Download History of the Earth`s climate

The history of Earth climate
In order to understand the history of the
Earth's climate, we must first understand
something about the age of the Earth and
how various events have been dated.
Some fundamental questions:
.How old is the Earth?
. How do we know the age of the Earth??
. What was the origin of the Earth's
atmosphere?
. What was the Earth's early climate like?
. How has the Earth's climate changed
over geologic time?
The Radiometric Time Scale: Key to age
of Earth and geologic time
. 1896, discovery of natural radioactive
decay of Uranium by Henry Becquerel,
French physicist
. 1905 British physicist Lord Rutherford
described the structure of the atom and
suggested radioactive decay for
measuring geologic time
. 1907 Yale Prof. B.B. Boltwood published
first chronology of Earth based on
radioactivity
. 1950... first more-or-less accurate age
dating.
Principles of radiometric dating
. elements distinguished by number of
protons in the nucleus. Proton + neutron
combination = nuclide
. same no. protons but different no.
neutrons = isotopes. e.g., isotopes of C
with 6, 7, or 8 neutrons designated 12C,
13
C, 14C.
. radioactive decay of less stable nuclides
is statistically determined.
. The number of radioactive nuclei that
decay in a given unit of time is directly
proportional to the number of nuclei
present at that time—first-order kinetics.
∂N
= − λN
∂t
where
∂N
= rateofchangeovertimeofradioactivenuclei( N )
∂t
λ = decayconst., uniqueforeachradioactivenuclide
Re arrangeto :
∂N
= − λ∂t
N
Integratefromtimeototimet2 toget :
ln( Nt 2 / No ) = − λt
Nt 2
= e − λt
No
takingnatural log ofbothsides :
ln Nt 2 = ln No − λt
A plot of ln N versus time will give a
λ
straight line with slope = -λ
With radioactive nuclides, we often speak
of half-life, the length of time required to
diminish the original # of radioactive
nuclei by 1/2:
N = 1/2No
1/2 = e-λt1/2
2 = eλt1/2
t1/2 = ln2 = 0.693/λ
λ
By various rearrangements, we can use a
measured concentration of a radioactive
element at the current time to determine the
age. After a time t has elapsed, N atoms of
the parent (p) will be left and No - N atoms
of the daughter (d) will be formed.
1 d 
t = ln + 1
λ p 
where d = number of daughter atoms present
today (N-No)
p = number of parent atoms remaining today
(N)
Some commonly used radioactive
elements, their decay products, and
currently accepted 1/2 lives:
Source: U.S. Geological Survey
Parent Isotope
Uranium-238
Uranium-235
Thorium-232
Rubidium-87
Potassium-40
Samarium-147
Stable Daughter Product
Lead-206
Lead-207
Lead-208
Strontium-87
Argon-40
Neodymium-143
Currently Accept. 1/2-Life
4.5 billion years
704 million years
14.0 billion years
48.8 billion years
1.25 billion years
10.6 billion years
Based on different groups of rocks over
geologic time and radioactive dating, a
Geologic Time scale has been developed:
Geologic time scale figure here.
Problem set handed out in class due
March 28th in class
So, how old is the Earth?
Ancient rocks exceeding 3.5 billion years
in age are found on all of Earth's
continents. Oldest rocks on Earth found
to date = Acasta Gneisses in
northwestern Canada (4.03 billion yrs)
and the Isua Supracrustal rocks in West
Greenland (3.7 to 3.8 billion yrs). These
ancient rocks are not from any sort of
"primordial crust" but are lava flows and
sediments deposited in shallow water, so
that Earth history began before these
rocks were deposited. In Western
Australia, single zircon crystals found in
younger sedimentary rocks have
radiometric ages 4.3 billion years. The
Earth is at least 4.3 billion years old.
The best age for the Earth (4.54 Ga) is
based on the Canyon Diablo meteorite. In
addition, mineral grains (zircon) with UPb ages of 4.4 Ga have recently been
reported from sedimentary rocks in westcentral Australia.
