THE IRIDIUM ANOMALY AT THE K-T BOUNDARY
By Phillip Bigelow
Iridium (77Ir) is a "platinum-group" metallic element that is very uncommon in the earth's crust. The platinum-group metals include platinum, iridium, palladium, rhodium, rhenium, and osmium. On the Periodic Table, these metals are also in the same columns as the "Group 8" elements, which include iron, nickel, and cobalt. Group 8 elements all have a natural chemical affinity for each other, and therefore they tend to collect together in nature. The greatest concentration of iron on Earth is at its core. This is also where the greatest concentration of Earth's iridium resides. The platinum group elements are called "siderophile" elements. Translated, "siderophile" means "love of iron".
But at the surface of the Earth, there are only a few environments that contain more than a trace of iridium. Volcanic and plutonic rocks that are rich in iron minerals, such as peridotite, some basalts, and some gabbros, occasionally contain enough platinum-group metals to be profitable to mine. However, these ores are rare.
To give you an idea of how rare the element is in sedimentary rocks, in a randomly-selected rock weighing one gram, the amount of iridium contained within it would be less than 1 to 2 BILLIONTHS of a gram (1-2 nanograms). In fact, the amount of iridium is often so small that it sometimes cannot be measured at all using today's scientific equipment (it is estimated to be as low as 0.05 nanograms/gram of sediment).
In the clay layer at the Cretaceous-Paleogene boundary, the concentration of iridium is still unbelievably diluted, but it is abundant enough to be easily measured by instruments. Some of the iridium occurs as submicron-sized dust grains in a solid-solution with the other platinum-group metals, but most of the platinum group metals are probably mixed with iron metal and nickel metal and iron oxide and nickel oxide in the dust grains.
In meteorites (iron meteorites + the stony meteorites), platinum-group metals are significantly more abundant than they are in rocks in the Earth's crust, often hundreds to thousands of times more concentrated (averaging about 500 nanograms/gram of meteorite in a type of stony meteorite called a chondrite) (Alvarez et al., 1980). Iron meteorites contain much higher concentrations of these elements. Platinum-group metals are also found in cometary nuclei, but in a somewhat lesser concentration than in chondrites. These facts led Alvarez et al. (1980) to propose that the unusual concentration of iridium found in the Cretaceous-Paleogene boundary clay in Italy could be explained by air-fall settle-out from a dust cloud caused by the impact of a large extraterrestrial body. Originally, their idea was not widely accepted by the paleontological community (although non-paleontologists seem to have recognized its merits with less consternation almost immediately after the publication was released). Today, nearly all paleontologists have now accepted Alvarez et al.'s claim that an impact fallout layer exists at the K-T boundary, but not all of them have accepted Alvarez et al.'s suggestion that the mass extinction was caused by the impact event. For a good summary of the problems encountered in trying to associate the extinction event with the impact layer, see Signor and Lipps (1982) and Williams (1994).
Iridium is one of the most corrosion-resistant metals known, (it is only slightly soluble in one of the most potent acids available, aqua regia). Therefore, it is also insoluble in groundwater. This means that iridium tends to stay in place in the rock strata, rather than becoming mobilized and transported in solution throughout the rock. Hence, the long-term stability of the iridium in the K-T Boundary clay layer in the Hell Creek Formation.
Was the impactor an asteroid or a comet? The late Dr. Gene Shoemaker thought that the object was a comet, but there is still much uncertainty on this point. At least one researcher believes that he may have found a piece of the impactor, itself. Dr. Frank Kyte of the University of California, Los Angeles has found a small piece of a highly weathered stony meteorite in a deep sea sediment core that was collected from the central Pacific Ocean (Kyte, 1998). Dr. Kyte suspects that the Yucatan impactor was an asteroid. The meteorite fragment from the Pacific Ocean was found in the thin K/T boundary layer containing air-fall settle-out from the impact (Kerr, 1996). Kyte believes that the odds of such an association are too unlikely to be coincidence, although he admits that his discovery is not proof that the meteorite fragment is part of the Yucatan impactor. If it was a part of the object, it may have been blasted away from the Yucatan impact site and deposited in the Pacific Ocean (alternatively, it may not have been part of the main mass, and instead impacted into the Pacific Ocean separately) (but see Robin et. al, 1993).
Here's more information on the meteorite fragment from the K-T boundary.
Alvarez, L.W., W. Alvarez, F. Asaro, and H.V. Michel. 1980. Extraterrestrial cause for the Cretaceous-Tertiary boundary extinction. Science 208:1095-1108.
Kerr, R.A. 1996. A piece of the dinosaur killer found? Science 271:1806.
Kyte, F.T. 1998. A meteorite from the Cretaceous/Tertiary boundary. Nature 396: 237-239.
Robin, E., L. Froget, C. Jehanno, and R. Rocchia. 1993. Evidence for a K/T impact in the Pacific Ocean. Nature 363:615-617.
Signor, P.W., and J.H. Lipps. 1982. Sampling bias, gradual extinction patterns and catastrophes in the fossil record, p. 291-296, in Silver, L.T., and P.H. Schultz (eds.). Geological implications of impacts of large asteroids and comets on the Earth. Geological Society of America Special Paper 190.
Williams, M.E. 1994. Catastrophic versus noncatastrophic extinction of the dinosaurs: Testing, falsifiability, and the burdon of proof. Journal of Paleontology 68: 183-190.
