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. Because the number of indepen- there were no sufficiently long igneous rock-based dent sites was lower than 3, we used sample means for magnetostratigraphies that could be used in this study. magnetozones 28, 27 and 26. The mean directions were then converted into VGPs. The polar wander, magneto- 2.2. Magnetostratigraphic records from outcrops zone by magnetozone, is shown in Fig. 1a.

2.2.1. African Plate, Italy (Gubbio) (Table 1a in 2.3. European Plate the Appendix) Three Italian sections located near Gubbio in the 2.3.1. Aix-en-Provence (France) (Table 1b in the Appendix) Apennines (Fig. 1a) together provided the best-defined Cojan et al. (2000), Cojan and Moreau (2006) sequences of reversals for the Paleocene and Eocene. documented a Paleocene magnetostratigraphy from The Bottaccione section provided data for magneto- continental successions in the Aix-en-Provence Basin. zones 33 to l8, (Roggenthen and Napoleone, 1977; Chron 27N was not defined in this section, because of a Napoleone et al., 1983; Rocchia et al., 1990), the sedimentary hiatus during magnetozones 27 and 26 in Contessa Highway section for magnetozones 29 to 16, which the boundary between these magnetozones can be and the Contessa Road section for magnetozones 25 to placed. This makes it possible to show polar wander 18 (Lowrie et al., 1982). For the Bottaccione section, the between the bottom and top of this part of the section, paleomagnetic directions of magnetozones 25–19 i.e. between magnetozones 27 and 26. On the other (Napoleone et al., 1983) and magnetozones 29–28 hand, the boundary between magnetozones 29 and 28 (Rocchia et al., 1990) were defined using stepwise was not defined, since in this zone the presence of demagnetisation and principal component analysis. The numerous remagnetisations makes it impossible to mean direction was calculated for each chron (a normal situate it; therefore, a common direction for magneto- or reversed polarity interval); however, for Bottaccione zones 29 and 28 was defined. In total, five VGPs, related section magnetozones 27–26, (Roggenthen and Napo- to magnetozones 29–28, 27, 26, 25 and 24, were leone, 1977) and both Contessa sections (Lowrie et al., computed (see Figs. 1a and 2). 1982), paleomagnetic directions were defined using the first demagnetization step for which the direction of 2.3.2. Basque Country (Spain) (Table 1c in the Appendix) magnetisation was stable. These were then published in Two magnetostratigraphies from hemipelagic succes- the form of a graph. We digitised the directions sions in the Basque Country have been published for the (declinations and inclinations), eliminating those that Paleocene; one at Trabakua, covering magnetozones 29 to deviated more than 45° from the mean direction. 24 (Pujalte et al., 1995), and the other at Zumaya,

Fig. 1. a) Apparent polar wander path (APWP) and site positions for Gubbio, Italy (Gb, stars), Aix-en-Provence , France (AP, crosses) and Basque Basin, Spain (BB, circles); b) Example of a calculation of a global mean pole for a given magnetozone using a small circle intersection method (in this case magnetozone 25). The poles for NAm=North America, SAf=South Africa and Austr=Australia can move freely along a small circle centred on each respective site. c) Mean APWPs calculated (Table 3) at the magnetozone level for anomalies 29 to 19 (65.5and 40.4 Ma) from our selected magnetostratigraphies transferred onto Africa. The confidence cones are shown with alternate light and dark shading for more clarity. c1 — dots and their associated confidence cones refer to data not corrected for shallowing; c2 — stars and associated confidence cones refer to data corrected for shallowing. d) Mean APWPs calculated using data grouped together into four periods (magnetozones 19–21, 22–24, 25–26, 27–29), see also

