Helen V. Michel

Extraterrestrial Cause for the Cretaceous-Tertiary Extinction

Platinum metals are depleted in the earths crust relative to their cosmic abundance; concentrations of these elements in deep-sea sediments may thus indicate influxes of extraterrestrial material. Deep-sea limestones exposed in Italy, Denmark, and New Zealand show iridium increases of about 30, 160, and 20 times, respectively, above the background level at precisely the time of the Cretaceous-Tertiary extinctions, 65 million years ago. Reasons are given to indicate that this iridium is of extraterrestrial origin, but did not come from a nearby supernova. A hypothesis is suggested which accounts for the extinctions and the iridium observations. Impact of a large earth-crossing asteroid would inject about 60 times the objects mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record. One prediction of this hypothesis has been verified: the chemical composition of the boundary clay, which is thought to come from the stratospheric dust, is markedly different from that of clay mixed with the Cretaceous and Tertiary limestones, which are chemically similar to each other. Four different independent estimates of the diameter of the asteroid give values that lie in the range 10 � 4 kilometers.

Spheroids at the Cretaceous-Tertiary boundary are altered impact droplets of basaltic composition

Sand-size spheroids of K-feldspar in the Cretaceous-Tertiary (C-T) boundary clay at Caravaca, southern Spain, were interpreted by Smit and Klaver as having solidified from a melt resulting from the impact of a large extraterrestrial body. Sand-size spheroids of K-feldspar, glauconite, and magnetite-quartz have been found in the C-T boundary clay in northern Italy, and spheroids of K-feldspar and pyrite were found in the boundary clay at Deep Sea Drilling Project Site 465A, in the central Pacific. These spheroids have textures similar to those of rapidly crystallized feldspar and mafic silicates. They are interpreted as diagenetically altered microcrystalline spherules of basaltic composition produced by the impact of a large asteroid in an ocean basin at the end of the Cretaceous. They are analogous to the glassy microtektites produced by impacts on more siliceous target rocks. 21 references, 4 figures.

Impact Theory of Mass Extinctions and the Invertebrate Fossil Record

There is much evidence that the Cretaceous-Tertiary boundary was marked by a massive meteorite impact. Theoretical consideration of the consquences of such an impact predicts sharp extinctions in many groups of animals precisely at the boundary. Paleontological data clearly show gradual declines in diversity over the last 1 to 10 million years in various invertebrate groups.Reexamination of data from careful studies of the best sections shows that, in addition to undergoing the decline, four groups (ammonites, cheilostomate beyozoans, brachiopods, and bivalves) were affected by sudden truncations precisely at the iridium anomaly that marks the boundary. The paleontological record thus bears witness to terminal-Cretaceous extinctions on two time scales: a slow decline unrelated to the impact and a sharp truncation synchronous with and probably caused by the impact.

Iridium Anomaly Approximately Synchronous with Terminal Eocene Extinctions

An iridium anomaly has been found in coincidence with the known microtektite level in cores from Deep Sea Drilling Project site 149 in the Caribbean Sea. The iridium was probably not in the microtektites but deposited simultaneously with them; this could occur if the iridium was deposited from a dust cloud resulting from a bolide impact, as suggested for the anomaly associated with the Cretaceous-Tertiary boundary. Other workers have deduced that the microtektites are part of the North American strewn tektite field, which is dated at about 34 million years before present, and that the microtektite horizon in deep-sea cores is synchronous with the extinction of five radiolarian species. Mass extinctions also occur in terrestrial mammals within 4 million years of this time. The iridium anomaly and the tektites and microtektites are supportive of a major bolide impact about 34 million years ago.

The End of the Cretaceous: Sharp Boundary or Gradual Transition?

Evidence indicates that the Cretaceous-Tertiary boundary is very sharp, and, within the limits of resolution, it is apparently synchronous at the various boundary localities. Arguments to the contrary, particularly those of Officer and Darke, are shown to invalid.

The Precursor of the Cretaceous-Tertiary Boundary Clays at Stevns Klint, Denmark, and DSDP Hole 465A

Results of detailed mineralogical, chemical, and oxygen isotope analyses of the clay minerals and zeolites from two Cretaceous-Tertiary (K/T) boundary regions, Stevns Klint, Denmark, and Deep Sea Drilling Project (DSDP) Hole 465A in the north central Pacific Ocean, are presented. In the central part of the Stevns Klint K/T boundary layer, the only clay mineral detected by x-ray diffraction is a pure smectite with > 95 percent expandable layers. No detrital clay minerals or quartz were observed in the clay size fraction in these beds, whereas the clay minerals above and below the boundary layer are illite and mixed-layer smectite-illite of detrital origin as well as quartz. The mineralogical purity of the clay fraction, the presence of smectite only at the boundary, and the δ18O value of the smectite (27.2 � 0.2 per mil) suggest that it formed in situ by alteration of glass. Formation from impact rather than from volcanic glass is supported by its major element chemistry. The high content of iridium and other siderophile elements is not due to the cessation of calcium carbonate deposition and resulting slow sedimentation rates. At DSDP Hole 465A, the principal clay mineral in the boundary zone (80 to 143 centimeters) is a mixed-layer smectite-illite with ≥90 percent expandable layers, accompanied by some detrital quartz and small amounts of a euhedral authigenic zeolite (clinoptilolite). The mixed-layer smectite-illite from the interval 118 to 120 centimeters in the zone of high iridium abundance has a very low rare earth element content; the negative cerium anomaly indicates formation in the marine environment. This conclusion is corroborated by the δ18O value of this clay mineral (27.1 � 0.2 per mil). Thus, this mixed-layer smectite-illite formed possibly from the same glass as the K/T boundary smectite at Stevns Klint, Denmark.

