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SPU-22: The Unity of Science from the Big Bang to the Brontosaurus and Beyond. Lecture 17 7 April 2014 Science Center Lecture Hall A. Coming Attractions.
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7 April 2014
Science Center Lecture Hall A
Soon to play at a lecture hall near you (in fact today):
Mystery of dinosaurs’ demise. Detective story for the ages
Many examples of unity of science
What did in dinosaurs?
- What are some theories?
- Fascinating story of main theory
- Is case open and shut?
Many theories (see next slide)
Most agreed prior to 1970: Dinosaur demise desultory (took a long time, perhaps millions of years). Sparseness of fossil record didn’t allow fine distinctions
One contrary opinion, from astronomers: Nearby supernova explosion killed many organisms (more later)
Impact of asteroid/comet
Nearby supernova explosion
Failure to diversify
Mammals ate eggs
Walter Alvarez (geologist, son)
Luis Alvarez (physicist, father)
Frank Asaro (nuclear chemist)
Helen Michel (Plutonium chemist)
Detective, Walter Alvarez, first searches old strata in Italy for evidence of changes in direction of TRM in rocks. Why? See next slide
How have Italy’s Appenine mountains rotated in last ~100 million years?
What light shed by TRM? Little: Rock layers twisted, preventing useful deductions. Original goal thus not attainable
Geologists changed goal and sought record of magnetic reversals and uncovered unexpected anomaly
As with Pasteur’s dictum, prepared mind allows puzzling discovery to yield enormous dividends, as we shall see
Alvarez became fascinated by:
A 1-cm-thin layer of clay sandwiched between two thick layers of limestone; “forams” missing from the thin layer and of different kinds in the two thick layers. Layer seems to be at boundary between two eras, Cretaceous (older) and Tertiary – now Paleogene - (younger) [Thanks to Int’l Commission on Stratigraphy].
Why is this thin layer of clay here and why are there no forams in it?
Foram is short for forminafera. This creature roamed earth from c. 180 MYA, judging from fossil record
Foram sizes varied from c. 1 mm up to c. 20 cm, with most at small end of size distribution
Choice of problem to tackle is critical
Alvarez learned about dinosaur disappearance occurring at approximate time of KPgboundary
Decided consciously to therefore pursue implications of thin clay layer: Dinosaur disappearance was major problem in search of solution
Why clay layer so thin? Why only clay? Why no forams?
Was layer deposited unusually slowly or was it deposited at “normal” speed?
Why should one care about answers?
1. Rate of deposit of clay was constant (and normal), with limestone deposits halted by extinction of forams, for some as yet unknown reason, resulting in lack of limestone
2. Rate of deposit of limestone was normal, but that of clay was abnormally high for short period, due for example to rapid erosion for some as yet unknown reason. (But why no forams at all?)
Enter Luis Alvarez, father and Nobel-prize winning physicist:
Need for judge, substance also deposited, but at knowable constant rate (i.e., proxy time scale). What?
10Beryllium could provide uniform time scale:
Produced at roughly constant rate by very high energy cosmic rays striking oxygen and nitrogen atoms in atmosphere
Textbook half-life was 2.5 million years, allowing, in principle, useful determination of 10Beryllium concentration after ~65 million years, using new, more sensitive mass spectrometer
10Be could not be detected! Why? Half-life was wrong.Corrected value turned out to be 1.5 million years; using this proxy therefore became hopeless. Why?
What could be another useful proxy? How about platinum element, reasoned Luis, borne by meteoritic dust? Concentration far exceeds normal concentration in earth’s surface, because iron absorbs these elements and dragged them down within earth when it was molten (or near molten)
Used sensitive neutron activation analysis (what is this?), since concentrations were predicted to be 0.1 ppb if deposit was slow and undetectable if it were rapid. Challenge extreme for this technique; samples chosen very carefully from the clay layer, above and below it, and far below it.
Results of Frank Asaro’s team astounding: after a nine-month (!) effort, found 9 ppb – 90 times expected concentration of iridium for rapid deposit. Explanation??
Maybe due to a nearby supernova. (Astronomers had suggested that radiation from such an event could kill many organisms, including dinosaurs.)
How to check: Look for other consequences. Luis and physics again to the fore. Radioactive 244Pu, half-life 83 million years, would accompany supernova and not normally be present in clay layer. For this search, Helen Michel joined Frank Asaro; she was expert plutonium chemist
Test of clay layer sample via neutron activation analysis yielded positive results. Amazing: dinosaurs done in by supernova!
Announce to world? Planned on doing so at upcoming conference in Europe
Deputy Lab director: Make independent check before making any announcement
Result: Not a trace of 244Pu! Theory dead on arrival
New idea needed. Again, Luis to the fore: asteroid or comet impact?
