Wednesday, August 29, 2007

Some bits on the Eocene Ocean & PETM Extinction

I finished reading Under a Green Sky by Dr Peter D Ward. It's an interesting book, but not as deep as his other latest one that I read, Out of Thin Air. This has more travelogue and discussion of the see-sawing on the Permian Extinction's announcements - Becker et al vs most everyone else on the Permian Extinction root cause and mechanism, frex - and other bits of paleo historia. However, it does have a lot of good bits that are well worth reading.

One of those good bits is the chapter on the Eocene called "The Overlooked Extinction." More specifically it's really about the PETM and the associated extinctions. He goes on to link a flood basalt eruption to the PETM kick off: the Brito-Arctic Province I believe is the one he's fingering. He paints a picture of the chain of events that are a lot like the Permian Mass Extinction but on a much smaller scale. I find the evidence as presented as interesting and possibly compelling, but as with other extinction events, I think it's premature to say that there's a Grand Unified Mass Extinction Theory[1].

However, there is an interesting bit that seems to have come out of the research on the PETM and more specifically the Eocene Oceans. Doug Muir once commented that the water column for the Eocene Ocean was warm, damn warm from top to bottom (Doug, was this on HDTD, SHWI, or here somewhere? I can't seem to find it!). It wasn't quite as warm as the ocean surface temperatures in the Permian, but the Permian had uber stratified oceans, so the depths were colder. This contrasts with the PETM oceans. They were warm and not stratified, at least vertically. The mixing - from top to bottom - still happened, but in a way that seems wrong to those of us that grew up with our oceans and their circulation.

It seems that there were cold provinces - if I may use that word here - in the Arctic and Antarctic. However, they were more like cool provinces since they were ice free as far as we can tell (-1.5 C is the average temperature recorded for the Arctic waters at this point). Whereas the tropical surface temperatures were approximately 8 C. To a lay person, this would seem to say that the oceans would have a normal circulation albeit a sluggish one due to the smaller differential. However, the reality is that it had a completely opposite direction than our own.

Our oceans have the warm waters sweep to the north and then get cold and sink. The water then flows along the ocean basins and upwells in the tropics where it is once again heated and sent back to the north and south. This keeps the waters oxygenated and the bottoms of the oceans from becoming anoxic. Mostly. However, during the PETM it seems that the oceans had the opposite pattern. The upwelling was in the higher latitudes and the sinking was in the tropics. The reason being is that the water evaporation of the very warm tropical waters would produce very salty water which would sink and displace the cooler waters of the south and north along the top to the tropics by upwelling at the mid-latitudes or Arctic depending on the model used. If the warm bottom waters upwelled in the mid-latitudes that means that Arctic and Antarctic Seas might have been isolated in terms of water circulation...which is interesting. In my mind, it almost gives the ocean current circulation a model that looks like how the atmospheric circulation looks today.

That's interesting, very interesting, and has a lot of implications. I haven't thought them through as yet: the PETM and it's associated extinction are so far down pipeline that it might not ever see the light of day. However, if you guys have some thoughts be sure to post (*cough*doug*cough*carlos*cough*Zach*cough*Brian*cough*) Since I finished Ward's book and found it worthwhile I thought I'd share it. I've done so before and I'll do it again. There are some other juicy bits in there as well, but this one came up first. Now I have to get the Late Triassic Extinction rewrite done (Open Office core dumped yesterday's work) and clear up some bits with work. Woo.

1. I am going to have to do a post on why I find GUMETs annoying at some point. While we are making progress in that direction, I'm unconvinced we've arrived there whatsoever.

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