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

Abstract

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-