Chapter 6 Changing Life in a Changing World

UPDATES FOR CHAPTER 6.
  1. January 2005. Rethinking the great mass extinctions. This refers to a long paper by Bambach et al. 2004, which re-analyzed the Sepkoski data base. The major conclusion is that there were only three great mass extinctions: the end-Ordovician, end-Permian, and end-Cretaceous. For more information, see this mini-essay

NOTES AND LINKS FOR CHAPTER 6.

The Global Diversity Gradient

Global Tectonics and Global Diversity

Plate tectonics and past geography

For world geography in Ediacaran times (around 650 Ma), see Map of the Vendian world

The supercontinent continued to break up during the Cambrian and Ordovician, to form a set of small continents that were generally distributed in lower latitudes.

Map of the Late Cambrian world around 514 Ma

Map of the Middle Ordovician world around 458 Ma

Continental collisions should decrease diversity, just as continental splitting increases it. There were several continental collisions from the Middle Paleozoic through the Permian, as the fragments of Rodinia coalesced.

Map of the Middle Silurian world around 425 Ma

Map of the Early Devonian world around 390 Ma

Map of the Early Carboniferous world

Map of the Late Carboniferous world around 306 Ma

The great extinction at the end of the Permian took place shortly after the continents finally merged into a giant supercontinent, Pangea, composed of a large northern land mass, Laurasia, and a southern land mass, Gondwanaland.

Map of the Late Permian world around 255 Ma

The rise in diversity that began in the Triassic and continued into the Cenozoic coincides very well with the progressive break-up of Pangea.

Map of the Early Triassic world around 237 Ma

Map of the Early Jurassic world around 195 Ma

Map of the Late Jurassic world around 152 Ma

Map of the Early Cretaceous world around 94 Ma

Map of the Late Cretaceous world around 69 Ma

Map of the Middle Eocene world around 50 Ma

Map of the Middle Miocene world around 14 Ma

Footnotes

Page 71. Another look at Sepkoski's data base. A paper in PNAS. I think this is a really good paper, assessing Sepkoski's data base on biological/functional criteria rather than simply numbers. The results are surprising, suggesting that Earth's marine ecosystems, once established, are VERY resistant to collapse. Only twice, at the Permo-Triassic and at the K-T boundaries, were global ecosystems strikingly altered in balance. You need to read the paper rather than the press release: PNAS 99: 6854-6859. May 2002

Mass Extinctions

Site on mass extinctions at the Hooper Virtual Museum at Carleton University. These are student-constructed Web pages, most of them very well done.

P. 75. The Ordovician Mass Extinction

P. 75. The Late Devonian (F-F) Mass Extinction
From the Hooper Virtual Museum site. This is now considered to be a large "normal" extinction rather than a great "mass extinction".

Comparing the Ordovician and Devonian extinctions
Jack Sepkoski was concerned to present as interpretation-free a picture as possible of mass extinctions. So he presented diversity measures based on best available taxonomy. However, ecology counts too: in other words, not all taxa are equal, now or ever. Now Mary Droser of UC Riverside and her colleagues have compared the end-Devonian and end-Ordovician extinctions. They were about the same size, but the Ordovician extinction did not disrupt global ecology anything like the way that the end-Devonian extinction did. Numbers don't tell all the story, in other words. How nice it is to have this firmly documented, rather than having a gut feeling! Note, however, that Bambach et al. 2004 have downgraded the end-Devonian extinction as less than a "mass extinction". Both groups could be right; Bambach et al. are discussing size (numbers of taxa); Droser et al. are discussing ecological restructuring (read their title). Droser, M. L. et al. 2000. Decoupling of taxonomic and ecologic severity of Phanerozoic marine mass extinctions. Geology 28: 675-678. You won't find it freely available on the Web.

P. 75. The end-Triassic Mass Extinction: a mass extinction at or near the Triassic-Jurassic boundary?
This is now considered to be a large "normal" extinction, certainly not a great "mass extinction".

The Latest Paleocene Thermal Maximum
This is a climatic "spike" with some extinction resulting. About 55.5 Ma, in the latest Paleocene, a strange set of events occurred. The oceans warmed very quickly: high latitude surface waters and deep ocean water warned by perhaps 4 to 8° C. Many deep-sea organisms, as detected in microfossil samples, became extinct. On the continents, many new mammal groups appeared et about this time. This event is not a major extinction event like others described here, but it is extraordinary.

The clue is provided by carbon isotope analysis of ocean sediments et the time. Very suddenly, it seems, huge amounts of carbon with a high carbon-12 isotope ratio were added to the oceans. In theory, this could have occurred if very large amounts of methane hydrate (clathrate) escaped from seafloor sediments, putting a large amount of methane gas into atmosphere and ocean.

