DOI: 10.1126/science.1084114 , 791 (2003); 300Science et al.Eske Willerslev,Holocene and Pleistocene SedimentsDiverse Plant and Animal Genetic Records from www.sciencemag.org (this information is current as of October 20, 2009 ):The following resources related to this article are available online at http://www.sciencemag.org/cgi/content/full/300/5620/791version of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, http://www.sciencemag.org/cgi/content/full/1084114/DC1 can be found at: Supporting Online Material http://www.sciencemag.org/cgi/content/full/300/5620/791#otherarticles, 6 of which can be accessed for free: cites 18 articlesThis article 66 article(s) on the ISI Web of Science. cited byThis article has been http://www.sciencemag.org/cgi/content/full/300/5620/791#otherarticles 25 articles hosted by HighWire Press; see: cited byThis article has been http://www.sciencemag.org/cgi/collection/paleoPaleontology : subject collectionsThis article appears in the following http://www.sciencemag.org/about/permissions.dtl in whole or in part can be found at: this articlepermission to reproduce of this article or about obtaining reprintsInformation about obtaining registered trademark of AAAS. is aScience2003 by the American Association for the Advancement of Science; all rights reserved. The title CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience on October 20, 2009 www.sciencemag.orgDownloaded fromity. The transition could thus be accompaniedby phase separation between iron-rich HSand magnesium-rich LS ferropericlases. Thiscould explain x-ray diffraction observationssuggesting a breakdown of ferropericlase(24), which were observed in the same pres-sure range as our spin transition.The observed transitions indicate that thelower mantle would be segregated into twodifferent layers characterized by different ironpartitioning between perovskite and ferroperi-clase. The transition pressures are consistentwith the depths at which lower mantle layeringhas been proposed (1) and provide a mineralphysics basis for an Earth’s lower mantle madeof two distinct layers. Moreover, observationsthat the iron-free olivine (forsterite) is moreviscous than iron-bearing phases (25, 26) implythat, similarly, an iron-free perovskite is likelyto be more viscous than an iron-rich one. Thisidea is also corroborated by the fact that viscos-ity scales in many materials (27) with T/Tm(theratio of temperature to melting temperature)and that the melting point of an iron-free per-ovskite is much higher than that of an iron-bearing one (28, 29). In that sense, becauseperovskite is the major lower-mantle phase, thetransition could have a fairly strong rheologicalsignature as inferred from geophysical observa-tions (30) and could affect the geodynamics inthe lowermost mantle. Further studies couldconstrain geodynamical interpretations (2, 3)ofthe seismic observations and could enablequantification of the effect of such a viscouslayer on the dynamics of plumes. Note that sucha layering model requires no isolated convec-tion cells, because the chemistry of the twolayers is reversible as a function of depth (thetransition is reversible upon decompression);uplifted materials will recover the partitioningproperties of the top layer.Iron-free perovskite is stable to very highpressures and temperatures. It was speculatedthat the breakdown of iron-bearing perovskite(6) at the core-mantle boundary (CMB) wasresponsible for the chemical heterogeneitiesobserved in the D⬙ layer. We suggest, how-ever, that the iron-free end member is the onethat is most likely to be present at thosedepths, and that the interaction of iron-richferropericlase with the liquid outer coreshould instead be taken into consideration.Geodynamical modeling of this lowermostlayer could contribute to our understanding ofcore-mantle interactions because dominantiron-depleted perovskite could create an elec-trically, thermally (31), and rheologically in-sulating lid above the CMB.References and Notes1. R. van der Hilst, S. Ka´rason, Science 283, 1885 (1999).2. T. Lay, Q. Williams, E. J. Garnero, Nature 392, 461(1998).3. L. H. Kellogg, B. H. Hager, R. D. van der Hilst, Science283, 1881 (1999).4. E. Knittle, R. Jeanloz, Science 235, 668 (1987).5. S. E. Kesson, J. D. Fitz Gerald, J. M. Shelly, Nature 393,253 (1998).6. G. Serghiou, A. Zerr, R. Boehler, Science 280, 2093(1998).7. G. Fiquet et al., Geophys. Res. Lett. 27, 21 (2000).8. D. Andrault, J. Geophys. Res. 106, 2079 (2001).9. S.-H. Shim, T. S. Duffy, G. Shen, Science 292, 2437(2001).10. D. M. Shermann, J. Geophys. Res. 96, 14299 (1991).11. J. Brodholt, unpublished data.12. R. E. Cohen, I. I. Mazin, D. G. Isaak, Science 275, 654(1997).13. The sample (Mg0.83Fe0.17)O was prepared by anneal-ing of a mixture of MgO and Fe2O3maintained at1673 K for 24 hours under a controlled oxygenfugacity between 10⫺9and 10⫺11atm. These sam-ples were characterized by x-ray diffraction andshown to be pure ferropericlase.14. See supporting data on Science Online.15. G. Peng et al., Appl. Phys. Lett. 65, 2527 (1994).16. F. M. F. de Groot et al., Phys. Rev. B 51, 1045 (1995).17. X. Wang et al., Phys. Rev. B 56, 4553 (1997).18. J.-P. Rueff et al., Phys. Rev. Lett. 82, 3284 (1999).19. J. Badro et al., Phys. Rev. Lett. 83, 4101 (1999).20. J.-P. Rueff et al., Phys. Rev. B 60, 14510 (1999).21. F. Sette et al., Phys. Rev. Lett. 75, 850 (1995).22. V. Malavergne, F. Guyot, Y. Wang, I. Martinez, EarthPlanet. Sci. Lett. 146, 499 (1997).23. The critical parameter for this type of transition isdensity; in the absence of structural phase transi-tions, the effect of temperature is to increase thetransition pressure to compensate for the thermalexpansion. It is therefore not possible to expressprecisely the transition pressures in terms of depthswithin Earth, because the thermal expansion coeffi-cient at those pressures is not known.24. L. Dubrovinsky et al., Eur. J. Mineral. 13, 857 (2001).25. W. B. Durham, C. Goetze, J. Geophys. Res. 82, 5737(1977).26. W. B. Durham, C. Froideveaux, O. Jaoul, Phys. EarthPlanet. Inter. 19, 263 (1979).27. J. Weertman, J. R. Weertman, Annu. Rev. Geophys. 3,293 (1975).28. P. E. van Keken, D. A. Yuen, A. P. van den Berg, J.Geophys. Res. 100, 15233 (1995).29. A. Zerr,
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