PTYS 214 – Spring 2011

Next week is Spring Break – NO CLASSES

Class website: http://www.lpl.arizona.edu/undergrad/classes/spring2011/Pierazzo_214/

Useful Reading: class website “Reading Material” http://www.pnas.org/content/96/20/10955.full http://en.wikipedia.org/wiki/Mass-independent_fractionation http://en.wikipedia.org/wiki/Fossil_record_of_fire http://en.wikipedia.org/wiki/Great_Oxygenation_Event

Announcements

Midterm

Total Students: 30

Class Average: 72.6

Low: 35

High: 103

Midterm is worth 20% of the grade

5 0 6 0 7 0 8 0 9 0

0

5

10

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tude

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GradeE D C B A

Early Life Summary

Evidence of the earliest life on Earth is difficult to prove:

– 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)

Atmospheric Oxygen

All terrestrial life requires energy, carbon and nutrients, and liquid water

Why is atmospheric oxygen important for life?

1. All terrestrial multicellular life requires high O2 CH2O + O2 → H2O + CO2 + energy

2. Almost all terrestrial life requires some protection from UV

Ozone: the Good and the Bad

90%

10%

Stratospheric OzoneMost of the ozone is in the stratosphere (above 15 km)

Production:O2 + (UV radiation < 240 nm) → 2 O

O + O2 → O3

Destruction:O3 + (UV radiation 240-310 nm) → O2 + O

O3 + O → 2O2

The Good Stratospheric ozone absorbs part of the UV spectrum

(<310 nm) where other gases do not absorb That’s why ozone in stratosphere is good for us

UV-C

UV-BUV-C

Tropospheric OzoneIn the troposphere ozone is the result of pollution:

OH + COpollution → H + CO2

H + O2 → HO2

HO2 + NOpollution → OH + NO2

NO2 + hν → NO + O

O + O2 → O3

Net reaction: CO + 2O2 → CO2 + O3

The BadOzone is a very chemically active gas

and can cause eye and respiratory problems

Both O2 and O3 are important to the biosphere but O3 cannot form without O2

What are natural sources of O2?

Volcanoes: NO Major volcanic gases are H2O, CO2, SO2 etc.,

but no O2

Today the major source of O2 is LIFE

H2O + CO2 → CH2O + O2

Atmospheric Oxygen!

Mt. Pinatubo eruption, 1991

Oxygen Sources

CO2+H2O O2 + CH2O

2H2O+hν O2 + 4H

Space

Ocean

Atmosphere

Hydrogen escape

Organic carbon burial

Photosynthesis

Water dissociation(minor)

Oxygen Sinks

Oxidation of reduced gasesO + H2O H2O2

SO2 + H2O2 H2SO4

Oxidative weathering of rocksFe2+ Fe3+

(FeO Fe2O3)

AtmosphericO2

Outgassing (volcanoes)

SO2, H2S, H2

Aerobic Respiration CH2O+O2 CO2 + H2O

Methane OxidationCH4 + O2 CO2 + 2H2

OceanLand Land

Changes in Oxygen Abundance

Oxygen abundance in the atmosphere is a result of the balance between sources and sinks

The atmosphere does not have much mass

Any lack of balance in sources vs. sinks results in the immediate changes of the

atmospheric oxygen

When did life start to produce O2?

Molecular biomarkers

Earliest biomarkers for cyanobacteria and eukaryotes: ~ 2.5 -2.7 Gyr ago

Maybe some photosynthetic O2 flux occurred 2.7 Gyr ago

Geologic Evidence Atmosphere with low oxygen until about 2.3 Gyr ago:

– BIFs (Banded Iron Formations)– Detrital Uraninite and Pyrite– Paleosols and Redbeds– Sulfur Isotope Ratios

2.47 Gyr old Brockman IronFormation, Western Australia

Alternating iron-rich layers and iron-poor shale or chert layers

Iron-rich: include iron oxides (Fe3O4 or Fe2O3) formed in the oceans by combining oxygen with dissolved iron

Iron-poor: deep ocean should have been anoxic, causing deposition of shales and cherts

BIFsVarying O2

amount

Rounded detrital uraninite from ca. 2.7Ga Witwatersrand Basin, South Africa

Rounded detrital pyrite from ca. 2.6 GaBlack Reef Quartzite, South Africa

Uraninite (UO2) and pyrite (FeS2) are unstable under high O2 levels in the atmosphere

If in contact with the atmosphere (detrital), they can only form in an O2-poor atmosphere

Detrital Uraninite and Pyrite

Hekpoort Paleosol, South Africa (about 2.22 Gyr old)

Paleoproterozoic Redbeds, ON, Canada

Reddish color is due to hematite (Fe2O3) presence of O2

Oldest Redbeds are about 2.3 Gyr old

Paleosols prior to 2.3 Gyr agolost their iron (no oxygen to form hematite)