Table 2a. In d1 and d2, the conventions are the same as for Fig. 1c. 156 M.-G. Moreau et al. / Earth and Planetary Science Letters 260 (2007) 152–165 covering magnetozones 29 to 26 (Roggenthen, 1976). The digital data from Leg 74 (latitude −28.5° N, longitude These two sections are about 20 km apart. We digitised the 2° E) has not been published, and therefore, we chose to graphs of variations in inclination and declination in digitise the graphs from boreholes 525A, 527, 528 and accordance with the stratigraphic altitudes published by 529. Parts of the section where perturbations were the authors, and ensured that, for each of these sites, the reported were avoided. Inclination evolution with time mean directions obtained with the digitally redefined data for magnetozones 29 to 25 is shown in Fig. 2 and Table were consistent with those of the authors. For magneto- 2b1 in the Appendix. zones 29, 28, 27 and 26, the recalculated paleodirections Tauxe et al. (1984) kindly provided us with digital take into account the data from both sites and the 95% data from Leg 73, hole 523 (latitude −28.6°, longitude confidence cones were defined using the total number of 357.7°) and hole 524 (latitude −29.5°, longitude 3.5°), samples. At Trabakua, as at Aix-en-Provence, chron 27N enabling us to calculate the mean inclination at each was not defined, and the boundary between magneto- magnetozone. Inclination wander with time for magne- zones 26 and 27 was placed within the sampling hiatus. tozones 29 to 23 and 20–19 is shown in Fig. 2 and Table For magnetozones 25 and 24, the data originated from a 2b2 in the Appendix. single site. Six VGPs, for magnetozones 29 to 24, were therefore defined (see Figs. 1a and 2). 2.4.3. Australia (Table 2c in the Appendix) The magnetostratigraphy of ODP Leg 121 (latitude 2.4. Magnetostratigraphic records from drill cores −31°, longitude 93.5°) located off Australia was published by Gee et al. (1991), who kindly sent us the In the absence of other useable outcrop-based magne- digital data enabling us to calculate the mean inclination tostratigraphic records, magnetostratigraphic records from at each magnetozone. Only the best data (category A drill cores are employed. Here, we calculated the mean according to the authors) were used. The error bars were inclination for each magnetozone (Enkin and Watson, sometimes quite large, and the author believes that there 1996), and defined its variation through time. is a shallowing of the inclination, depending on the There are many Ocean Drilling Program (ODP) Legs depth of burial. Although according to the author the on the Antarctic plate as well as some in the Atlantic, data from this site are of poor quality, we have used north of latitude 60°, and for some of these, magnetos- them because this is the only available magnetostrati- tratigraphies stretching from 65 to 42 Ma have been graphy for the Australian plate. Inclination evolution for published. However, in both these regions, the inclina- magnetozone 28 to 24 and 20, is shown in Fig. 2. tions are very high, which is problematic because the mean inclination estimations are always underestimated 2.5. Calculation of the mean polar wander path when the declinations are unknown. These magnetostratigraphies were therefore not used in the present The magnetostratigraphies used in this study in order study. The magnetostratigraphy of site 577 of Leg 83 on to calculate the mean APWP belonged to four major the Pacific plate (Bleil, 1985) was also not considered plates. All VGPs were transferred onto the Africa plate, because of the poor quality of the data, as acknowledged which was chosen as the common reference frame. by the authors, and difficulty in kinematically connect- Gubbio belongs to Apulia, which is a promontory of ing the Pacific plate to the Indian Ocean. Africa. It was therefore necessary to define the rotation of Apulia in relation to Africa. Accordingly, the mean 2.4.1. North America (Table 2a in the Appendix) Gubbio VGP between magnetozones 29 (65.6 Ma) and 19 Digital data of excellent quality were available from (40.6 Ma) was calculated and then compared to the the ODP Leg 171 cores off Florida (latitude 30° N, wander pole for Africa at 55 Ma (Besse and