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

20 March 2009

Permo-Triassic Boundary NOT the Event of the Season

ResearchBlogging.orgHave you ever had one of those moments, where you read a paper, and you realize the paper is saying...pretty much everything you have been working on for the past few seasons? I just had one of these precious moments. Fortunately, I don't think negates my efforts though. In this case, the paper's location is on the opposite side of the world and approximately 190 Ma earlier. So I think I am safe. *Phew*

The paper in question is Gastaldo et al, 2009 (availble here). This was published in the March issue of Geology, which means you will have to meander over to your local library/university computer to gain access, or just join the GSA.

The deposits spanning the terrestrial Permo-Triassic (P-Tr) boundary are fluvio-lacustrine, with the placement of the boundary based on the position of a succession of laminated beds. Gastaldo et al. seek to determine the legitimacy of using lithostratigraphic means to evaluate the position of the P-Tr boundary. For those not in the know, lithostratigraphy is a subset of stratigraphy which focuses only on characteristics of the rocks themselves to define stratigraphic relationships. In other words, sandstones correlate with sandstones, mudstones with mudstones, etc. Lithostratigraphy is a fairly useful tool for some purposes, but it is inadequate for other purposes. So if you are using lithostratigraphy to help examine some aspect of the geologic record, you better make certain that lithostratigraphy can do what you think it can do.

In the case of fluvial (rivers) deposits and lacustrine (lake) deposits, lithostratigraphy is rife with internal discontinuities. To put it another way, rivers move and lakes can dry up. The upshot of this is that moving laterally in these depositional environments might mean that you are crossing timelines as well. If workers don't take this into account when they are recording their observations, they will have transcribed errors into their data set. For some studies, this is a trivial problem. However, when workers are examining an extinction event, like the one that happened at the P-Tr boundary, these subtle internal discontinuities can lead to erroneous interpretations.

And, in point of fact, the authors determine that the P-Tr boundary (the laminated beds) are diachronous. Which means that these beds were not laid down at the same time, and should not be used to interpret a basin-wide event.

The only problem I have with this paper is how they established their stratigraphic framework. The authors set out to evaluate the utility of lithostratigraphy as a proxy for basin-wide events. Yet, when they set up their stratigraphic framework, they based it on.... lithostratigraphy. To me, this seems akin to using a word to define itself. It doesn't invalidate the conclusions, but I think they could have made a better point by evaluating the lithostratigraphy with some other form of stratigraphy. For example, this paper would have been a much stronger argument if they set up the stratigraphic framework based on some chronostratigraphic criteria.

Chronostratigraphy is essentially "Time Stratigraphy". By looking at deposits that could only have been deposited at approximately the same time (or, in the case of ash deposits, at the same time), it is possible to evaluate the timing of depositional events throughout a basin. A commonly used chronostratigraphic marker, in a terrestrial setting, is palynology (looking at pollen preserved in the rocks). This will provide some "fuzziness" in the data set, because the pollen can be reworked, but this is a better chronostratigraphic framework than nothing. The best chronostratigraphic markers are event beds, like an ash deposit. These can be radiometrically dated, and best of all, they can't be reworked without diluting the ash to the point where it is unrecognizable (it turns out ash deposits have a VERY low preservation potential in most terrestrial settings).

That critique aside, this paper still makes a valid point. Lithostratigraphy, on its own, is an inadequate tool to evaluate the timing of non-localized events (events limited to ~ 1km radius) in terrestrial settings.

citation:

Gastaldo, R., Neveling, J., Clark, C., & Newbury, S. (2009). The terrestrial Permian-Triassic boundary event bed is a nonevent Geology, 37 (3), 199-202 DOI: 10.1130/G25255A.1

4 comments:

Eric said...

I agree (as you know) with the whole "lithostrat is kinda a bad stratagy for basin-wide isochronous event interps". HOWEVER, what do you think about the large-scale basin architecture of the Karoo? There is a MARKED change, at the scale of 100's of m, from the lower, low-diversity sand-rich macroforms and green overbank mudrock to the higher-diversity macroform architecture and dominate red-mudrock in the upper portion of these strata.

At that scale, there is clearly some sort of basin re-organization going on that is being recorded.

Bryan said...

I agree that there is a marked change in lithology basin wide. Indicating that something geologically significant is going on. However, I don't know if there is enough information to make an argument about basin-wide timing of the change in lithology.

I think it was Smith, 1995 (I have to read it again to make sure, but I think that is right) who was acknowledging this is a decent, but not perfect, criterion for marking the P-Tr boundary in the field, as Lystrosaurs and Dicynodonts are found on either side of this change. So use it as a first order approximation of where the boundary could be. Similar to the lithostrat K-Pg. But Smith (once again, I think) pointed out that while this does mark a basin reorganization, it isn't necessarily possible to use this as a chronostratigraphic constraint. He even made the argument that this transition might not even represent an extinction event so much as an extirpation event, which gives extinction studies a double whammy.

For most studies, I can't see this making much of a difference. However, for studies that hinge on highly precise placement of a temporal event, lithostrat is probably a tool that you don't want to rely on too heavily.

Here is the smith citation (I know it is on LANL's server, but every time I try hot linking there something hinky goes on):

Smith, R. M. H., 1995 Changing Fluvial Environments across the Permian-Triassic boundary in the Karoo Basin, South Africa and possible causes of tetrapod extinctions. Palaeogeography, Palaeoclimatology, and Palaeoecology. v. 117 no 1-2 p. 81-104.

Eric said...

It's always interesting that all these discussions of the Karoo almost NEVER discuss what's going on in the Parana in South America, which is the same damn basin! At around the end-Permian, there is a big ass epieric seaway in the Parana that is draining away; by the start of the Tr, it's all gone, replaced by nonmarine in the Parana. I wonder if that doesn't explain the whole basin-scale facies shift.

Like you say, I suspect that the "event" is probably more related to basin-evolution, rather than the whole "all plants died at the end permian" that Ward et al. want to evoke for these lithofacies changes.

Bryan said...

That seems in line with what Smith, 1995 was arguing. It seems that the Karoo used to be a nice fluvial habitat and then it just dried up. That is why he argues that it might just be an extirpation event. The early Dicynodonts were more adapted to fluvial, wettish environments. When the ecology changed, they might have left the area. The area was then subsequently taken over by the Lystrosaurs who were more adapted to the dry conditions.

Really, this sort of pattern might have implications for a majority of interpreted extinction events. Add the Signor-Lipps effect, and it becomes incredibly difficult (if not impossible) to identify the exact timing of an extinction event.

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