GEOPHYSICS, VOL. 63, NO.5 (SIPTEMBER-OCTOBER 1998); P 1585-1594,8 FIGS., 2 TABLES. Gauss's theorem, mass deficiency at Chicxulub crater (Yucatan, Mexico), and the extinction of the dinosaurs J. 0. Campos-Enriquez *, H. F. Morales-Rodriguez$, F. Dominguez - Mendez, and F. S. Birch ** ABSTRACT Using Gauss's theorem, we estimated the mass de- ficiency of the Chicxulub impact structure (Yucatan, Mexico) from its gravity anomaly. The mass deficiency obtained from the residual gravity anomaly map ranges between 1.06 x 10 16 and 1.67 x 10 16 kg. Because the gravity anomaly has approximately radial symmetry, we also estimated the mass deficiency from selected profiles. In this way, we obtained slightly lower mass deficien- cies (6.16 x 10 15 to 1.35 x 10 16 kg). The central gravity high, which is supposed to be associated with the central structural high, has a mean excess mass of 1.93 x 10 14 kg. By assuming a mean density contrast of 100 kg/m 3 be- tween the country rock and the sedimentary and brec- ciated rocks, we estimated the equivalent total mass (1.60 x 10 17 to 4.34 x 10 17 kg) and volume (6.16 x 10 13 to 1.67 x 10 14 m3 ) of breccias and sedimentary rocks re- sponsible for the gravity anomaly. These figures repre- sent lower bounds on the mass and volume ejected from the impact crater. They represent estimates made from geophysical principles and data, and compare well with independent estimates based on other principles such as scaling relations. According to actual estimations of the sulfur dioxide mass generated by the Cretaceous- Tertiary impact and our results, only a small fraction (about 1%) of the anhydrite in the target strata was vaporized. INTRODUCTION Why did the dinosaurs become extinct? Possible answers to this question are meteorite impact (Alvarez et al., 1980), and intense volcanism (Officer and Drake, 1985; Hallam, 1987; Rampino, 1987). Here, we will only address the impact theory. Discovery of an iridium anomaly during a paleomagnetic study carried out on the Botaccione sequence near the town of Gubbio (Italy) to study the transition from the Cretaceous to the Tertiary (K/T) led Alvarez et al. (1980) to propose a theory relating the massive biota extinction to global cli- matic changes due to the impact of a major meteorite. At the same time, gravity and magnetic studies in the Yucatan platform (Mexico) revealed conspicuous circular anomalies that Penfield and Camargo (1981) interpreted as the signa- ture of an impact. These two far-reaching scientific works were not linked immediately. Indeed, the catastrophic theory of Alvarez et al. (1980) needed support. This support derived from subsequent geologic, geochemical, and geochronologic studies conducted during the 1980s (Hildebrand et al., 1991; Sharpton et al., 1992; Blum et al., 1993; Koeberl, 1993; Krogh et al., 1993). In particular, the discovery and documentation of tsunami deposits enabled scientists to confine the impact site to the area of the Gulf of Mexico and the Caribbean (Hildebrand and Boyton, 1990). The search for the specific impact site(s) was launched (Hildebrand and Boyton, 1992). Hildebrand et al. (1991) revised the 10-year-old Penfield/Camargo inter- pretation, concurred with it, and further linked the Chicxulub impact crater and the massive biota extinction at the end of the Mesozoic. Current studies by Mexican, United States, and Canadian institutions also seem to confirm this in- terpretation (Sharpton et al., 1993; Pilkington et al., 1994; Campos-Enriquez et al., 1994; Hildebrand et al., 1995; Espindola et al., 1995; Urrutia-Fucugauchi et al., 1996), al- though arguments against it continue (Meyerhoff et al., 1994). Even if the existence of an impact structure in the sub- surface of northern Yucatan (Figure 1) has achieved near consensus, several questions still remain. In particular, the subsurface features of this impact structure need to be estab- lished. Because the Chicxulub impact structure lies under a cover of 300-1000 m of Tertiary sediments, geophysical meth- ods have been used to map its features. This task, however, is Manuscript received by the Editor February 07, 1997; revised manuscript received January 27, 1998. *Instituto de Geofisica, UNAM, Delegacidn Coyoacan, 04510 Mexico D.F., Mexico. E-mail: ocampos@tonatiuh.igeofcu.unam.mx. $IPN-ESIA-Ticoman, Ciencias de la Tierra, Mexico D.F., Mexico. E-mail: hmorales@moomsa.com.mx. E-mail: fdm@starmedia.com. **University of New Hampshire, College of Engineering and Physical Sciences, Department of Earth Sciences, James Hall, 56 College Road, Durham, New Hampshire 03824-3589. E -mail: fsb@hopper.unh.edu. © 1998 Society of Exploration Geophysicists. All rights reserved. 1585 Downloaded 02/08/16 to 132.248.6.197. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/