Courtillot, longitude 283.5° E) (Ogg and Bardot, 2001). The mean 2002). The rotation was determined as 30°, which is fairly inclination was therefore calculated for each magneto- similar to the value of 25° calculated by Lowrie and zone from 29 to 19, and the variation with time was Alvarez (Lowrie and Alvarez, 1975). The VGPs for the defined as shown in Fig. 2. Basque and Aix-en-Provence Basins were also transferred to Africa. An arbitrary declination of 90° was used to 2.4.2. Africa (Table 2b in the Appendix) calculate the corresponding VGPs from drill cores using There are several DSDP (Deep Sea Drilling Project) actual inclinations. We then transferred these VGPs onto and ODP Legs off Africa. ODP Leg 208 apparently has no the African plate. Because the magnetic declination was available discrete inclination data, but data from Legs 74 arbitrarily set, we treated these poles using only inclination (Chave, 1984) and 73 (Tauxe et al., 1984)wereuseable. data, i.e. as small circle constraints using the method M.-G. Moreau et al. / Earth and Planetary Science Letters 260 (2007) 152–165 157

Fig. 2. Paleodirections curve as a function of time (and magnetozones number) for: Gubbio, Aix-en-Provence, Basque Basin, North America DSDP site, South Africa DSDP sites, Australia DSDP site. The curves with black diamonds show the data from each site; their error bars are shown as dotted lines (Tables 1 and 2). Values recalculated using our global pole are shown with red crosses; and values recalculated using poles from all the sites, except the considered sites with green inclined crosses. The blue open triangles show values corrected for shallowing for the Basque Country, Leg 74 South Africa and Australia, and blue open circles with shows values expected using the global pole recalculated after correcting for inclination. 158 M.-G. Moreau et al. / Earth and Planetary Science Letters 260 (2007) 152–165 derived from McFadden and McElhinny (McFadden and African plate Leg 73 matched well, and in Leg 74, McElhinny, 1988). As an example, the resulting small the observed and expected inclinations ran parallel. circles for magnetozone 25 are shown in Fig. 1b. A mean Indeed, the measured inclinations were too low, geomagnetic pole for each magnetozone could then be probably because of the presence of clay since, as in defined from the average between fixed PGV (outcrops) the Basque Country, the section consists of limestone– and paleo-colatitude small circles (cores) using the marl alternations. In Australia, as previously noted (Gee Paleomac program (Cogné, 2003) The mean poles were et al., 1991), the measured inclinations were also far too defined using 5 sites for magnetozone 29, 7 sites for low to represent the Paleocene; however, the point in the magnetozones 28, 27, 26 and 25, 5 sites for magnetozone Eocene was satisfactory. 24, 3 sites for magnetozone 23, 20 and 19, and only 2 sites for magnetozones 22 and 21, precluding computation of 3.2. Impact of inclination errors the confidence cone. The global geomagnetic polar wander path determined using data from sites located in The phenomenon of inclination shallowing in sedi- Africa, North America, Europe, and Australia and defined ments and sedimentary rocks has frequently been studied. with respect to Africa is shown by closed circles in Fig. 1c1 A method to separate contributions of non-dipolar and in Table 1a. components and shallowing due to compaction has been proposed (Tauxe and Kent, 2004) but was not useful in our 3. Discussion: reliability of the calculated polar case since declinations from drillings were not available. wander path Anson and Kodama (Anson and Kodama, 1987)also proposed a model for calculating the original inclination, 3.1. Comparison of the master APWP to local Inc0, as a function of the measured inclination Incm: measurements tanðIncmÞ¼ð1 adVÞtanðInc0Þ For each site and for each magnetozone in the Paleocene Fig. 2 compares the paleomagnetic directions where dV is the relative variation in volume during measured at each site, the expected