Indian Iridium

By the mid-'980s, even the fiercest opponent of the Alvarez theory had to admit that the iridium anomalies, however much they spread above and below the K-T boundary and whatever their cause, did mark the position of the K-T boundary and must reflect a global event. If Deccan volcanism was the source of that iridium, it might then be possible to find an iridium-rich K-T boundary layer amidst the Deccan basalt flows. An intensive search for iridium by the French team came up empty, however. Finally a group of Indian geologists, led by N. Bhandari of the Physical Research Laboratory in Ahmedabad, discovered the iridium needle in the Deccan haystack.'3

The basalt flows that make up the traps alternate with layers of sediment called intertrappeans. The alternating stratigraphy of the basalts and the intertrappeans tells us that volcanism began, stopped long enough for sediments to accumulate, resumed, stopped again while more sediments collected, and so on until a layer cake of basalt and sediment built up.

The Indian geologists sampled in the Anjar region of Gujurat State, where seven basalt flows are recognized, each separated from the next by intertrappean sedimentary layers several meters thick. The third intertrappean bed from the bottom, ITIII, contains bones and eggshells of dinosaurs. The Indian geologists used the argonargon method to date the lava beds designated Fill and FIV, which lie above and below ITIII, at 65.5 ± 0.7 and 65.4 ± 0.7 million years, identical to the 65.0-million-year date for the K-T boundary. (This evidence also shows that dinosaurs were still alive up to the very end of the Cretaceous.) Since the third lava bed is 65 million years old, the two earlier ones must be older, as the magnetic reversal results indicated they are, confirming that Deccan volcanism started well before the K-T boundary.

Within sedimentary layer ITIII, just above the highest dinosaur fossil, there are three chocolate-brown layers each less than a centimeter thick. In one of these thin layers, Bhandari and colleagues found a sharp iridium peak reaching 1,271 ppt, compared to a background of less than 10 ppt in the basalts (the low levels in the basalt were later confirmed by Schmitz and Asaro) and to less than 100 ppt in the nearby intertrappean sediments. Osmium levels are high in the layer and the osmium:iridium ratio is the same as in meteorites and the mantle. The remarkable perseverance and skillful detective work of the Indian geologists clearly confirm that this thin, unremarkable layer is the K-T boundary clay. Even when you know where to look and have good age control, finding an iridium-rich ejecta layer is difficult.

The exemplary findings of the Indian geologists and the accumulated knowledge of the chronology of the Deccan traps lead to several conclusions:

• The magnetic results obtained by the French geologists show that the Deccan eruptions began at least 1 million years before K-T time and lasted for at least 1 million years after it, far too long an interval to be consistent with the considerable evidence that the K-T event was rapid.

• Contrary to the claims of some paleontologists, and others who have opposed the Alvarez theory, the dinosaurs did not die out well before the K-T boundary but lived right up to it. (And some claim they lived on into the Tertiary!)

• Since three Deccan trap flows lie below the layer that contains dinosaur remains, the dinosaurs, and presumably the other species that were exterminated in the K-T event, survived at least the first few phases of Deccan volcanism. Thus the eruption of the Deccan volcanoes was not immediately inimical to life, even when the volcanoes were right next door.

• Some geologists have speculated that a major meteorite impact might have released so much energy that the earth's mantle below ground zero melted, initiating a period of volcanism. In this view, impact might have precipitated Deccan volcanism. But that idea does not work for the Deccan eruptions, which began at least a million years too early.

• Since the K-T boundary is located near the middle of sedimentary layer ITIII, which itself was deposited well after the volcanism that produced basalt flow III had ceased, it is hard to understand how Deccan volcanism could have been the source of the K-T iridium—the iridium was deposited after the Deccan volcanoes had stopped erupting (though they did resume).

• Since none of the other intertrappeans have high iridium, the element apparently was not produced in the normal course of Deccan volcanism and intertrappean sedimentation. Its presence in one thin layer among many suggests, if it does not demand, that the iridium has a special origin, unconnected with Deccan basalts and sediments. The extremely low concentrations of iridium in the Deccan basalts—among the lowest levels ever measured—make them a most unlikely source.

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  • karin
    Why is iridium found near volcanoes?
    9 years ago

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