Download 220 08Summary13 - Earth and Atmospheric Sciences

EAS 220
The Earth System
Spring 2008
Lecture 13
Evolution of the Atmosphere and Biosphere I: The Early Earth & the Origin of
Life
Geologic Time
For Prelim 2:
Learn the names of the Eons and Eras of the Geologic Time Scale.
Know the approximate (rounded to 2 non-zero digits) times of the boundaries
between them.
Absolute Geologic Time & Radioactive Decay
Measurement of absolute geologic time is based on radioactive decay.
Actually, it is usually based on the build-up of the new atoms produced by
radioactive decay (easier to measure what is there than what is not; 14C is an
exception).
Radioactive decay is a process that occurs at rates that are absolutely* independent
of environmental influences (like temperature, pressure, neighboring atoms, etc.).
The probability of decay of a radioactive nuclide is given by the decay constant, λ.
Isotopes
It is not elements per se that undergo radioactive decay, but specific isotopes of
some elements.
Isotopes are atoms of an element that differ only in the number of neutrons in the
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Lecture 13
nucleus.
Isotopes are written as the chemical symbol, with the mass number presuperscripted.
Examples
12
C (6 protons, 6 neutrons) and 13C (6 protons, 7 neutrons) are stable, but 14C (6
protons, 8 neutrons) decays to 14N (7 protons, 7 neutrons).
238
U and 235U are both unstable and undergo a series of decays eventually
becoming 206Pb and 207Pb.
Mathematics of Radioactive Decay
Basic Equation of Radioactive Decay:
dN
= −λN
dt
Where N is the number or radioactive atoms and t is time.
Says that the rate of decay is proportional to the number of radioactive parent
atoms present and the decay constant, λ.
Rearranging, integrating, and rearranging again, we have: N = N 0e −λt
The number of daughters, D, produced in time t will be: D = N 0 − N = N(eλt − 1)
If there are some daughters already present, D0, then the number of daughters after
time t will be: D = D0 + N(eλt − 1)
It is easier to measure isotope ratios, so this equation would be written, in the
example of the Rb-Sr system, as 87 Sr/86 Sr =( 87Sr/86 Sr) 0 +( 87Rb/86 Sr)(eλt − 1)
We can only measure present isotope ratios, not initial ones, so this equation has
two unknowns: (87Sr/86Sr)0 and t. We can solve for both, provided we analyze two
or more samples for which these two unknowns are the same.
This equation has the form: y = a + bx
If we plot 87Sr/86Sr vs. 87Rb/86Sr, then a = (87Sr/86Sr)0 is the intercept and b = (eλt - 1)
is the slope.
The slope of the line is thus proportional to time, and is called an isochron. This is
the isochron method of dating.
Hadean Eon
Defined as the time of formation - time for which there is no geologic record (so end
of this eon could change).
Meteorite Bombardment (known from the Moon) makes the Earth very inhospitable
Minerals (zircons) of 4.1 to 4.4 Ga age indicate that crust was beginning to form and
liquid water was present.
Hadean Atmosphere
The Earth’s primitive atmosphere was probably lost - quite likely in the Giant Impact.
The early atmosphere was produced in the (early Hadean) by volcanic degassing of
the Earth’s interior.
This early atmosphere had no free oxygen. It was either
Modestly reducing (CO2 dominated, a little CH4, CO, NH3) or
Highly reducing (CH4, CO, H2, NH3 dominated)
Present “consensus” is the former.
Our Story Begins with the Archean
Because we know so little about the Hadean, our story really begins with Early
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Lecture 13
Archean, 4 Ga ago.
By 3.9 Ga, the Late Heavy Bombardment was ending, and Earth was becoming
more hospitable (but not to us – no O2).
Acasta Gneiss: Oldest Rocks 3.96 Ga
The Isua “Supracrustals” 3.8 Ga
Do they harbor evidence of life?
Carbon Isotope Fractionation
Autotrophic organisms (plants, photosynthetic bacteria, chemosynthetic bacteria)
preferentially use 12CO2 molecules to synthesize organic compounds because:
12
C-bearing compounds (e.g., 12CO2) diffuse slightly faster than 13C-bearing ones
because they are lighter.