directions obtained compaction. The authors show that for non-interacting based on the mean pole calculated for the seven sites single-domain grains of magnetite in a kaolinite matrix the studied, and the expected directions obtained based on coefficient “a” varies from 0.54 to 0.63 depending on the the mean pole calculated without the data from each site, shape of the magnetite grains. In order to take into account as well as showing compaction-corrected curves. heterogeneity in the shape of magnetic carriers in the For each site, the expected direction curves recalcu- sediments, the maximum value was used. For ODP sites lated with and without the data from each site showed a 752 and 753 on the Australian plate, measurements of clear match with the observed data. At Gubbio, the 3 porosity made it possible to estimate the variation in curves showed significant differences in declination for volume d V due to compaction of the sediment, giving a magnetozones 29 to 27, but were clearly matched from negligible value for magnetozone 20, a value of 40% for magnetozone 26 onwards. At Aix-en-Provence and in magnetozones 24 to 27, and 60% for magnetozone 28 the Basque Country, the declination curves correlated (Gee et al., 1991). The original inclination could thus be fairly well, while the inclination curve for the Basque recalculated. For the Basque Country and South Africa, Country shows an inclination which is around 10° lower porosity data was not available, but it is known that in than expected, when only 1 or 2° could be accounted for limestone–marl alternations water ceases to be expelled by tectonics (Rosenbaum et al., 2002). The most likely when the porosity lies between 40 and 60% (Ricken, explanation is that the Basque Country data originated 1992). Moreover, if the initial porosity is 72% and the from limestone–marl alternations containing much clay, final porosity close to 40%, the variation in volume is ⁎ since 46 to 72% insoluble minerals were found about 50%, which gives tan (Incm)=0.68 tan (Inc0). This at Zumaya (Mount and Ward, 1986). In North America, correction was therefore applied to the data from the the measured and recalculated inclination curves Basque Country and Leg 74 from South Africa. The were clearly matched, and there was a good correlation inclinations after correction for shallowing are shown in between the changes in declination at Gubbio Fig. 2 (open blue triangles). and in inclination in North America, which is satisfac- For each anomaly in the Paleocene, the mean pole tory given that the latitudes of both sites only differ by was recalculated after these corrections (see Table 1, b). 13°, with a difference in longitude of 90°. The measured For all poles, the size of the confidence cone slightly and recalculated inclination curves of the South decreased and the wander path varied very little with the M.-G. Moreau et al. / Earth and Planetary Science Letters 260 (2007) 152–165 159

Table 1 Table 2 Mean virtual geomagnetic poles in African plate coordinates Mean virtual geomagnetic poles for four periods in African plate coordinates, a) original data, b) corrected for shallowing Age Age n ΦλkA95 (Ma) (magnetozones) (° E) (° N) Age Age n S ΦλkA95 (magnetozones) (Ma) (° E) (° N) a) Measured data 65 29 5 212.0 74.3 51.7 12.2 a) Measured data 64 28 7 200.1 77.5 41.2 10.2 29–27 65.5–61.6 19 6 199.6 75.7 556.6 4.6 62 27 7 191.2 74.0 63.4 8.2 26–25 61.6–56.7 14 7 182.0 73.5 144.2 3.4 60 26 7 179.6 73.6 151 5.3 24–22 56.7–48.7 11 7 204.2 69.8 106.5 4.6 57 25 7 184.7 73.4 114 6.1 21–19 48.7–40.4 9 3 195.8 74.0 364.1 2.9 54 24 6 203.6 68.9 61.7 9.2 Mean 29–19 65.5–40.4 53 7 195.6 74.0 88.2 2.1 52 23 3 207.2 71.4 219 13.8 50 22 2 202.4 70.8 b) Corrected for shallowing 47 21 2 197.8 71.6 29–27 65.5–61.6 19 6 204.3 74.8 92.7 3.6 43 20 4 197.0 73.1 209 8.7 26–25 61.6–56.7 14 7 188.4 73.0 331 2.2 41 19 3 188.8 76.8 325 11.3 24–22 56.7–48.7 11 7 207.2 70.0 220 3.2 65.5–40.4 29–19 53 195.6 74.0 88.2 2.1 21–19 48.7–40.4 9 3 195.8 73.9 366 2.9 Mean 29–19 65.5–40.4 53 7 199.6 73.5 148 1.6 b) Corrected for shallowing data Ages are successively shown as magnetozones and Ma. Same 65 29 5 210.7 74.5 81.7 9.7 conventions as Table 1 except that n is number of poles originating 64 28 7 209.6 76.1 83.9 7.1 from S worldwide scattered sites. 