Iridium Cretaceous-Tertiary

The historical background is presented for the asteroid-impact theory that is based on the iridium anomaly found in rocks frm the Cretaceous-Tertiary boundary. Recent measurements of Ir, Pt, and Au abundances from such rocks in Denmark have shown that the element abundance ratios are different from mantle-derived sources and agree with values for chondritic meteorites within one standard deviation of the measurement errors (7-10%). Rare-earth patterns for these rocks are... 

There have been a number of suggestions in the past that an extraterrestrial object impacting on the. earth caused or could cause massive extinctions of life. E. J. Opik [11], for example, discussed the lethal effects which could be caused by the heat generated from such objects striking the earth, and H. C. Urey [12] stated specifically that a comet was probably the cause of the Cretaceous-Tertiary extinctions. There have also been science fiction stories and a movie relating to the effects. The events likely to occur if the sunlight were temporarily "turned off" have also been discussed [13]. Our deduction in contrast to the others is based on physical science data (the iridium anomaly) and is the only explanation we found which explained the Ir anomaloy could cause the massive extinction of life and was likely to occur in a period of 100 million years. 

Anomalous Iridium Levels at the Cretaceous-Tertiary Boundary at Gubbio, Italy Negative Results of Tests for a Supernova Origin, Geol. Soc. America Abstract 11. 378 (1979). 

Michel et al. (1990) have measured the iridium content in Ocean Drilling Project hole 690 sediments from the Weddel Sea across the Cretaceous-Tertiary (K-T) boundary (Table 8.1). Depth z is relative to an arbitrary level in the core. Compare the reduced iridium flux at each depth down the core assuming the bioturbated layer is either 4 or 8 cm thick. 

Like iridium, arsenic is enriched in Cretaceous-Tertiary boundary shales from New Zealand (Brooks et al., 1984 Strong et al., 1987). The iridium is believed to have originated from an asteroid impact that caused the massive extinction at the end of the Cretaceous period about 65 million years ago. In contrast, most of the arsenic in the boundary shales probably had a terrestrial origin (Strong et al., 1987). The extinction of marine organisms, especially plankton, from the impact may have been responsible for increased anoxic conditions in the oceans, which led to the precipitation of arsenic in the marine deposits (Brooks et al., 1984), 541. 

There are characteristic chemical properties of cosmic dust that have been involved in the study of sediment accumulation rates. The platinum group elements, such as iridium and osmium, offer good examples. Attempts to use iridium in this way have had the important result of indicating a giant meteorite impact at the Cretaceous-Tertiary boundary (Alvarez et ai, 1980) but it has not been proven important in determining chronometry. 

Cretaceous-Tertiary boundary—In the geological column of sediments, the transition from sediments of the Cenozoic era to those of the Mesozoic era. A thin layer of iridium-rich material at this boundary was probably laid down by the asteroid that created the Chixulub crater in Mexico. 

Nichols D.J., Jarzen D.M., Orth C.J., Oliver P.Q. (1986) Palynology and iridium anomalies at Cretaceous-Tertiary boundary, south-central Saskatchewan. Science 231, 714-17. 

The efficiency of this method was again demonstrated by Bekov et al. (1988) when detecting Rh traces in natural samples. Extraordinarily high concentrations of iridium and other siderophiles were discovered in the Cretaceous/Tertiary (K/T) boundary deposits and interpreted (Alvarez et al. 1980) as a result of a large extraterrestrial body falling upon the Earth, causing mass extinction of the biota dominant in the... 

In Gubbio, Italy, a 1 cm layer of clay between extensive limestone formations marks the boundary between the Cretaceous and Tertiary Periods. This clay layer was known to have been deposited about 65 million years ago when many life forms became extinct, but the length of time associated with the deposition was not known. In an attempt to measure this time with normally deposited meteoritic material as a clock, extensive measurements of iridium abundances (and those of many other elements) were made on the Gubbio rocks. Neutron activation analysis was the principal tool used in these studies. About 50 elements were searched for in materials like the earth s crust, about 40 were detected and about 30 were measured with useful precision [26-28]2. 

NAA has been used in many applications. One very special one is the determination of the iridium content in the earth s cmst and in a 1-cm thick clay layer round the earth. This day layer was formed 65 million years ago and defines the boundary between the Cretaceous and the Tertiary pericxls of geological history. Shells offoraminifera are found in all marine environments and in abundance in the limestone above and below the spedal day layer. But not in the layer itself. In addition it was shown by NAA that the iridium content of the limestone layers above and below is 0.3 ppb while the concentration in the layer is 10-40 pbb. This is the source of the theory of a giant asteroid collision with the earth at this time, a collision that brought the end of the dinosaur era. 

The mean contents in the earth s crust of all the platinum group elements are as mentioned very low. In the light of this fact it is astonishing that very high concentrations of iridium have been found in sedimentary rocks from the time of transition from the cretaceous to the tertiary geological period. At this time, about 70 million years ago, extinction of many biological species, including the dinosaurs, occurred. 

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