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Cretaceous-Tertiary Extinct Tulated Arctic Lake (12) 6 June 1980, Volume 208, Number 4448 SCIENCE extinctions (3, 4), and two recent meet- ings on the topic (5, 6) produced no sign of a consensus. Suggested causes in- clude gradual or rapid changes in ocean- ographic, atmospheric, or climatic con- ditions (7) due to a random (8) or a cy- clical (9) coincidence of causative fac- Extraterrestrial Cause for the tors; magnetic reversal (10); nearby supernova (11); and the flooding of the tion ocean surface by fresh water from a pos- Cretaceous-Tertiary Extinct tulated arctic lake (12). A major obstacle to determining the Experimental results and theoretical interpret.ation cause of the extinction is that virtually all the available information on events at the time of the crisis deals with biological Luis W. Alvarez, Walter Alvarez, Frank Asaro, Helen V. A 4ichel changes seen in the paleontological rec- ord and is therefore inherently indirect. Little physical evidence is available, and it also is indirect. This includes varia- on August 31, 2008 In the 570-million-year period for microscopic floating ainimals and plants; tions in stable oxygen and carbon isotop- which abundant fossil remains are avail- both the calcareous pAlanktonic forami- ic ratios across the boundary in pelagic able, there have been five great biologi- nifera and the calcarec us nannoplankton sediments, which may reflect changes in cal crises, during which many groups of were nearly extermin.ated, with only a temperature, salinity, oxygenation, and organisms died out. The most recent of few species surviving the crisis. On the organic productivity of the ocean water, the great extinctions is used to define the other hand, some groiups were little af- and which are not easy to interpret (13, boundary between the Cretaceous and fected, including the 1;and plants, croco- 14). These isotopic changes are not par- Tertiary periods, about 65 million years diles, snakes, mammalIs, and many kinds ticularly striking and, taken by them- www.sciencemag.org selves, would not suggest a dramatic crisis. Small changes in minor and trace Summary. Platinum metals are depleted in the earth's crust relattive to their cosmic element levels at the C-T boundary have abundance; concentrations of these elements in deep-sea sediments may thus in- been noted from limestone sections in dicate influxes of extraterrestrial material. Deep-sea limestones exposed in Italy, Den- Denmark and Italy (15), but these data mark, and New Zealand show iridium increases of about 30, 160, and 20 times, re- also present interpretational difficulties. spectively, above the background level at precisely the time of th e Cretaceous-Ter- It is noteworthy that in pelagic marine tiary extinctions, 65 million years ago. Reasons are given to indicatED that this iridium is sequences, where nearly continuous Downloaded from of extraterrestrial origin, but did not come from a nearby supernovfa. A hypothesis is deposition is to be expected, the C-T suggested which accounts for the extinctions and the iridium obseirvations. Impact of boundary is commonly marked by a hia- a large earth-crossing asteroid would inject about 60 times the obj4act's mass into the tus (3, 16). atmosphere as pulverized rock; a fraction of this dust would stay iin the stratosphere In this article we present direct phys- for several years and be distributed worldwide. The resulting darrkness would sup- ical evidence for an unusual event at ex- press photosynthesis, and the expected biological consequences nnatch quite closely actly the time of the extinctions in the the extinctions observed in the paleontological record. One prediction of this hypothe- planktonic realm. None of the current sis has been verified: the chemical composition of the boundary claLy, which is thought hypotheses adequately accounts for this to come from the stratospheric dust, is markedly different from that:of clay mixed with evidence, but we have developed a hy- the Cretaceous and Tertiary limestones, which are chemically sirriilar to each other. pothesis that appears to offer a satisfac- Four different independent estimates of the diameter of the asteroid give values that tory explanation for nearly all the avail- lie in the range 10 ± 4 kilometers. able paleontological and physical evi- dence. ago. At this time, the marine reptiles, thfe of invertebrates. Russell (2) concludes Luis Alvarez is professor emeritus of physics at flying reptiles, and both orders of dino- that about half of the genera living at that Lawrence Berkeley Laboratory, University of Cali- saurs died out (1), and extinctions oc- time the extinction fornia, Berkeley 94720. Walter Alvarez is an associ- perished during ate professor in the Department of Geology and curred at various taxonomic levels event. Geophysics, University of California, Berkeley. Frank Asaro is a senior scientist and Helen Michel is among the marine invertebrates. Dra- Many hypotheses have been proposed