62 27 7 197.6 72.8 83.2 7.1 60 26 7 186.8 73.3 235.3 4.2 57 25 7 191.1 72.7 406 3.20 magnetozone 24. We therefore computed mean poles for 54 24 6 208.0 69.3 154 5.8 the following grouped magnetozones: 29–27 (≈early 52 23 3 209.0 70.2 194 14.7 – ≈ – – 50 22 2 202.3 70.8 Paleocene), 26 25 ( middle late Palaeocene), 24 22 47 21 2 197.9 71.6 (≈early Eocene) and 21–19 (≈middle Eocene), (see 43 20 4 196.9 73.1 210 8.7 Table 2). Whether the calculation was carried out using 41 19 3 189.6 76.2 284 12.1 the original data (Fig. 1d1) or data corrected for 65.5–40.4 29–19 53 199.6 73.5 148 1.6 shallowing (Fig. 1d2), the mean poles for anomalies a) original data, b) corrected for shallowing. Ages are shown in Ma and 26–25 and 24–22 were significantly distinct according Φ λ magnetozone number; n is the number of sites used; , , longitudes to McFadden and McElhinny's significant test (1990) and latitudes of mean poles; k is Fisher's precision parameter; and A95 is the 95% confidence cone. (see Table 3). This strongly validates the existence of a backward loop in the global APWP during the Paleocene and early Eocene. The other mean poles corrections (Fig. 1c2, stars). Moreover, for the Paleocene taken two by two only differed after correction for and Eocene, 65–40 Ma, the mean pole before correction for shallowing (Φ=195.6° E, λ=74.0° N, A95 =2.1°) Table 3 was indistinguishable from that after correction Paleomagnetic tests for: a) original data, b) corrected for shallowing data (Φ=199.6° E, λ=73.5° N, A95 =1.6°) using the test of γγ McFadden and McElhinny (1990). The expected First pole Second pole 95% Test directions based on the mean poles recalculated for the (magnetozones) (magnetozones) whole Earth are shown in Fig. 2 (open blue circles), the a) Measured data shapes of which were nearly identical to those of the 29–27 26–25 5.3 5.8 identical – – uncorrected curves (red crosses). 26 25 24 22 8.0 5.3 distinct 24–22 22–19 5.0 5.3 identical 22–19 29–27 2.0 6.7 identical 3.3. Statistical tests b) Corrected for shallowing data The confidence cones associated with the poles 29–27 26–25 4.8 4.4 distinct – – calculated for successive magnetozones, taken two at a 26 25 24 22 6.8 3.6 distinct 24–22 22–19 5.4 4.0 distinct time, overlapped, indicating that the pairs of poles were 22–19 29–27 2.6 5.3 identical not significantly different (see Fig. 1c and c ). 1 2 First pole and second poles: Age range of the two poles to be However, the polar wander curve suggested three compared; γ angle between the poles to be compared; γ95 critical motion phases delimited by two major changes in angle below which mean directions are identical at the 95% level direction, one at magnetozones 26–25 and the other at (McFadden and McElhinny, 1990); test: result of the significance test. 160 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) 152–165 161 shallowing (see Table 3). In both cases, the oldest and well as mammals associated with tropical flora have youngest poles of anomalies 29–27 and anomalies 21– been found on Ellesmere and Axel Heiberg Islands 19 were identical. (Dawson et al., 1976; Estes and Hutchison, 1980; Wolfe, 1980; McKenna, 1980; Dawson, 1990; Marincovich 3.4. Correlation with climate data et al., 1990; Eberle and McKenna, 2002). These deposits occurred within the lower part of Member IV of the The geocentric dipolar nature of the Earth's magnetic Eureka Sound, which is dated from the Wasatchian field was first put forward by Irving (1956) on the basis (early Eocene). Few terrestrial mammals are found of paleolatitude determinations and zona climatic higher up in Member IV. Marine vertebrate fauna occurs comparisons. In the globally warm climate that affected in Member III on Ellesmere and Axel Heiberg Islands. the Earth at the end of the Paleocene and beginning of (Miall, 