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

Quiz #5

Total Students: 28

Class Average: 2.6

Low: 1

High: 4

Quizzes are worth 20% of the grade

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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, 2010Review 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

Recovering from a Snowball Earth

Atmospheric CO2

Volcanic CO2 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 50-60oC

CO2 is rapidly removed by silicate (and carbonate) weathering, forming cap carbonates

For the hard snowball Earth hypothesis, it would require huge amounts of CO2 in the atmosphere!

More realistic for a slushball Earth hypothesis

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 CH4 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

Duration of Snowball Events Estimates of the duration of the low-latitude glaciations have been

obtained by various approaches:

1. Amount of CO2 outgassing needed to overcome the glaciation:

4 – 40 million years

2. Variation in stable carbon isotope ratios (13C/12C)> 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

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

Lake Bonney (Taylor Valley, Antarctica)

Courtesy of Dale Andersen

Photosynthetic life thrives beneath ~5 m of ice

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

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: 12C – 13C – 14C12C: 6 protons, 6 neutrons – Stable13C: 6 protons, 7 neutrons – Stable14C: 6 protons, 8 neutrons – Unstable (decays to 14N with 7 protons & neutrons)

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

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

ln(2)

PD1ln

T T(age)2

1

Age Determination1. 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

Parent(radioactive source) Daughter Half Life

Carbon-14 Nitrogen-14 5,730 yrs

Potassium-40 Argon-40 1.25 billion yrs

Uranium-238 Lead-206 4.47 billion yrs

Thorium-232 Lead-208 14 billion yrs

Rubidium-87 Strontium-87 48.8 billion yrs

Radioactive Isotope Systems

Some radioactive isotopes are particularly good geologic clocks

Radiocarbon Dating

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 – 14C is best for dating objects up to 60,000 years old

Potassium-Argon Dating(T½ = 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 40Ar in the rock has to be produced by the decay of 40K

By measuring 40K (parent) and 40Ar (daughter) in the same rock we can find the age of that rock

Best for dating objects (events) more than 100,000 years old

Uranium-Lead Dating (T½ = 1.25 Gyr)

Usually performed on the mineral zircon (ZrSiO4)

Zircons can incorporate uranium (238U) into its crystalline structure when they form but reject lead

Any lead (206Pb) 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

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

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 = 1×109 years = 1 billion year

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

Evidence for Life

Carbon Isotopes

Natural carbon is a mix of 13C and 12C (1 13C every 99 12C)

On Earth the standard ratio is: 13C/12C = 0.01123722

In living organisms typical ratio is: 13C/12C ~0.0109563

Photosynthesis prefers 12C to 13C

One way to measure the change in carbon isotopic ratio is to determine 13C (measured in parts per thousands, or per mil):

standard12

13

standard12

13

sample12

13

13

CC

CC

CC

1000Cδ

Evidence at 3.8 Gyr is strongly questioned…

Oldest evidence of life!Oldest evidence of life!

What about the oldest rocks?

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…)

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)

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)

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.)

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

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)

Example: All eukaryotes use sterols (membranes)

steranes are biomarkers for eukaryotes

methylhopanes are biomarkers for cyanobacteria

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

Early Life SummaryEvidence 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!