Paleosols and Redbeds

Normally, isotopic ratios of an element follow a standard mass fractionation line (MFL):

33S 0.515×34S

Prior to 2.5 Gyr ago the isotope ratios fall off the MFL line! 33S = 33S - 0.515×34S 0

Sulfur Mass-Independent Fractionation

Farquhar et al. 2001

3.3 – 3.5 Gyr old samplesS-isotopes: 32S 95% 33S <1% 34S 4 % 36S trace

33S>0.51534S

33S<0.51534S

Kump (2008) Nature 451, p.277-278

Large Sulfur MIF effects are associated with photochemical reactions (involving UV radiation)

Sulfur MIF can only occur in an oxygen-free atmosphere

33S

=

33S

- 0

.515

×34

SSulfur Mass-Independent Fractionation

The O3 layer should have been absorbing most UV radiation by 2.3 Ga, as soon

as O2 levels began to rise

What About Ozone (O3)?

O2 rise causes O3

rise!

An O2 level of 1% PAL is

sufficient to create a

sufficient ozone screen

Oxygen was in the atmosphere by 2 Gyr ago

However, life was limited to unicellular organisms or very simple multicellular organisms until ~540 Myr ago

The oldest known possible multicellular

eukaryote is Grypania

(~1.9 Gyr old)

Slow Early Evolution…

All known complex multicellular organisms need at least 10-20% of

the present oxygen

Cambrian ExplosionAbout 540 Myr ago there was a seemingly rapid appearance of

complex multicellular organisms

(all we really know about it comes from two main locations!)

Forest Fires and Atmospheric OxygenCH2O + O2 → CO2 + H2O

Fires produce charcoal that is preserved in the geologic record

There has been a continuous record of charcoal in sediments younger than 360 million years old

O2 levels have not been lower than 15% during the past 360 million years

Present Atmospheric Level

0%

10%

20%

30%

Atmospheric oxygen

21%

15%

Fire

Summary of the O2 Constraints

(Goldblatt et al., 2006)

Great Oxidation Event

Low-Fe Paleosols Redbeds

Detrital Uraninite

Pyrite

BIFsEucaryotes

P.A.L. = Present Atmospheric Level

Atmospheric Oxygen Summary

~1ppm

No O2/O3

A few% 15-35%

Major steps in the evolution of lifePhanerozoic Eon (542 Myr ago - present) “Visible life” (macroscopic animals and plants)

Proterozoic Eon (2.5 – 0.54 Gyr ago) Mostly single-celled and some primitive multicellular organisms

Archean Eon (3.5? - 2.5 Gyr ago)

Single-celled organisms, prokaryotes (cyanobacteria)and some eukaryotes

Phanerozoic Eon

Paleozoic Era (250-540 Myr ago) - Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian periods - Age of sea life (trilobites)

Mesozoic Era (65-250 Myr ago) - Triassic, Jurassic, Cretaceous periods- Age of dinosaurs

Cenozoic Era (0-65 Myr ago) - Paleogene, Neogene periods- Age of mammals

The fossil record of biodiversity

Species: ability to interbreed, producing fertile offsprings

similar morphology (body shape) or DNA

Species always form and die due to genetic mutations and natural selection

On average, 10 - 25 new species

originate and become extinct each year

Change in number of species = origination rate - extinction rate

Logistic Growth Curve

No logistic growth curve in the fossil record!

Why?

Sampling bias!

There are much more recent rocks than ancient rocks available to study

Possible alternative: Minimize sampling bias

by looking at higher taxonomic groups

Species

Crust

Sediments

TaxonomySpecie: Homo Sapiens (all people)

Genus: Homo (humans and close relatives)

Family: Hominidae (“great apes”: humans, chimpanzees, gorillas, orangutans)

Order: Primates (all apes and monkeys)

Class: Mammalia (mammary and sweat glands)

Phylum(division): Chordates (vertebrates)

Kingdom: Animalia (moving consumers)

Domain: Eukarya (complex cells)

Complex Life

Earth-like complex life requires not only energy, water, nutrients and carbon but also oxygen and ozone (UV

protection)

Suppose the environment has everything indicated above (Phanerozoic eon)

Does it mean that the animal life will evolve smoothly?

No!

Mass ExtinctionSharp decrease in the number of species in a relatively

short period of time

1) It must be a rapid event (from less than 10,000 to 100,000 years)

2) A significant part of all life on Earth became extinct (use of families is more reliable than species; for example extinction of 18% of all families corresponds to about 40% of all genera and 70% of all species)

3) Extinct life forms must have came from different phyla, lived in different habitats, spread out over the whole world