Figure 1.

Sixty-five million years ago, a large asteroid or comet slammed into the Yucatan peninsula of southeastern Mexico. The impacting object was 10 to 15km in diameter. If it was an asteroid, it probably moved at a velocity of about 20km per second. If it was a comet, it could have moved as fast as 60km per second. These velocities are phenomenally high. They far exceed the speed of sound and are 20 to 60 times greater than the speed of a fast bullet.
     At a velocity of 20km per second or greater, the initial impact would have been over in less than a second. The initial release of energy by the impact would have converted the impacting object and some of the target to silicate and metal vapor with a temperature of several thousand degrees Celsius. Within a couple of minutes, the extraordinarily hot vapor would have spread over the better part of a continent. The next fraction of energy would be released by ejection of melted target from the impact site. Most of the remaining energy would have been quickly released by ejection of solid debris from the growing crater. Dust from that solid debris would have spread globally within hours.
     It is widely believed that this impact drove most species of animals to extinction. An extraordinary range of organisms went extinct at about that time including dinosaurs, ammonites (which resembled shelled squid or cuttlefish), most of the clams, oysters, and other animals on the continental shelf, and many kinds of mammals and marine plankton. This mass extinction traditionally defines a boundary between two major periods in Earth history, the Cretaceous age of dinosaurs and ammonites, and the Tertiary age of mammals.
     The causes of extinction would have varied roughly with distance from the crater. Last year, Peter Schultz (Brown University) and I suggested that the impacting object struck at a low angle from the northeast and that extinctions in North America (and perhaps northeastern Asia) resulted from vaporization of the landscape by the downrange vapor cloud. However, the tremendous extinction of marine organisms and of animals on other continents must have been caused by a different mechanism, because the hot vapor cloud probably did not extend over the entire earth. In 1980, Luis Alvarez and his colleagues at University of California-Berkeley first proposed that dust from the impact caused global darkness, leading to cessation of photosynthesis and the collapse of food chains throughout the world. Although this proposal is difficult to test, it has real explanatory power, because it provides a reasonable explanation for mass extinction on land and in the sea at all latitudes. Analyses of congealed impact melt by Haraldur Sigurdsson (GSO faculty) and his colleagues further strengthened global darkness and cooling scenarios by showing that tremendous quantities of sulfur dioxide (SO2 ) were released from the Yucatan target (see Maritimes, August 1991). High atmospheric concentrations of SO 2 would have greatly enhanced the darkness and cooling that followed the impact.
     Studies of Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sites have contributed to our understanding of this impact and its consequences in at least three different ways. First, DSDP and ODP drilling has provided detailed records of the relative timing of the impact and marine extinctions. For example, careful examination of sediments from DSDP and ODP sites shows that skeletal assemblages of calcite-secreting zooplankton a few millimeters below the impact debris typically contain 30 to 40 species. These assemblages closely resemble the assemblages that were deposited for millions of years before the impact. By contrast, assemblages a few millimeters above the impact debris are dominated by a single species and are completely different than the pre-impact assemblages. Such studies provide clear evidence that the Yucatan impact caused a global mass extinction of marine plankton. By contrast, the rarity of dinosaur skeletons renders it very difficult to demonstrate that dinosaur extinction exactly coincided with the impact.
     Second, DSDP and ODP drilling have greatly increased the number of sites where we can examine the geographic consequences of the impact. For example, studies of impact debris from DSDP and ODP sites strengthen our hypothesis of an oblique impact by showing that the grain size of shocked debris is larger in North America than it is in the Pacific and Atlantic Oceans equidistant from the Yucatan.
     Third, and perhaps most astonishing, studies of DSDP and ODP sites show that the Yucatan impact changed the chemical and biological structure of the oceans for millions of years. Such studies demonstrate that the impact had at least three long-term effects on the oceans and climate: the mean seafloor accumulation of planktic calcite (CaCO3 ) skeletons decreased by a factor of four, the rain of organic matter from the photic zone to the deep ocean decreased to nearly zero, and the composition of deep-sea sediments strongly oscillated on hundred-thousand-year timescales for a million years after the impact.