The lower mass of 12C results in it forming a slightly weaker bond so it is easier to
break the 12C—O bond.
Delta Notation
Isotopic variations in nature are quite small. For convenience, they are reported
as per mil (‰) variations from a standard, denoted as δ. (For carbon isotopes
the standard is the Pee Dee Belemite or PDB).
Significance of Carbon Isotope Values in isua
In general, carbon with highly negative δ13C occurs in nature only in biologically
produced compounds.
The negative δ13C values of graphite in the Isua rocks suggests the carbon is
ultimately of biological origin and therefore that life existed at this time.
However, this remains controversial.
Earliest Fossil Evidence of life
The first reasonably uncontroversial evidence of life comes from stromatolites of the
3.5 Ga Pilbara Formation in W. Australia.
Stromatolites still occur today in the hypersaline water of Shark Bay on the west
coast of Australia. They are layers of calcite precipitated by photosynthetic
cyanobacteria.
Apex Chert, W. Australia 3.45 Ga
The Apex Chert is located stratigraphically above the stromatolites. It contains
microstructures that look remarkably like modern bacteria.
Buck Reef Chert 3.42 Ga
Similar filamentous structures occur in the 3.42 Ga Buck Reef Chert of South
Africa.
Fig Tree Chert, South Africa 3.2 Ga
Summary: When Did Life Originate?
We presently have controversial indirect isotopic evidence of life at ~3.8 Ga.
There is very good trace fossil evidence of life at just more than 3.5 Ga.
There is abundant and convincing evidence of life and photosynthesis in rocks
3.5 Ga and younger. Many of these rocks not only contain biological structures,
but organic molecules of biological origin.
The Big Question: How did life originate?
The short answer is nobody knows
First life is likely to have been simpler than any organism we are familiar with
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A Simple Definition of Life: a self-organizing and self-reproducing system.
The Hadean World
Reducing Atmosphere - at least modestly
Key raw materials likely present: CH4, NH3, H2O, H2S, CO2, PO43Key components for life - organic compounds such as amino acids and
nucleotides derived from the above could have been more common and more
chemically stable than at present (albeit not necessarily abundant).
Amino acids and other complex organic molecules are present in some
meteorites
No organisms to decompose these molecules
Geologic Activity
High levels of geologic activity would have provided a wide variety of
environments (such as deep-sea hydrothermal vents) for abiotic catalysis of
organic molecules and mixing of compounds of contrasting oxidation state (and
consequently an energy source).
The macromolecule hypothesis & the “RNA World”
Most current ideas about the origin of life posit that the first “life” was a single
molecule that could do it all - assemble a copy of itself (growth, reproduction) and
provide the energy to do so (metabolism).
One macromolecule, RNA (ribonucleic acid) is the prime suspect because it can
Store, transmit and duplicate genetic information (like DNA)
Catalyze chemical reactions (unlike DNA)
Furthermore, certain riboenzymes have been shown to catalyze their own
synthesis under specific conditions.
This, however, is one idea of many. It remains controversial and there is not yet a
consensus on this matter.
Transition out of the RNA World
More complex life, multimolecular life, requires a barrier between itself and its
surroundings.
Present cell walls are composed of a bilayer of phospholipids -each of which has a
hydrophobic and hydrophilic end.
Another alternative is proteinoids - abiogenic protein-like amino acid chains that can
collapse into bubble-like structures.
Proteins and RNA then synthesize DNA.
Some Observations about the Origin of Life
Improbable or not, it did happen (in one way or the other) and happened relatively
quickly.
Possibly present 3.8 Ga ago - and arguably far more complex than mere
macromolecules. Even the fossils and trace fossils of 3.5 Ga are remarkably
similar to modern bacteria - they are significantly evolved beyond “first life”.
The Earth (and solar system) were likely quite hostile places for the first few
hundred million years.
The time scale for the origin of life is no more than a couple of hundred million
years - and quite possibly less than one hundred million years.
This leads some scientists to think that at least simple life might not be so
improbably after all!
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