COMETARY IMPACTS INTO OCEAN - THEIR RECOGNITION AND THE THRESHOLD CONSTRAINT FOR BIOLOGICAL EXTINCTIONS

The Montagnais impact crater is presently the only site in the ocean w here the effect of a meteorite fall on marine organisms has been studi ed. The impact crater is 45 km in diameter and was formed at 50.8 Ma b y a fall of probably an old cometary nucleus 3.4 km in diameter, into shallow (<600 m) ocean. Comparison of the impact structure and related deposits with those on land shows several major differences of which the most significant is the absence of an elevated crater rim. Instead , the crater perimeter is bevelled and eroded as a consequence of impa ct induced bottom currents and turbulent, return water flow into the e xcavating cavity. By this process most of the fall-out breccia is rewo rked back into the crater cavity where it accumulates in much larger t hicknesses than in impact craters on land. At a microscopic scale, the shock metamorphism features are about the same as those for land impa cts. Geochemically, impacts of comet nucleii, may not leave a recogniz able signature at the impact horizon except for a minor increase in ir idium. Thus stratigraphic horizons associated with extinctions and/or major changes in biota have to be closely examined for other impact in dicators, like the presence of tectites, glass spherules, and quartz g rains with shock features. Occurrence of megatsunami wave deposits, ex tensive erosion on continental margins, margin failures and faunal mix ing above erosional unconformities are other potential impact indicato rs. There is no single indicator that can provide sufficient proof of an impact event. Such interpretations have to be based on multiparamet er studies of global extent, since many of the impact indicators are o nly of regional extent. The lack of extinction of any marine plankton genera, or of bottom dwellers at the Montagnais impact site allows us to place a lower limit for biological extinctions caused by cometary i mpacts on those with nucleus >4 km in diameter. The calculated frequen cy for a cometary impact which could result in a 10% extinction of mar ine genera is about 6 x 10(-7) yr-1 and for the K/T boundary type of e xtinctions about 2 x 10(-8) yr-1. Even allowing for a large degree of uncertainty in these estimates, it is unlikely that the biological ext inction events for the last 250 Ma identified by Sepkoski (1990) could have been all caused by meteorite impacts.