The oldest dated moon rocks have ages
between 4.4 and 4.5 billion years and
provide a minimum age for the formation
of the moon. The moon formed when a
Mars-sized body collided with the
primitive Earth.
Origin of the atmosphere:
. As Earth accreted, it trapped gases in
roughly the proportion found in the sun.
(Note: Sun's main gases: H, He, O, Fe, N,
Mg, C, Si, plus other gases)
. Some of these gases were light enough to
accumulate in the atmosphere, but heavy
enough to be held by gravity.
.However, any early atmosphere would
have been burned off by the collision
mentioned above (temperatures to as high
as 16000oK). The lightest gases were
preferentially lost. However, the deeper
Earth contained volatile elements in
similar proportions to their original
source (i.e., same as the sun).
The overall effect of the impact was to
alter the mass fractionation of the
atmosphere, with lighter elements burned
off/lost and heavier gases left behind, but
many in solid Earth.
After impact: "Runaway Greenhouse"
.Under early conditions of high temp. at
Earth's surface, main gas = water vapor.
. Radiative cooling, water condensed into
oceans.
. Main source of warmth = sun, which
radiated less 4.5 billion years ago, by
~30%
. water turned to ice, further cooling.
. carbon-containing gases received from
carbonaceous chondrite meteorites.
. meteorite-related carbon gases caused
greenhouse warming.
. Initial high CO2 concentration in
atmosphere diminished gradually by
chemical weathering and formation of
carbonate in the oceans.
. This uptake of CO2 allowed temperature
to drop enough that life could evolve.
. Eventually, bacteria-like organisms
developed, including cyanobacteria
(stromatolites). Photosynthesis produced
Oxygen.
. There is some question as to when life
and especially photosynthetic life first
evolved. The oldest stromatolites are
about 3.45 billion years old, western
Australia.
Stromatolites are laminated structures
built mainly by cyanobacteria. They are
still found today, but were once much
more common. They dominated the fossil
record between about 1-2 by ago. Today,
they are found mainly in saline lakes or
hot spring environments. The best
example of living stromatolites is at
Hamelin Pool, Shark Bay, Western
Australia.
The bacteria precipitate or trap and bind
layers of sediment to make accretionary
structures (domical, conical or complexly
branching). Hence, make for excellent
fossil record. Range in size from cm to
many meters.
. Oxygen was used up as oxidation of Fe-S
minerals to form banded Fe formations,
which are ubiquitous in early
(precambrian) rocks.
. Eventually, photosynthesis won out over
oxidation, and atmosphere evolved to
more oxygen-rich.
. multicellular organisms appeared at
least as far back as 550 million years ago.
. multicellular organisms in the ocean
produced shells which took up alot of
carbon... eventually forming carbonate
sedimentary rocks and organic-rich
deposits.
Figure on change in atmosphere goes near
here.
. Let's compare Earth to other nearby
planets:
Gas
Early
Atmos.
CO2
98%
1.9%
N2
O2
trace
Ar
0.1%
o
C
290
Press. 60
(bars)
Earth
Today
0.033%
78%
21%
0.93%
16
1
Venus Mars
96.5%
3.4%
trace
0.01%
477
92
95.3%
2.7%
0.13%
1.6%
-53
0.006
Venus is the second planet from the sun
and receives intense solar radiation. Its
high CO2 concentration provides for a
runaway greenhouse.
Mars is the fourth planet from the sun
and receives less solar radiation than
Earth. However, the CO2 in the
atmosphere does keep the planet from
becoming excessively cold.
The atmospheres of Venus and Mars are
thought to be little changed over the last
4+ billion years, but Earth has changed
primarily because of evolution of life.
Figure on climate changes over geologic
time goes here.
A few notes:
Early Earth (Pre-Cambrian) very hot;
eventually cooled as runaway greenhouse
replaced by more oxygen-containing
atmosphere.
In the Late Paleozoic, glacial episodes
made for a cold, wet climate.
Early Eocene: Considerably warmer and
wetter than today.
Late Eocene: Global cooling began,
leading to glaciation and the interglacial
period we now are in.