a staff scientist in the Energy and Environment Divi- matic extinctions occurred among the to explain the Cretaceous-Tertiary (C-T) sion of Lawrence Berkeley Laboratory. SCIENCE, VOL. 208, 6 JUNE 1980 0036-8075/80/0606-1095$02.00/0 Copyright © 1980 AAAS 1095 Identification of Extraterrestrial and summarized other previous work. sa, has a matrix of coccoliths and cocco- Platinum Metals in Deep-Sea Sediments Considerations of this type (23) lith fragments (calcite platelets, on the prompted us to measure the iridium con- order of 1 micrometer in size, secreted This study began with the realization centration in the 1-centimeter-thick clay by algae living in the surface waters) and that the platinum group elements (plati- layer that marks the C-T boundary in a rich assemblage of foraminiferal tests num, iridium, osmium, and rhodium) some sections in the Umbrian Apen- (calcite shells, generally in the size range are much less abundant in the earth's nines, in the hope of determining the 0.1 to 2.0 millimeters, produced by crust and upper mantle than they are in length of time represented by that layer. single-celled animals that float in the sur- chondritic meteorites and average solar Iridium can easily be determined at low face waters). system material. Depletion of the plati- levels by neutron activation analysis In some Umbrian sections there is a num group elements in the earth's crust (NAA) (24) because of its large capture hiatus in the sedimentary record across and upper mantle is probably the result cross section for slow neutrons, and be- the C-T boundary, sometimes with signs of concentration of these elements in the cause some of the gamma rays given off of soft-sediment slumping. Where the se- earth's core. during de-excitation of the decay prod- quence is apparently complete, forami- Pettersson and Rotschi (17) and Gold- uct are not masked by other gamma rays. nifera typical of the Upper Cretaceous schmidt (18) suggested that the low con- The other platinum group elements are (notably the genus Globotruncana) dis- centrations of platinum group elements more difficult to determine by NAA. appear abruptly and are replaced by the in sedimentary rocks might come largely basal Tertiary foraminifer Globigerina from meteoritic dust formed by ablation eugubina (16, 26). This change is easy to when meteorites passed through the at- Italian Stratigraphic Sections recognize because G. eugubina, unlike mosphere. Barker and Anders (19) the globotruncanids, is too small to see showed that there was a correlation be- Many aspects of earth history are best with the naked eye or the hand lens (Fig. tween sedimentation rate and iridium recorded in pelagic sedimentary rocks, 1). The coccoliths also show an abrupt concentration, confirming the earlier which gradually accumulate in the rela- change, with disappearance of Cre- suggestions. Subsequently, the method tively quiet waters of the deep sea as in- taceous forms, at exactly the same level was used by Ganapathy, Brownlee, and dividual grains settle to the bottom. In as the foraminiferal change, although Hodge (20) to demonstrate an extra- the Umbrian Apennines of northern pe- this was not recognized until more re- terrestrial origin for silicate spherules in ninsular Italy there are exposures of pe- cently (27). deep-sea sediments. Sarna-Wojcicki et lagic sedimentary rocks representing the In well-exposed, complete sections al. (21) suggested that meteoritic dust ac- time from Early Jurassic to Oligocene, there is a bed of clay about 1 cm thick on August 31, 2008 cumulation in soil layers might enhance around 185 to 30 million years ago (25). between the highest Cretaceous and the the abundance of iridium sufficiently to The C-T boundary occurs within a por- lowest Tertiary limestone beds (28). This permit its use as a dating tool. Recently, tion of the sequence formed by pink bed is free of primary CaCO3, so there is Crocket and Kuo (22) reported iridi- limestone containing a variable amount no record of the biological changes dur- um abundances in deep-sea sediments of clay. This limestone, the Scaglia ros- ing the time interval represented by the clay. The boundary is further marked by a zone in the uppermost Cretaceous in which the normally pink limestone is www.sciencemag.org white in color. This zone is 0.3 to 1.0 me- ter thick, varying from section to sec- tion. Its lower boundary is a gradational color change; its upper boundary is abrupt and coincides with the faunal and floral extinctions. In one section (Con- tessa) we can see that the lower 5 mm of Fig. 1. Photomicro- the boundary clay is gray and the upper 5 Downloaded from graphs of (a) the basal mm is red, thus placing the upper bound- bed of the Tertiary, of the zone in the middle of the clay showing Globigerina ary eugubina, and (b) the layer. top bed of the Cre- The best known of the Umbrian sec- taceous, in which the tions is in the Bottaccione Gorge near largest foraminifer is Gubbio.
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