1. PTYS 214 – Spring2011  Homework 6 DUE in class TODAY  Reminder: Extra Credit Presentations (up to 10pts) Deadline: This Thursday! (must have selected a paper)  Study Guide for Midterm available for download on the website  Class website: http://www.lpl.arizona.edu/undergrad/classes/spring2011/Pierazzo_214/  Useful Reading: class website  “Reading Material” http://en.wikipedia.org/wiki/Radioactive_decay http://www.mnsu.edu/emuseum/archaeology/dating/radio_carbon.html http://en.wikipedia.org/wiki/Oldest_rock Announcements

2. Quiz #5  Total Students: 28  Class Average: 2.6  Low: 1  High: 4 Quizzes are worth 20% of the grade

3. Some recent interesting articles… Atmospheric circulations of terrestrial planets orbiting low-mass stars, by A. Edson et al. – Icarus 212, p. 1-13, 2011 Investigate the atmospheric circulation of idealized planets of various rotation periods around low-mass stars, with surfaces of all land or all water, but with an Earth-like atmosphere and solar insolation The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary, by P. Schulte et al. – Science 327, p. 1214-1218, 2010 Review the data available for the end-Cretaceous, and the connection between the mass extinction event (e.g., dinosaurs) and the Chicxulub asteroid impact, in Mexico Ozone perturbation from medium-size asteroid impacts in the ocean, by E. Pierazzo et al. – Earth and Planetary Science Letters 299, p. 263-272 Investigate the depletion of atmospheric ozone after the impact of mid-size asteroids in the ocean, and the related increase of UV radiation at the Earth’s surface

4. Recovering from a Snowball Earth Atmospheric CO 2  Volcanic CO 2 builds up in the atmosphere until the greenhouse effect becomes big enough to melt the ice  The meltback is very quick (a few thousand years)  Surface temperatures climb briefly to 50-60 o C  CO 2 is rapidly removed by silicate (and carbonate) weathering, forming cap carbonates For the hard snowball Earth hypothesis, it would require huge amounts of CO 2 in the atmosphere! More realistic for a slushball Earth hypothesis

5. Recovering from a Snowball Earth Alternate Theories  Destabilization of substantial deposits of methane clathrates (solid form of water with methane in its crystal structure) locked up in low-latitude permafrost  as the clathrates melt, large amounts of CH 4 are released in the atmosphere (Kennedy et al., Nature 453, p. 642-645, 2008)  Large impact on the thick ice could release large amounts of water vapor and sea salts in the atmosphere, changing its chemistry and circulation, and strongly increasing the atmospheric greenhouse effect

6. Duration of Snowball Events Estimates of the duration of the low-latitude glaciations have been obtained by various approaches: 1.Amount of CO 2 outgassing needed to overcome the glaciation: 4 – 40 million years 2.Variation in stable carbon isotope ratios ( 13 C/ 12 C) > 6 – 10 million years 3.Amount of extraterrestrial material (Ir) accumulated during glaciation (catastrophically accumulated in sediments after the ice melted) 3 – 12 million years

7. How could photosynthetic life survive during extreme glaciations?  Thick ice (~1 km; hard snowball) –Life could survive in tidal cracks, meltwater ponds, tropical polynyas (areas of open water surrounded by ice)  Thin ice (several meters; weak snowball) –Tropical ice remains thin due to penetration of sunlight and photosynthesis can continue in the ocean

8. Lake Bonney (Taylor Valley, Antarctica) Courtesy of Dale Andersen Photosynthetic life thrives beneath ~5 m of ice

9. Stay Tuned…. The low-latitude Earth glaciations are still puzzling Main problems: - limited data, - low temporal resolution, - limited knowledge of early Earth conditions, - multiple interpretations (theories) Slowly, new data is being acquired, and uncertainties may be somewhat reduced

10. What are Isotopes?  Atoms of a chemical element with the same atomic number (Z), but different mass number (A) Some isotopes are stable, others are not Example: 12 C – 13 C – 14 C 12 C: 6 protons, 6 neutrons – Stable 13 C: 6 protons, 7 neutrons – Stable 14 C: 6 protons, 8 neutrons – Unstable (decays to 14 N with 7 protons & neutrons)

11. Half Life Amount of time it takes for one-half of the radioactive atoms in a sample (“parent” isotope) to decay to the “daughter” isotope

12. Radiometric Dating Independent of heat, pressure, or any condition other than the presence of a radioactive source  Young objects have many parent nuclei and few daughter nuclei  Old objects have few parent nuclei and many daughter nuclei

13. Age Determination 1.Count how many parent atoms are present, P 2.Count how many daughter atoms are present, D 3.Assume that over time no other parent or daughter atoms were added! P D

14. Parent (radioactive source) DaughterHalf Life Carbon-14Nitrogen-145,730 yrs Potassium-40Argon-401.25 billion yrs Uranium-238Lead-2064.47 billion yrs Thorium-232Lead-20814 billion yrs Rubidium-87Strontium-8748.8 billion yrs Radioactive Isotope Systems Some radioactive isotopes are particularly good geologic clocks

15. Radiocarbon Dating

16. What do we date with radiocarbon? Human Evolution (last ~60,000 yrs) Mammoth Extinction (~12,000 yrs ago) Fossil Shells What about the earliest life on Earth? NO – 14 C is best for dating objects up to 60,000 years old

17. Potassium-Argon Dating (T ½ = 4.47 Gyr)  Argon is a (noble!) gas therefore when a rock is melted all Ar escapes  After a rock becomes solid (think volcanic rocks) any 40 Ar in the rock has to be produced by the decay of 40 K  By measuring 40 K (parent) and 40 Ar (daughter) in the same rock we can find the age of that rock Best for dating objects (events) more than 100,000 years old

