Ramblings about what I encounter within the realm of the geosciences, as well as the occasional rant about nonsense.

27 February 2008

We will start with the end...

Well, here I am. I figure a good opener would be to discuss (rant) about a side feature of my ongoing and nearly finished (hopefully) thesis. I am working with the Cretaceous-Tertiary (KT) Boundary within the Williston Basin in central MT. Initially it was broader in scope, but has essentially been whittled down to one aspect.

I am focusing on comparing lithostratigraphy (stratigraphy based upon rock characteristics) and chronostratigraphy (stratigraphy based upon time). Essentially, I am comparing the two methods as it applies to the terrestrial end Cretaceous (end-K) event. Unbelievably straight forward, I know, but that is it. Lack of any significant funding (the bank of the parents were my primary benefactors) forced me into a project which I now jokingly refer to as a 4th grade science project on steroids.

But the state of my thesis is not necessarily what I wish to discuss. I am curious about the whole obsession with the end-K event itself. I was routinely turned down for funding for not addressing causal mechanisms for the extinction, despite only wanting to evaluate lithostratigraphy and chronostratigraphy. I have read up on extinction of the dinosaurs as it applies to the KT boundary, but I am always amazed at the "jump" that individuals make from showing an event happened to saying that it was what caused the extinction event. Aside from the problems of obtaining an age estimate of the boundary (I have seen error bars as small as ± 40,000 years), people have yet to acknowledge problems related to timing the actual extinction of the dinosaurs.

Signor and Lipps addressed this problem in the 80s (if I find a link I will add it, like this). They demonstrate that it will be generally impossible to distinguish a gradual extinction event from a catastrophic extinction event. This was published in the early days of the Bolide impact hypothesis, but it didn't stop anyone from jumping to conclusions. Alvarez et al. 1980 (subscription required for the full text .pdf) do a superb job of showing a bolide collided with the earth at approximately the time of the dinosaurs extinction. However, they never show any evidence to link the two events except through a variant of the Post hoc ergo propter hoc argument. They happened around the same time, therefore one caused the other.
note: They also make a passing remark about this sort of event causing mass extinctions in general (an unfalsifialble hypothesis as they state it, because they indicate if there is no evidence of an impact, it would be the result of an ice-rich comet being the impactor).

As Signor and Lipps' paper demonstrates, we don't know exactly when the two events occurred in relation to each other. For all we can gather from the fossils, we will never be able to know which fossil is the last dinosaur. We will never even be able to know if the last dinosaur was preserved. Without knowing this information, attempting to determine the sequence of events (even if we know a specific date for an impact event) is meaningless.

Another problem facing the whole extinction concept is a lack of definition. Despite talking reason and making perfect sense, Signor and Lipps have overlooked a key point. As has the rest of the extinction centered community. Nobody has ever bothered to define a gradual extinction, and nobody has bothered to define a catastrophic extinction. I have been involved in discussions among the Paleo grad students where half the group want to define it as a rate and the other half think it should be magnitude. Personally, I think of it as rate. Because, by definition, every extinction has a magnitude of 100% for that given species. But as for how quick or slow an extinction is.... I don't think anyone would be able to make an argument as to where to put the boundary between gradual and catastrophic that is based on anything other than opinion.

So I am forced to conclude that until someone can address and surmount the problems addressed by Signor and Lipps, the mechanism of every mass extinction event (including the dinosaurs) remains an intractable question. We simply cannot know when a species went extinct in relation to a supposed cause. Unfortunately, a good many people are so focused on finding "the answer", they aren't stopping to ask if the question (as we currently understand it) even makes sense.

2 comments:

Jeannette said...

I wonder... if it is possible to identify natural groupings of rates of extinctions. I don't know nearly enough about biology and paleontology to begin to brainstorm how to answer this, but it would be interesting to compare rates of extinctions to differentiate between 'gradual' and 'immediate' without being subjective (i.e. by using natural populations).

Welcome to the geo-fold, Einme!

Einme said...

The closest thing I have seen to what I think you are proposing is discussing overall variety within an ecosystem.

This was used to help with the Signor-Lipps effect, but it is also not necessarily valid.

Back to dinosaur extinction (sorry, it is what I am somewhat familiar with), it was initially argued that the overall diversity of the dinosaurs was dropping by the end-K (one major therapod genus, a handful (2-3) ornithopod genera, etc.). This was interpreted to represent a gradual extinction signal. However, in subsequent years, it has been shown that this was a result of an incomplete data set.

As it stands right now, I wouldn't be confident using this method to determine a gradual vs catastrophic extinction. There may be other ways to deal with this problem, but currently I am stymied.

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Between Los Alamos,NM and White Rock, NM

Between Los Alamos,NM and White Rock, NM
The photo of the travertine spring was taken in the small opening in the center of the image.

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