1986; Marincovich et al., 1990). The age of the Eocene (Marincovich et al., 1990; Kennett and Stott, Member III is late Paleocene, probably corresponding to 1991), the temperature was probably relatively equable magnetozones 26–25 (59 Ma) (Tauxe and Clark, 1987; along a wide range of latitudes, suggesting that the Tarduno et al., 1997). The positions of Ellesmere Island development of warm-climate flora and fauna at high and Axel Heiberg Island with respect to the polar circle latitudes in both hemispheres was conditional on are shown in Fig. 3a at 64, 59, 53 and 44 Ma. In Fig. 3c, insolation, which depends only on the Earth's orbital the paleolatitude of fossil sites on these islands (long.= parameters. The polar-night boundary is the most 278° and lat.=79°) was calculated for mean poles at objective criterion for luminosity, even if atmospheric 65.5–61.6 Ma, 61.6–56.7 Ma, 56.7–48.7 Ma and 48.7– refraction permits weak winter daylight beyond the 40.4 Ma (bold line). We added on the same figure polar circle. We thus analyzed only fossil and paleocli- paleolatitudes computed at the magnetozone level (thin matic evidence versus paleolatitude determinations at line). The astronomical oscillation zone of the polar latitudes above or close to the polar circle. The polar- circle resulting from the Earth's obliquity is shown with night boundary is currently at a latitude of 66°33′ and is the first appearance of vertebrates. Even though error in determined by the obliquity of the ecliptic. Over a the paleolatitude with respect to the amount of motion 30 myr period spanning the last 20 Ma and projecting does not permit us to consider the simultaneousness of forward 10 Ma, the obliquity calculated using Laskar's these events as certain, it is noteworthy that vertebrates solution (Laskar et al., 1993) varies between 22°37′ and appeared when the site was within the astronomical 24°13′, corresponding to a polar-night boundary oscillation zone of the polar circle. between 65°47′ and 67°23′. Obliquity is thought to The presence of these various species on these have remained within this range during the Cenozoic. islands thus suggests migration, synchronous with migration of the pole, rather than adaptation of these 3.5. Northern hemisphere species to the polar night.

Fig. 3a shows reconstructions of the northern part of 3.6. Southern hemisphere the northern hemisphere at 64, 59, 53 and 44 Ma in relation to North America. In each diagram, the The same reconstructions as conducted for the minimum and maximum latitude of the polar circle, northern hemisphere are shown in Fig. 3b. It can be 65.5° and 67.2°, has been drawn around the seen that the Adelie Land part of Antarctica (between 90 corresponding pole. and 180° longitude) as well as Australia are permanently Terrestrial vertebrate fossil deposits consisting of outside the polar circle, whereas the central part of terrestrial tortoises, snakes, lizards and crocodilians as Antarctica always remains inside. The head of the

Fig. 3. Correlation of APWP with climatic data, at 64 Ma (red), 59 Ma (green), 53 Ma (blue) and 44 Ma (orange). — a) and b) Polar part of the North and south hemispheres respectively reconstructed onto North America and Antarctica kept fixed, with astronomical oscillations zone of polar circle around the corresponding pole. For each age, the zoom shows fossiliferous site and consider the polar circle position. Ellesmere and Axel Heiberg Islands (Eureka group) were within the astronomical oscillations zone of the polar circle between 59 Ma and 53 Ma. The tip of the Antarctic Peninsula is inside polar circle at 59 Ma. — c) and d), Paleolatitude of fossiliferous sites vs. time for both mean pole at 65.5–61.6 Ma, 61.6– 56.7 Ma, 56.7–48.7 Ma and 48.7–40.4 Ma (bold line) with confidence interval (pink colour) and poles related to magnetozones (thin line). Astronomical oscillations zone of the polar circle is grey shaded. Biostratigraphic data associated are shown above. c) — In Eureka group, 1 for Mollusk and Ostracodes, 2 for Chondrichthyes and Osteichthyes; and 3 for Amphibia, Reptilia, and Mammalia (Marincovich et al., 1990) dotted line indicate sparse mammal fossils. Vertebrates appear with the southward shift of the site within the astronomical oscillations zone of the polar circle. d) Size of conifer growth rings (bold line) with standard deviation (thin line) vs. time; dotted line indicate the possible discontinuities between formations (see text). There is drastic diminution of tree growth with higher latitudes. 162 M.-G. Moreau et al. / Earth and Planetary Science Letters 260 (2007) 152–165