     Several GSO scientists have played an important role in these DSDP/ODP-based discoveries. While at URI, James Zachos (GSO alumnus) and Michael Arthur (former GSO faculty) were the first to document the mean global decrease in calcite accumulation. Also while at URI, Zachos, Arthur, Lowell Stott (GSO alumnus), James Kennett (former GSO faculty), Danielle Luttenberg (GSO alumnus), and I all played active roles in documenting and interpreting the post-impact decrease in the deep-sea rain of organic matter. In the 1980's, researchers generally interpreted these decreases in organic rain and calcite accumulation as having resulted from low levels of biological production in post-impact oceans. More recently, Percy Donaghay (GSO marine scientist) and I re-interpreted these changes as the result of pronounced shifts in marine ecosystem structure.
     Regardless of their cause, these changes in organic rain and calcite sedimentation probably changed oceanic alkalinity, atmospheric CO2 concentrations, and ocean-climate interaction. For example, our recent studies suggest that in the South Atlantic, organic rain to the deep ocean did not recover for three million years after the impact. This long-lasting decrease in organic rain must have significantly decreased the export of carbon dioxide (CO2 ) from the surface ocean and atmosphere to the deep ocean. On time scales greater than a decade or two, this would have increased atmospheric CO 2 concentrations and may have increased global mean temperatures by a couple of degrees Celsius.
     Despite such predictions of mean climate change, previous studies have failed to find consistent changes in post-impact sea surface temperatures. A recent study of South Atlantic sediments by John King (GSO faculty), Carol Gibson (GSO technician), and me suggests a way to reconcile the expectation of climate change with a general lack of evidence for such change. In short, our discovery of 100,000-year oscillations in sedimentary composition suggests that the general state of the climate and oceans may have similarly oscillated for a million years. The 100,000-year period of these oscillations is suspiciously similar to the 100,000-year period of changes in eccentricity of earth's orbit. The timing, amplitude, and persistence of the post-impact oscillations suggest that the impact enhanced sensitivity of the oceans to orbital forcing for almost a million years. If so, the previous failure to identify long-term, post-impact changes in sea-surface temperatures may have simply resulted from failure to sample sediments at close enough intervals.
     URI researchers continue to use DSDP and ODP cores to refine our understanding of the Yucatan impact and its consequences. Last year, for example, Haraldur Sigurdsson (GSO faculty), Lew Abrams (GSO alumnus), Steven Carey (GSO faculty), King, Scott Rutherford (GSO student), and I took part in an ODP cruise in the Caribbean Sea. We successfully recovered debris from the Yucatan impact at two sites. Sigurdsson and Carey are studying the impact debris for further evidence of the impact's immediate consequences. Abrams, King, and I are examining geophysical logs of the drill holes and sediment compositions of the recovered cores to resolve two issues. The first issue is the duration of the interval characterized by low mean calcite accumulation (our preliminary results suggest that it may have lasted as much as five million years after the impact). The second issue is the geographic extent of the 100,000-year oscillations in post-impact marine sediments. If there were indeed a consequence of global oscillations in the oceans and climate, we should see them in the Caribbean cores.
     Whatever the outcome of these Caribbean studies, URI studies of DSDP and ODP cores have already shown that events occurring in less than a heartbeat can change the entire world for millions of years.

Recommended Reading
D'Hondt, S., J. King, & C. Gibson, 1996. An oscillatory marine response to Cretaceous/Tertiary impact.Geology 24, 611-614.
D'Hondt, S., T.D. Herbert, J. King, and C. Gibson, 1996. Planktic foraminifera, asteroids, and marine production: death and recovery at the Cretaceous-Tertiary boundary. In New developments regarding the K/T event and other catastrophes in earth history, Geological Society of America Special Paper 307, ed. by G.T. Ryder, D.E. Fastovsky, and S. Gartner, 303-317.
Schultz, P., and S. D'Hondt, 1996. The Cretaceous/Tertiary (Chicxulub) impact angle and its consequences. Geology 24, 963-967.
Sheehan, P.M., D.E. Fastovsky, R.G. Hoffmann, C.B. Berghaus, and D.L. Gabriel, 1991. Sudden extinction of the dinosaurs: Latest Cretaceous, Upper Great Plains, USA. Science, 254, 835-839.
Sigurdsson, H., S. D'Hondt, and S. Carey, 1992. The impact of the Cretaceous/Tertiary bolide on evaporite terrane and generation of a major sulfuric acid aerosol. Earth and Planetary Science Letters, 109 (3/4), 543-559.
Zachos, J.C., M.A. Arthur, and W.E. Dean, 1989. Geochemical evidence for suppression of pelagic marine productivity at the Cretaceous/Tertiary boundary. Nature, 337, 61-64.