Iridium anomalies of late Eocene age at Massignano (Italy), and ODP Site 689B (Maud Rise, Antarctic)

Iridium anomalies of Late Eocene age are here reported in sequences from northern Italy and Weddell Sea, Antarctic. We have analyzed 270 samples of marly limestones from the Eocene/Oligocene boundary stratotype section at Massignano, Italy, ranging from 35.6 to 34.5 Ma (NP 18 Zone to NP 19/20 Zone). We discovered a distinct Ir peak of 156 ± 19 parts per trillion (ppt) in the lower calcareous nannofossil NP 19/20 Zone, corresponding to the mid-lower part of foraminiferal P16 Zone, and the top of magnetic Chron 16n2. The age of this Ir anomaly derived from several dated volcanic ashes found in the same section is 35.7 ± 0.4 Ma

High-Precision Chemical Characterization of Major Obsidian Sources in Guatemala

The important Mesoamerican obsidian source El Chayal has been carefully explored and sampled. Forty new source specimens have been subjected to neutron activation analysis, and the composition patterns are recorded and compared. The westernmost of the known Guatemalan obsidian sources, Tajamulco, has also been studied. Precise analytical data are also reported for the important Ixtepeque and Rio Pixcaya sources. In addition, the complete analytical data for a group of artifacts in Belize excavated by Norman Hammond are reported and compared with the source compositions. The establishment in the literature of a library or collection of precise and intercomparable composition data for the sources of this general region is suggested, and the present paper is offered as a first contribution toward this objective. Elemental analysis of obsidian has been shown to be useful in reconstructing ancient trade routes. A sizable amount of work has been done on Mesoamerican sites and sources (Cobean et al. 1971; Hammond 1972; Heizer et al. 1965; Stross et al. 1976a). In this paper we propose to discuss 2 problems that have naturally arisen in these studies and to suggest at least partial solutions. reliability of the results have tended to differ from author to author. The second problem concerns the cost of reliable analysis and sampling, which is perhaps far higher than necessary. Previous research has shown that in some cases a correlation of an artifact with its source can be made by analysis of only 2 or 3 elements, but in other instances a far greater number of elements must be determined to reach a firm conclusion with regard to the source of the glass. This is true not only for the obvious reason that the chance of overlapping patterns increases with the increasing number of supply sources and archaeological sites, but because a priori statements on the chemical composition pattern of an unanalyzed obsidian source are impossible to make. An obsidian source or supply area may have several outcrops, which may or may not be chemically similar. However, even an individual outcrop may show a systematic variation within its own boundaries (Bowman et al. 1973). A source thus has to be adequately sampled so that its homogeneity may be demonstrated, or if it is found to be heterogeneous, so that the character of the variation in its composition may be defined by analysis. It is thus desirable to develop libraries of composition patterns for all sources of archaeological obsidian in a given region. These composition records should be as complete, accurate, and precise as possible. They should be published in units and in a form that makes it convenient to compare them, in order to be of use to all interested in identifying artifacts according to source. It is also desirable that the data be published in journals that are generally known and accessible to those working on the subject. Comparison of the composition patterns of the obsidian of a given cultural area often reveals that determination of the abundances of a limited number of elements is sufficient to distinguish the sources within the region in question. The determination of these few elements in the artifacts collected within this area should be sufficient to assign provenience to these artifacts, and this would be far less expensive than a complete analysis of the samples. When a generally applicable, detailed catalog of composition patterns is available, it is not too difficult to determine the most economical method for matching artifacts with their respective sources. Existence of

PRECISE CHARACTERIZATION OF GUATEMALAN OBSIDIAN SOURCES, AND SOURCE DETERMINATION OF ARTIFACTS FROM QUIRIGUA

LBL-12252 Revised Preprint eley L boratory ALIFORNIA Submitted to American Antiquity PRECISE CHARACTERIZATION OF GUATEMALAN OBSIDIAN SOURCES, AND SOURCE DETERMINATION OF ARTIFACTS FROM QUIRIGUA ... Fred H. Stross, Payson Sheets, Frank Asaro, and Helen V. Michel November 1981 TWO-WEEK LOAN COPY This is a Library Circulating Copy which may be borrowed for two weeks. For a personal retention copy, call Tech. Info. Diu is ion, Ext. 6782 Prepared for the U.S. Department of Energy under Contract W-7405-ENG-48

Ecuadorian Obsidian Sources Used for Artifact Production and Methods for Provenience Assignments

A study by neutron activation analysis and X-ray fluorescence measurements of 116 Ecuadorian obsidian artifacts as well as primary and secondary source samples has shown that the material originatedfrom two Andean sources in north-central Ecuador, a flow in the valley of Mullumica, and deposits fou-nd on Yanaurco Chico and the nearby Quiscatola crest. The Yanaurco-Quiscatola deposit complex, homogeneous within our ability to measure, is represented by 18 percent of the artifacts. The Mullumica source has variable, apparently linearly related element abundances that may have resultedfrom the incomplete mixingof two magmas within a magma chamber. Seventy-eight percent of the samples studied are deduced to have originatedfrom that source. Five artifacts from La Chimba cannot be assigned an origin, but four of them have the same provenience. To test the validity of the mixed-magma theory, we present equations to calculate the abundance of any measured element from that of iron. On the average, agreement within about 3 percent is obtained for artifacts.