Antarctica peninsula, on the other hand, almost leaves formation. From the Eocene onwards, the climate started to the polar area at 53 Ma (Fig. 3b). Major fossiliferous deteriorate, eventually leading to the gradual glaciation of plant and vertebrate deposits located on King George the Antarctic continent (Dingle and Lavelle, 1998). and Seymour Islands at the tip of the Antarctic peninsula in the west of the continent (Dingle and Lavelle, 1998; 3.7. Polar wander and conclusion Poole et al., 2005) allowed for reconstruction of the climatic history of the region from the Maastrichtian to Magnetostratigraphies have the advantage of display- Eocene. The climatic change inferred from analysis of ing an APWP for each examined site and, in ideal 600 samples of fossil wood from angiosperms and conditions, all APWPs transferred onto a single plate are conifers shows that there was gradual cooling during the identical. Here, we computed poles derived from Maastrichtian and lower Paleocene. The latter period worldwide ‘stacked’ magnetostratigraphic data only. (Sobral formation) is described by Poole et al. (2005) as This method allowed for computation of global VGPs at being cool to cold, but not glacial (Dingle and Lavelle, the magnetozone time resolution level (b4 my in our case) 1998). The increase in tree-ring density in the fossil from a natural and as unbroken as possible record of woods discovered in the Sobral formation (Francis and directions, and is probably one of the best methods for Poole, 2002) further strengthens the case not only for a detecting small amplitude (b∼10°) or rapid (b∼10 myr) cold climate but also for a decrease in daylight, leading pole motion. The present study revealed a loop in our to biological stress. Dingle and Lavelle (1998) interpret Paleocene–Eocene master African APWP with a maxi- this cooling as a response to the formation of a cold mum amplitude of 6.8° (Table 3b). Many factors may current, which led to cooling of surface water in the account for small apparent polar motion, including the ocean surrounding the Antarctic Peninsula. However, a existence of a main dipolar field non-aligned with the time lag of over 5 my between these two climatic events, earth rotation axis, non-dipolar components of the Earth's one terrestrial and the other marine, appears to argue magnetic field, and errors in the sedimentary record of against this hypothesis. magnetization such as inclination errors linked to As with the northern hemisphere, Fig. 3d shows the compaction. These points have often been addressed in paleolatitude of King George and Seymour islands the literature in order to explain the frequently encoun- (long.=295° E, lat.=−65° S) as well as the astronomical tered anomalies of inclination in Asia during the Tertiary oscillation zone of the polar circle as a function of time. (Wilson, 1970; Westphal, 1993; Van der Voo and Torsvik, We also indicated the size of conifer growth rings for 2001; Kodama and Dekkers, 2004; Tauxe and Kent, four geological formations (La Meseta Fm, Cross Valley 2004; Hankard et al., 2006). Moreover, our loop only Fm, Sobral Fm and Lopez de Bertodano Fm) related to relies upon a limited set of magnetostratigraphies. the studied period (Francis and Poole, 2002). These However, despite this, time-dependant coeval evolution formations are superimposed, but the age of transition of our virtual magnetic pole displacement with vertebrate between them is not as precise as that of the magneto- occurrence