Cometary impacts into ocean: their recognition and the threshold constraint for biological extinctions

摘要: The Montagnais impact crater is presently the only site in ocean where effect of a meteorite fall on marine organisms has been studied. 45 km diameter and was formed at 50.8 Ma by probably an old cometary. nucleus 3.4 diameter, into shallow (<600 m) ocean. Comparison structure related deposits with those land shows several major differences which most significant absence elevated rim. Instead, perimeter bevelled eroded as consequence induced bottom currents turbulent, return water flow excavating cavity. By this process fall-out breccia reworked back cavity it accumulates much larger thickness than craters land. At microscopic scale, shock metamorphism features are about same for impacts. Geochemically, impacts comet nucleii, may not leave recognizable signature horizon except minor increase iridium. Thus stratigraphic horizons associated extinctions and/or changes biota have to be closely examined other indicators, like presence tectites, glass spherules, quartz grains features. Occurrence megatsunami wave deposits, extensive erosion continental margins, margin failures faunal mixing above erosional unconformities potential indicators. There no single indicator that can provide sufficient proof event. Such interpretations based multiparameter studies global extent, since many indicators regional extent. The lack extinction any plankton genera, or dwellers allows us place lower limit biological caused cometary >4 diameter. calculated frequency could result 10% genera 6 × 10−7 yr−1 K/T boundary type 2 10−8 yr−1. Even allowing large degree uncertainty these estimates, unlikely events last 250 identified Sepkoski (1990) all impacts.