be to use only magnetostratigraphies principal component analysis; these were the only data spanning the entire period of interest. Unfortunately, in available for magnetozone 28. At magnetozone 27, the the present case, only two magnetostratigraphies data from two sections were poor and differed con- respond to this criterion, and therefore, we pragmatically siderably. Slump and highly perturbed stable isotope selected those with at least 5 consecutive magnetozones. data have been reported in the Bottacione section This appears to be the best compromise between the (Corfield et al., 1991), and in the Contessa sections the quality of published magnetostratigraphies and mini- mean direction relating to this magnetozone is only mum time continuity, but even with this lax criterion, defined by 7 samples.
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