on Ellesmere Island and the tree ring growth zones, and accordingly, age gaps do exist. These rate in Western Antarctica was observed for the 65–40 Ma uncertainties are represented by dotted lines in the figure. period (magnetozones 29–19). This agreement lends The size of growth rings was clearly correlated with the weight to the simplest interpretation, which favours global paleolatitude determinations; they progressively de- motion of the lithospheric plates during this period. creased during poleward motion (up to 600 km inside However, given the limited set of magnetostratigraphies the polar circle during magnetozones 26 and 25), probably currently available, our interpretation is primarily a because of winter daylight reduction, then significantly plausibility argument. increased as our site moved southward at the time of Many questions arise from our findings; most magnetozone 24. importantly, what causes this motion? Greff-Lefftz When the climatic history of the Antarctic Peninsula is (2004) predicted that a super plume at high latitudes compared with the polar wander curve it can be seen that could slightly alter the rotation of the Earth. If so, the this region lay well within the polar circle during the formation at high latitudes, between 61 Ma and 53 Ma Paleocene, particularly the late Paleocene (Fig. 3b, 59 Ma, (magnetozones 26–24), of the North Atlantic Igneous and 3d). This is in accordance with the upper Paleocene Province (White and Lovell, 1997) might have triggered fossiliferous deposits, which attest to climatic stress during a small episode of true polar wander, causing the this period (Sobral formation). A shift of 7° northwards observed motion. Another question concerns the rela- was observed during magnetozone 24 (Fig. 3d), correlating tionship between this wander and the Paleocene–Eocene with the climatic improvement observed in Cross Valley thermal maximum. Kirschvink and Raub (2003) already M.-G. Moreau et al. / Earth and Planetary Science Letters 260 (2007) 152–165 163 proposed that Cambrian true polar wander drove as a basis for continental–marine correlation. Geology 28, – catastrophic clathrate release. If our early Eocene TWP 259 262. Corfield, R.M., Cartlidge, J.E., Premoli-Silva, I., Housley, R.A., 1991. is correct the 6.8° polar motion for Africa translates into a Oxygen and carbon isotope stratigraphy of the Paleogene and 7.5° change of latitude of Antarctica after plate motion Cretaceous limestones in the Bottacione Gorge and Contessa correction, causing thus high-latitude margins of this Highway sections, Umbria, Italy. Terra Nova 3, 414–422. plate to migrate to lower latitudes between 58 and 52 Ma. Dawson, M.R., 1990. Terrestrial vertebrates from the Tertiary of Albeit of much smaller amplitude than the Cambrian Canada's Arctic Islands. In: Harington, C.R. (Ed.), Canada's missing dimension: science and history in the Canadian Arctic event, our proposed change in latitude would possibly islands. Canadian Museum of Nature, vol. 1, pp. 91–104. trigger a catastrophic phenomenon of methane release. Dawson, M.R., West, R.M., Hutchison, J.H., 1976. Paleogene For instance, Thomas et al. (2002) invoked that a gradual terrestrial vertebrates: northernmost occurrence, Ellesmere Island, warming of surface waters followed by downwellings in Canada. Sciences 192, 781–782. the deep ocean could be responsible for that phenomenon. Dickens, G.R., O'Neil, J.R., Rea, D.K., Owen, R.M., 1995. Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene. Paleoceanography Acknowledgments 10, 965–971. Dickens, G.R., Fewless, T., Thomas, E., Bralower, T.J., 2003. Excess This work was funded by INSU (ECLIPSE). We thank barite accumulation during the Paleocene–Eocene thermal Max- L. Tauxe and J. Gee for generously sending us data from imum: Massive input of dissolved barium from seafloor gas hydrate reservoirs. Geol. Soc. Am. Spec. Pap. 369, 11–23. Legs 73 and 121; R. Enkin and I. Cojan for their fruitful Dingle, R.V., Lavelle, M., 1998. 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