width of the immiscibility gap in the MgO- FeO system at high pressure, nor can we determine whether ferropericlase might dis- sociate completely into MgO and FeO after a sufficiently long time of heating. However, the ferropericlases at least partially dissociate into a phase with lower density (Mg-rich, 6.1 g/cm 3 ) and a phase with higher density (Fe-rich, 7.8 g/cm 3 ) at 85 GPa, correspond- ing to a depth of 1900 to 2000 km (Prelimi- nary Reference Earth Model). Such dissocia- tion of ferropericlase along with phase tran- sitions in silica (23) and possible dissociation of (Mg,Fe)SiO 3 -perovskite (10, 17, 23) may lead to the heterogeneity of the lower mantle. References and Notes 1. M. Rosenhauer, H. K. Mao, E. Woermann, Yearb. Carnegie Inst. Washington 75, 513 (1976). 2. P. Richet, H. K. Mao, P. M. Bell, J. Geophys. Res. 94, 3037 (1989). 3. D. G. Isaak, R. E. Cohen, M. J. Mehl, J. Geophys. Res. 95, 7055 (1990). 4. Y. Fei, H. K. Mao, J. Shu, J. Hu, Phys. Chem. 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At Uppsala Lab we have obtained powder x-ray dif- fraction data with a Siemens x-ray system consisting of a Smart charge-coupled device (CCD) area detec- tor and a direct-drive rotating anode as x-ray gener- ator (18 kW ). The Mo K  radiation (tube voltage 50 kV, tube current 24 mA, cathode gun 0.1  1 mm) is focused with a capillary x-ray optical system to 40 m full width at half-maximum. At ESRF we collected the powder diffraction data with a fine incident x-ray beam (approximately rectangular shape with dimen- sions 8 m by 9 m or less) of wavelength 0.3738 Å on the FastScan imaging plate. The collected images were integrated to obtain conventional diffraction spectra. 19. A. C. Larson and R. B. Von Dreele, GSAS (General Structure Analysis System) LANSCE, MS-H805 (Los Alamos National Laboratory, Los Alamos, NM, 1994), p. 86. 20. S. Stolen, R. Glokner, F. Gronvold, T. Atake, S. Izumi- sawa, Am. Mineral. 81, 973 (1996). 21. E. Hålenius, H. Annersten, S. Jo ¨nsson, Geochem. Soc. Spec. Pub. 5 (1996), p. 255. Mo ¨ssbauer experiments were performed using a conventional transmission geometry. Measurements were made on the quenched samples mounted on the diamond and leading the gamma rays through the gasket hole. A point source of 57 Co in Rh, nominal value 60 mCi, was used in the experiment and calibrated against -Fe at room temperature. 22. Chemical analyses were obtained using the CAM- EBAX SX-50 electron microprobe with a phi-rho-Z correction procedure (operating conditions: acceler- ating potential, 20 kV; beam current, 50 nA; beam diameter, 1 to 2 m). Counting times were 10 s. The following analyzing crystals were used: TAP for Mg K  , Al K  , and Si K  ; LiF for Mn K  and Fe K  . 23. S. K. Saxena, P. Lazor, L. S. Dubrovinsky, Mineral. Petrol. 69, 1 (2000). 24. Supported by the Swedish Natural Science Council and the Wallenberg and Crafoords Funds. The syn- chrotron x-ray studies were conducted at ESRF, Grenoble, on the ID30 beam line. The comments of two anonymous referees were useful. 6 March 2000; accepted 30 May 2000 Pattern of Marine Mass Extinction Near the Permian-Triassic Boundary in South China Y. G. Jin, 1 * Y. Wang, 1 W. Wang, 1 Q. H. Shang, 1 C. Q. Cao, 1 D. H. Erwin 2 The Meishan section across the Permian-Triassic boundary in South China is the most thoroughly investigated in the world. A statistical analysis of the occur- rences of 162 genera and 333 species confirms a sudden extinction event at 251.4 million years ago, coincident with a dramatic depletion of  13 C carbonate and an increase in microspherules. The end-Permian mass extinction eliminated over 90% of all marine species and had a significant impact on land species as well (1, 2). However, geochronologic results from South China reveal that the main extinction occurred over a period of less than 500,000 years (3), coincident with the eruption of the Siberian flood basalts (4, 5) and with a sharp shift in  13 C carb (6 ). Although there are claims for multiple pulses of extinction, in- cluding at least three at the classic Meishan sections in South China (7, 8) [probably the most thoroughly studied Permian-Triassic (P-T) marine boundary section in the world], the cause of the extinction remains enigmatic. Here we examine sampling and preservation effects (9) using a statistical analysis of spe- cies’ stratigraphic ranges (10, 11) to demon- strate the extreme rapidity of the extinction. 1 Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing 210008, China. 2 Depart- ment of Paleobiology, MRC-121, National Museum of Natural History, Washington, DC 20560, USA. *To whom correspondence should be addressed. E- mail: ygjin@public1.ptt.js.cn A B Fig. 3. Examples of images collected (A) on the ID30 beam line at ESRF with monochromatic 0.3738 Å radiation with the Fast Scan image plate, and (B) with in-house x-ray facilities at Uppsala Lab (Mo K  radiation, Smart CCD area detector) demonstrating the splitting of ferro- periclase reflections after long heating at tem- peratures above 950 K and pressures above 80 GPa. Inset shows backscattered electron image of the recovered sample (black, iron-rich parts; white, magnesium-rich parts). R EPORTS 21 JULY 2000 VOL 289 SCIENCE www.sciencemag.org 432