Comment on ''Stability of the Earth with respect to the spin axis for the last 130 million years'' by J.A. Tarduno and A.Y. Smirnov (Earth Planet. Sci. Lett. 184 (2001) 549^553)

In a recent issue of Earth and Planetary Science Letters, Tarduno and Smirnov [1] claim that no true polar wander (TPW) occurred over the last 130 Ma. This claim is in contradiction with the conclusion of several studies devoted to this topic [2^8], including our recent EPSL paper [9]. In this paper [9] we showed that some 110 þ 10 Ma ago the Earth’s spin axis shifted by 18 þ 4‡ (at a rate larger than 1‡/Ma, or possibly even larger than 5‡/ Ma) with respect to the main hotspots presently located in the Atlantic and Indian oceans. We interpreted this shift as a displacement of the Earth’s spin axis with respect to the global solid Earth (i.e. TPW). Tarduno and Smirnov [1] contest this conclusion and a⁄rm that TPW is an artifact. Their argument relies on the fact that a few North American paleomagnetic poles not included in our selection do not ¢t the trends predicted by two simplistic models. However, we show below that these additional poles very nicely ¢t our TPW model. Tarduno and Smirnov’s reasoning [1] is based upon the consideration of seven North American poles labeled A^G (¢gure 2 in [1]). Note that Pre¤vot et al.’s whole database [9] is now available through anonymous FTP at ftp.dstu.univmontp2.fr ; use your email address as the password and type cd pub/paleomag/epls2000. Our own selection criteria, discussed in detail in [9], are particularly stringent, which led us to reject, in a completely objective manner, three of the seven poles of Tarduno and Smirnov: especially we rejected poles B, D and G on the basis of too large Fisher concentration parameter (U), poles obtained mainly from syenitic intrusions (not granites, as mentioned in [1]), and Sierra Nevada batholith, respectively. In particular, Sierra Nevada obviously is not part of the North American craton, and this pole cannot be considered a reliable datum for the present purpose. We list in Table 1 the selected poles of Pre¤vot et al. [9] for the period between approximately 80 and 130 Ma. Tarduno and Smirnov [1] believe they can check the reality of the TPW shift observed around 110 Ma [9] by comparing, in the 40 Ma long bounding interval between 90 and 130 Ma, the paleolatitudes computed from these seven paleomagnetic poles with the paleolatitude curves predicted from two distinctly diierent models: (i) Model 1 (curve 1 in the present Fig. 1) assumes that the North American plate moved with respect to the hotspot system as described by Muller et al. [10] while the Earth’s rotation axis remained in its present posi-