Accretion from a Planetary Ring Around Earth as a Possible Explanation of the Shape of the IR Peak at the K/T Boundary
At most K/T boundary localities the Ir distribution exhibits shoulders, i.e., a gradual increase in the Ir content of the boundary material immediately before the K/T event and a gradual decrease in the Ir content after the K/T event. Here we investigate a version of the impact hypothesis that accounts both for the central peak and the shoulders of the Ir distribution. If the Ir is of extraterrestrial origin, the shoulders of the Ir distribution can be explained in two ways. The shoulders may reflect that boundary material has been redistributed by diffusion or bioturbation. But, as local chemical and biological environments must have been widely different at various localities, the fact that the shape of the Ir peak, with shoulders on both sides of the peak, is ubiquitous makes redistribution by bioturbation  or by diffusion  unlikely. Alternatively the shoulders of the Ir peak can be interpreted as a primary signal from the accretion of a planetary ring around the Earth. The evolution of a planetary ring encompasses two phases, first a fast and violent phase where all orbital elements change rapidly, and then a second much longer and more quiet phase. In the second phase of the development, the ring is in quasi-equilibrium . The ring particles are thought to be remnants of a decomposed asteroid inside the Roche limit of the Earth. Accretion profiles from a temporary ring around the Earth was shown to have similarities to the Ir concentration at the K/T boundary . We have now produced accretion profiles by a three-dimensional computer simulation of the dynamics of the quiet phase of a ring around the Earth. In the model the ring particles interact with one another in partly inelastic collisions and with the atmosphere of the Earth through a drag force. The collisions between the ring particles are too gentle to cause large-scale fragmentation of the particles. Figure 1 shows a typical accretion profile from a planetary ring around the Earth with two particle radii, calculated by our model. We assume that the asteroid was fully differentiated. The major part of the ring particles will be small Ir-poor silicate particles and only a minor part will be larger Ir-rich metallic particles. The main part of the silicate particles will accrete first, and after several ky the metallic particles will accrete onto the Earth constituting the central Ir peak. Finally the remaining silicate particles will accrete together with the very few remaining metallic particles, and thereby create the upper shoulder of the Ir peak. The variation in magnetic susceptibility of marine and terrestrial deposits at several K/T boundary localities indicate that the duration of deposition of the K/T boundary material is ca. 40 ky . This timescale is in good agreement with the duration of accretion from a planetary ring around the Earth. Acknowledgment: This work was supported by the Carlsberg Foundation. References:  Dyer B. D. et al. (1989) Geology, 17, 1036-1039.  Tredoux M. et al. (1989) J. Geol., 97, 585-605.  Brahic A. (1976), J. Computational Phys., 22, 171-188.  Stage M. and Rasmussen K. L. (1992) Meteoritics, 27, 292.  Hansen H. J. et al. (1992) Meteoritics, 27, 230. Figure 1, which appears in the hard copy, shows the distribution of accreted particles with two different radii, rp= 1 m and 10 m.
- Pub Date:
- July 1993
- CRETACEOUS-TERTIARY BOUNDARY