PHYSICAL PROPERTIES OF IRIDIUM (source: Handbook of Chemistry and Physics)
Atomic number: 77
Isotopes: There are two stable isotopes, iridium-191 and iridium-193. Iridium-193 is more abundant (see table, below).
|Stable Isotope||Percent Of Total Of Both Stable Isotopes In Earth's Crust|
In addition to the two stable isotopes, twenty-two short-lived radioactive iridium isotopes are known. They are constantly being created by cosmic rays and by terrestrial radiation. Due to their geologically short half-lives, they are so uncommon that they are hard to detect in most isotopic analyses. They are:
Ir-182 15 minutes
Ir-183 0.9 hours
Ir-184 3.2 hours
Ir-185 14 hours
Ir-186 16 hours
Ir-186 1.7 hours
Ir-187 10.5 hours
Ir-189 13.3 days
Ir-190m2 3.2 hours
Ir-190m1 1.2 hours
Ir-190 11 days
Ir-191m 4.9 seconds
Ir-192m2 greater than 5 years
Ir-192m1 1.4 minutes
Ir-192 74 days
Ir-193m 12 days
Ir-194 17.4 hours
Ir-195 4.2 hours
Ir-196m 84 minutes
Ir-196 52 seconds
Ir-197 7 minutes
Ir-198 50 seconds
Atomic weight (weighted average of isotopes): 192.22
Melting Point: 2410 degrees Centigrade
Boiling point: 4130 degrees Centigrade
Specific Gravity: 22.42 (at 17 degrees Centigrade)
Valences: 3 or 4
IRIDIUM ANOMALIES IN THE K-T BOUNDARY CLAY FROM AROUND THE WORLD
Note: This list represents only a minuscule fraction of the known K-T sections with iridium anomalies
Snow Creek area, Herpijunk Promontory, Montana (Hell Creek Formation): Iridium anomaly is "11.7 nanograms/gram of sediment." Samples from Hell Creek Formation NON-K-T boundary layers in the same area show an iridium concentration that is below the detection limit (1 to 2 nanograms/gram of sediment). These non-boundary samples probably reflect the "normal" concentration (background concentration) of iridium in the Hell Creek Formation in Montana.
Reference: Smit, J., and S. van der Kaars. 1984. Terminal Cretaceous Extinctions in the Hell Creek Area, Montana: Compatible with Catastrophic Extinction. Science 223: 1177-1179.
Lance Creek, Wyoming (Lance Formation): Iridium anomaly is "21 parts per billion".
Reference: Bohor, B.F., D.M. Triplehorn, D.J. Nichols, and H.T. Millard, Jr. 1987. Dinosaurs, spherules, and the "majic" layer: A new K-T boundary clay site in Wyoming. Geology 15: 896-899.
Hojerup Church site, Stevns Klint, Denmark (Fish Clay): "...the Ir in the boundary layer residue rises by about a factor of 160 over the background level (the background level is ~0.26 ppb). A 1-cm thickness of this layer would have about 72 X 10-9 gram of Ir per square centimeter." Reference: Alvarez et al. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208(4448):1095-1108.
Bottaccione Gorge, Acqualagna, Petriccio, and Contessa sites, Gubbio, Italy (Scaglia Rossa Formation): "...iridium increases by a factor of about 30 in coincidence with the K-T boundary....iridium abundance is 5.5 parts per billion (ppb) up to 9.1 ppb....with a steady [iridium] background level of ~0.3 ppb...." The average iridium concetration in all of the Gubio boundary clay sites was slightly greater than 6 ppb. Reference: Alvarez et al. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208(4448):1095-1108.
Gosau Basin, Austria (Upper Gosau Formation): "Iridium anomaly of 14.5 p.p.b. (parts per 1 X 109)...The marly limestone underlying the K/T boundary layers....has an iridium content of less than 0.5 p.p.b......A marked increase of carbon, as determined by microanalyses, is also apparent in the K/T boundary layers." Reference: Preisinger et al. 1986. The Cretaceous/Tertiary boundary in the Gosau Basin, Austria. Nature 322:794-798.
Deep Sea Drilling Project (DSDP), Hole 576. 320 21.4' N; 1640 16.5' E. North Pacific ocean floor subsurface. Coordinates near Hawaii. "Broad iridium anomaly....smeared across 30 cm (of thickness).....smearing due to bioturbation." This is the core in which Frank Kyte found the fossil meteorite in the K-T boundary layer. Reference: Kyte, F.T. 1998. A meteorite from the Cretaceous/Tertiary boundary. Nature 396: 237-239.
Bjala Section, Bjala, Bulgaria. Situated on the coast of the Black Sea, 35 km south of Varna. This is the first recognized K-T boundary section that occurs in hemipelagic marine sediments The boundary section was discovered in 1991, and is located in the Luda Kamchiya unit, lying between the Balkan chain in the south and the Moesian platform in the north. Three post-K-T event markers are found in the section: fall-out sediments, boundary clay sediments, and reworked Cretaceous sediments.
Reference: Preisinger et al. 1993. Cretaceous/Tertiary boundary sections on the coast of the Black Sea near Bjala (Bulgaria). Palaeogeography, Palaeoclimatology, Palaeoecology: 104: 219-228.
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