A hasty end to the Cretaceous period

A hasty end to the Cretaceous

It is now quite widely accepted that the catastrophic extinction at the end of the Cretaceous (the K-T extinction – 65.0 m.y. ago) was the result of the impact of major extra-terrestrial body (either a comet or a meteorite) - as was first proposed by Alvarez and others in 1980. On the other hand, one of the popular alternative explanations – a 500,000 year long period of eruptive activity of the Deccan basalts in India – still has some life in it. Much of the available evidence can be used to support either hypothesis, and most of that evidence comes from a thin layer of clay (several cm to several tens of cm thick) found within K-T boundary sedimentary rocks around the world.

	At several of the K-T boundary locations, such as Gubbio in Italy – a thin layer of shale is sandwiched within beds of otherwise monotonous limestone. The lowest part of the shale (a few mm at Gubbio) is characterized by anomalously high levels of iridium. Iridium is not common in sedimentary rocks, but it is present at higher levels in extra-terrestrial bodies, and also in some volcanic magmas, and the high levels in the shale could have been derived from either source.
The shale also contains microscopic glass spheres, which again might have been produced by a meteorite impact or by volcanism. The additional presence of microscopic diamonds in the shale is consistent with the extreme pressure of a major impact, but not with Deccan-type volcanism.
	On the other hand, it is evident that the K-T extinction was not an immediate event. Many species became extinct hundreds of thousands of years before the end of the Cretaceous, and many survived for hundreds of thousands of years into the Tertiary. This evidence favours the volcanism hypothesis.

Geologists from California and Italy have taken another approach to resolving the K-T boundary question by attempting to estimate the elapsed time represented by the shale layer at exposures in Italy and Tunisia. They have determined the rate of accumulation of ³He within the sediments, and compared this with known rates of ³He influx from interplanetary dust. Based on the helium ratio data Mukhopadhyay et al. (2001) argue that the K-T boundary clay in both Italy and Tunisia represents an interval of around 10,000 y, and hence must be the product of a relatively quick event like an impact, as opposed to a long-term global disturbance such as the 500,000 y Deccan eruptions. They describe the scenario as follows:

Interplanetary dust (IPD) has ³He/⁴He ratios which are several thousand times higher than terrestrial material, and the rate of IPD influx is relatively constant. ³He/⁴He in sediments can be used to estimate sediment accumulation rates. Low ratios indicate high sedimentation rates (because the IPD is swamped by ³He-poor terrestrial material) and high ratios indicate low rates. Large objects – such as the purported K-T impact body – do not contribute significantly to the ³He/⁴He flux because they are vaporized on impact (Mukhopadhyay et al., 2001).

The massive clouds of dust raised by the impact blocked sunlight sufficiently to produce global darkness for as much as a few years.
Both within the oceans and on land was life decimated to the extent that only around 25% of species survived.
With near-zero oceanic productivity at the sites studied, deposition of limestone ceased, and the only deposition was the slow accumulation of suspended clay particles.
After several thousand years the ecosystem recovered and limestone deposition was re-established.
There was a high turnover rate for planktonic species to begin with, but after several thousand more years populations began to stabilize.

The data of Mukhopadhyay et al. support the concept that a single brief event – such as an extra-terrestrial collision - was responsible for the K-T shale layer, and was probably the primary cause of most of the K-T extinctions. The observation that the enrichment of iridium is confined to the lowermost part of the boundary layer strongly supports this argument. It is important to reiterate, however, that the Deccan eruptions also took place at the end of the Cretaceous. They may have played an important role in the environmental disruption of the time, and in the extinction of many species.

References

Alvarez, L., Alvarez, W., Asaro, F. and Michel, H., Science, V. 208, p. 1097. (1980)

Mukhopadhyay, S., Farley, K. and Montanari, A., A short duration of the Cretaceous-Tertiary boundary event: evidence for extraterrestrial helium-3, Science, V. 291, p. 1952-1955 (April 2001)

Steven Earle, 2000. Return to Earth Science News