Clathrates: BBC News OnLine, 1998.

In summary, methane hydrate is a compound of a methane molecule trapped inside a lattice of water molecules. The whole compound forms a gel (rather than a gas, liquid, or solid), but only if it is cold AND under pressure. Heat it, or bring it to normal atmospheric pressure, and it breaks up into water and methane. Meanwhile, in its gel form it can be set on fire, burning to form water and carbon dioxide.

Experiments from a submersible in Monterey Bay have proved that one can squirt methane gas into cold deep seawater, and have it emerge from the tank as clathrate as it reacts immediately with seawater. In nature, the methane is produced from organic sediment by methanogenic bacteria, and if the sediment is buried at the right depth, the methane that would otherwise be "natural gas" is trapped in the sediment as a clathrate gel.

In the real world, clathrates are most abundant in the permafrost of high latitudes (especially the Arctic), and in the thick sedimentary layers on the continental shelves of the world. Clathrates probably contain more energy than all the oil and gas deposits of the world, but in engineering terms it is difficult and dangerous to try to exploit them.

Clathrates present a very large source of energy and of greenhouse gases (methane immediately, and carbon dioxide when the methane burns or is oxidized in air). And they are unstable: fairly small changes of temperature and pressure may release them, perhaps very quickly or even catastrophically. A change in pressure on seafloor sediments can occur if sealevel changes, and a climatic change of only a few degrees can warm the seafloor sediments too. Perhaps, then, a slight warming could result in a release of clathrate that would generate more warming, and more clathrate release, in a runaway reaction that could give a large, sudden, global temperature crisis.

This process was suggested for the Late Paleocene Thermal Maximum, and recent research has found major disturbances in continental shelf sediments at exactly 55.5 Ma off the coast of Florida. This beautiful confirmation of the clathrate-release hypothesis for this event makes one wonder whether there have been others. In fact, the clathrate-release hypothesis had already been suggested as a factor in the Permo-Triassic extinction, and has now been suggested in an extravagant (and I think foolish) version for the Cretaceous-Tertiary extinction.

A Jurassic clathrate event?
Not a mass extinction. AP story from the Seattle Post-Intelligencer, July 27, 2000, on an article in that week's Nature. A moderate extinction during the Jurassic (at 183 Ma) occurred at exactly the same time that there were massive volcanic eruptions associated with the breakup of Gondwanaland. Oxygen isotope values in marine fossils changed too. The scenario is that the eruptions warmed shallow waters in the oceans, releasing clathrates that generated significant global warming, killing off some marine groups.

The Eocene-Oligocene Extinction

Not a mass extinction. During the late Eocene there were at least two significant asteroid impacts, one in the Chesapeake Bay region, and the other at Popigai, in Siberia. There was climatic cooling that lasted at least 100,000 years, and evolutionary change in planktonic organisms as far south as Antarctica. Note that this is NOT at the end of the Eocene. Reference: Vonhof, H. B., et al. 2000. Global cooling accelerated by early late Eocene impacts? Geology 28, 687­690.

The Permo-Triassic (P-T) Extinction

Volcanism at the P-T boundary: the Siberian Traps This massive basalt field is about twice the volume we had thought: easily the largest basalt eruption documented on Earth. And this gigantic eruption took place very rapidly.

A methane belch at the Permo-Triassic boundary. I've read this paper, (it is in Geology) and IMHO it is the bad paper of the decade (so far: and there are already many candidates). First, this is not a new idea: for example, my Davis colleagues Dan Dorritie and Gary Vermeij published a much cleaner methane hypothesis in Science several years ago. So, to start with, this is SHODDY scholarship, to put it mildly. Second, there is no justification for the claim that methane would be released practically instantaneously and would then explode. Third, if methane was involved in Noah's Flood, there should be a carbon isotope spike ‹ has the author looked for it (NO!). And fourth, whoever said (for the press release) that a single mammal swimming in the ocean could set off a methane disaster may well be unaware that there were NO mammals (count them) in the Permian, let alone swimming ones, and is likely influenced by the science FICTION idea that a butterfly flapping its wings can set off a hurricane. Who reviewed this??? Press release from Northwestern University, August 28, 2003.

Sulfide poisoning at the P­T boundary? Press release from Penn State, November 5, 2003. Well, yes, but you will search in vain for any EVIDENCE. In fact, the authors say that they are starting to think about maybe looking for some. Do I need to say anything more?

Impact at the Permo-Triassic boundary? This possibility gained exciting new evidence in 2003 and 2004

Recovery After Extinctions

Extraterrestrial Impacts and Meteorite Craters

The reference list for Chapter 6, 4th Edition, with associated Web links

Page last updated January 18, 2005.

Links last checked October 1, 2005.

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