18. Uranium-Lead Dating (T ½ = 1.25 Gyr)  Usually performed on the mineral zircon (ZrSiO 4 )  Zircons can incorporate uranium ( 238 U) into its crystalline structure when they form but reject lead  Any lead ( 206 Pb) observed in zircons has to come from the decay of the uranium that was initially present in the zircons Best for dating objects (events) that are more than 100,000,000 years old

19. Problems with radiometric dating  The “system” has to remain closed – no input of “parent” atoms and no escape of “daughter” atoms  We cannot perform radiometric dating on sedimentary rocks… WHY? How to date sedimentary rocks

20. Oldest Known Ancient Rocks (on the surface of the Earth)  Nuwuagittuq (N Quebec, Canada) > 4.2 Gyr old  Akilia (SW Greenland) > 3.85 Gyr old  Isua (W Greenland) 3.7-3.8 Gyr old  Pilbara (NW Australia) ~3.52 Gyr old  Swaziland (South Africa) ~ 3.5 Gyr old 1 Gyr = 1×10 9 years = 1 billion year

22. Looking for the Earliest Life: Challenges  Ancient rocks are rare (buried, eroded, subducted, ejected into space during the late heavy bombardment)  Surviving rocks are changed by metamorphism (pressure and heat), strongly affecting fragile biological signatures  Not every rock can contain evidence for life (no life in igneous rocks)  No bones or shells! Single-celled prokaryotic organisms that may have been very different from life today  Contamination: younger rocks mixed with older rocks

23. Evidence for Life

24. Carbon Isotopes  Natural carbon is a mix of 13 C and 12 C (1 13 C every 99 12 C)  On Earth the standard ratio is: 13 C/ 12 C = 0.01123722  In living organisms typical ratio is: 13 C/ 12 C ~0.0109563 Photosynthesis prefers 12 C to 13 C One way to measure the change in carbon isotopic ratio is to determine  13 C (measured in parts per thousands, or per mil):

26. Evidence at  3.8 Gyr is strongly questioned… Oldest evidence of life! What about the oldest rocks?

27. Evidence for Life 1: Carbon Isotopes in Ancient Rocks The overall carbon isotope record older than 1 Gyr is similar to the carbon isotope record of modern time  Accepted Result: Autotrophic organisms were likely to be already present about 3.5 Gyr ago Claims of older ages are highly debatable! (problem with sedimentary rocks…)

28. Evidence for Life 2: Microfossils  Preserved remains of microbial organisms  Small! Up to a few tens of microns, either simple spheroids (“balls”) or simple filaments (“sticks”)  Best preserved in cherts (fine grained sedimentary rock that resists weathering and metamorphosis)

29. Oldest Microfossils 1.85-1.9 Gyr old: Colonies of spheroidal cells similar to blue-green bacteria (Belcher Group, Arctic Canada) and simple multicellular eukaryotes (Negaunee Form.,Michigan) 2.55 Gyr old: ellipsoids, spheroids, tubular filaments, 0.2 to 20  m in size (Transvaal Supergroup, South Africa)

30. 3.46 Gyr old CONTROVERSIAL! Dark, curved filaments interpreted as cyanobacterial microfossils They could also be: 1.Abiogenic (some shapes seem to follow crystal ghosts, or are part of complex branching structures) 2.Contaminants (must be sure they are deposited with the sourrouing rocks) Brasier et al. (2002) Nature 416, p.76 Warrawoona Group, Apex Chert, Pilbara, Australia Cyanobacteria (Anabaena sp.) Blue-green algae (Spirulina sp.)

31. Stromatolites  Laminated sedimentary structures accreted as a result of a microbial growth (trace fossils of microbial activity)  If stromatolites are biogenic then they represent fossils of colonial photosynthesizing microbes (cyanobacteria) that build reefs similar to corals  The most ancient “biological” stromatolite is 3.46 Gyr old (Warrawoona, Australia) The process is still going on today Shark Bay, Australia

32. Evidence for Life 3: Biomarkers  Certain hydrocarbon molecules found in ancient organic matter (like kerogen or oil) are recognizable derivatives of biological molecules  When these molecules have a specific biological source (they are associated with a specific bacterium) they are called biomarkers  Difficult to measure and contamination is a major problem  Biomarkers of eukaryotes and cyanobacteria have been found in 2.5-2.7 Gyr old rocks (Hamersley, NW Australia)

33. Example: All eukaryotes use sterols (membranes)  steranes are biomarkers for eukaryotes methylhopanes are biomarkers for cyanobacteria

34. The absence of fossils, biomarkers, etc. does not mean those organisms did not exist:  Preservation requires specific circumstances  Some organisms don’t have identifiable markers  Tectonic and geologic processes can eliminate or alter signals  Sampling location and biases may affect findings It is generally safe to assume that an organism existed long before it appears in the fossil record Word of Caution: No Markers ≠ No Life

35. Early Life Summary Evidence of the earliest life on Earth is difficult to prove (lack of samples, low preservation, contamination) and requires several techniques and lines of evidence: –Isotopic evidence seems to date it back to about 3.5 Gyr (Pilbara craton, Australia) –Oldest stromatolites are about 3.46 Gryr old –Earliest microfossils (accepted) date back to about 2.55 Gyr (Transvaal Supergroup, South Africa) –Earliest molecular biomarkers date back to about 2.5-2.7 Gyr old rocks (Pilbara, Australia) Absence of physical and chemical evidence does not mean life did not exist; preservation is limiting!