Impact on earth of asteroid or comet could account for enhanced iridium concentration
Why would effects be worldwide (i.e., why would dinosaurs be wiped out all over)?
Krakatoa 1883 analogy. Dust and fire could do wonders to make impact a killer. Nuclear winter…
Next step: Confirm that enhanced iridium phenomenon was worldwide (see next slide plus following two)
Suitable exposed layers in Denmark (see next slide): Analysis yielded positive results for iridium
Further, independent, confirmation from Spain, New Zealand, and even Colorado (see next slide plus one). But not everywhere…
Note: Initially hard to find comparably-aged strata properly exposed
From where did impactor come?
Arguing from evidence of craters on surfaces of other bodies in solar system, we can be quite sure that origin was asteroid or comet from our solar system, inner or outer part
Where is smoking gun (or at least gun)? If there was impact so enormous, why don’t we see physical effects, e.g., huge crater? (See next slide for enormous energies involved.)
Even without gun, smoking or otherwise, announcement of impact as cause of dinosaur extinction was world-class news and press made most of it. Many scientists’ nerves became quite raw. Published papers reached feverish pitch
Based on iridium concentration, Luis (and later others) estimated amount of energy released by impactor:
First, assumed characteristics: ~10 km diameter; 3 gm/cm3 density; 20 km/sec impact speed; …
Energy released: ~ 5 x 1023 Joules ~ 1014 tons of TNT, ~ 2 x 106 times greater than largest bomb explosion on earth (largest known volcano explosion ~ 1/400th as energetic)
Frequency: ~ once per 100 million years
Forward and backward shock waves; seismic waves in solid earth; possibly huge tsunami
White-hot ejecta spread around globe causing horrific fires
Earth enveloped with dust, possibly resulting in initial dramatic decrease in temperature
Release of greenhouse gases possibly resulting in subsequent dramatic increase in temperature
Consequence for life? Mass extinctions
[Much of above is based on numerical simulations; little supporting evidence other than that here presented.]
Bigger they are, harder they fall:
Large creatures disappeared completely; small creatures survived (how is unclear; that is clear)
Why didn’t seeds and roots allow more plant species to survive? No one knows; there exist speculations
Be all that as it may…
Maybe subducted? No way to tell.
First, exhaust other possibilities. How?
Search worldwide. Dogged search
led to discovery of anomalies near
Brazos river in Texas and evidence of
tsunami at relevant time period from south
In 1991, circular pattern of gravity anomalies was discovered under Yucatán Peninsula. Its diameter was about 200 km, as expected for asteroid of estimated size - about 10 km diameter - to have caused observed iridium concentration
In 1981, it then turned out, two oil geologists, working for PEMEX, Mexico’s national oil enterprise, had reported discovery of that same crater. No one on either “side” was aware of other side, although ironically both sides once presented papers at same geological conference, but at different sessions
What about age - a critical question?
Crater inaccessible; indirect means needed and found: For example, rocks and other materials (see, also, next two slides) near surface melted by impact showed age to be just about right for dinosaur demise, c. 65 million years
Case closed? Not necessarily
Impact very likely caused crater, but did it also cause mass extinction?
Was there any other possibility? Yes, massive volcanism
Massive volcanism can release poisonous gases of various sorts which could lead to massive extinction of life
Is there evidence that such massive volcanism as produced Deccan Traps occurred about 65 million years ago? Yes! Age from radioactive dating seems about right
Standing over 2,000 m high and covering about 500,000 km², original lava flows spread over three times that area, and spewed out over 500,000 cubic kilometers. By comparison Mount St. Helens disgorged about one cubic kilometer of lava in its eruptions late in 20th century
Key point: Temporal extent of eruptions; effect very different if in close proximity vs. very well spread out. Why?
Is there standoff? Not according to most scientists who believe that Deccan Traps out of running
“The correlation between impact-derived ejecta and paleontologically defined extinctions at multiple locations around the globe leads us to conclude that the chicxulub impact triggered the mass extinction that marks the boundary between…(geological) eras ~65.5 million years ago.”
Again, not necessarily: To my knowledge Deccan Traps camp has not yet raised white flag. Until it does, stay tuned
Of course, Nature is what it is whether or not flag is raised. My own opinion? Unless there is unexpected pertinent discovery, amazing asteroid/comet impact theory will most likely be seen by “history” as correct one. There is (remote?) possibility of coordination: seismic connection idea
Other problems: 1. How did mammals and birds survive? 2. Of five “major” extinctions none of others attributable to impact; what caused others?? Relevant to this one???
Next slide shows this reason