Earth and Planetary Sciences 101, Fall, 2002
THE
WAY THE EARTH WORKS
I.
Introduction
A. notion of different systems to the earth—
interior
lithosphere
atmosphere
hydrosphere
biosphere
B. earth processes and rates.
Uniformitarianism—“guiding”
principle back in the early years of geological science. “Present is the key to the past”. Is that really the case???? Do processes and rates of processes change
with time?
II.
Formation of Earth
Systems
III.
--descent from the stars????
A. interior and lithosphere
--thermal and gravitational
equilibrium….
--result is plate tectonics
--product is a mineral-rich
substrate for life to develop, evolve, and advance
B. Atmosphere and hydrosphere
--planetary outgassing!!!
--additional contributions
from comets and meteorites (and little green men and women)
--which gases were not able
to escape? Carbon dioxide, water vapor,
nitrogen, sulfur species,
--when did oxygen enter into
the picture???? With the development of photosynthetic organisms
--with cooling of the planet,
water vapor starts condensing……
--what are the four water
reservoirs? Oceans 1370 million cubic
km of water. Ice about 30 , surface
waters about 8 to 19 million, and moisture in the atmosphere 0.01 million!!!
C. Biosphere
--by 4 billion years ago, the atmosphere and
hydrosphere had formed
--relatively light gases had
escaped; heavier gases capable of retaining heat from the planet, while
allowing ultraviolet solar radiation through the atmosphere, to warm the planet.
--greenhouse effect, with
radiant energy from the sun converted to infrared radiation returning to the
atmosphere, but carbon dioxide and water molecules do not allow this radiation
to escape back to the outer space.
--the greater the carbon dioxide
in the atmosphere, the greater the greenhouse effect. Look at Venus, with 96.5 percent carbon dioxide in its
atmosphere.
--early life? Evidence???
Fossils of bacteria, probably in a marine setting, in sedimentary rocks
as old as 3.5 billion years. First step
to synthesize large organic molecules from methane and ammonia.
--next organic molecules
aggregate and form some means for growth and metabolism, but not capable of
self-reproducing…..possible location would be hydrothermal vents at mid-ocean
ridges…appropriate water temperatures, etc.
--next would be the first
truly self-replicating molecule (ribonuceic acid, RNA), and eventually DNA
(deoxyribonucleic acid)…DNA a more complex molecule
--alternative hypothesis is
that live arrived with incoming comets and therefore is truly extraterrestrial.
--what would have happened on
the earth’s surface? Weathering? Despite the fact that the early atmosphere
did not have a lot of oxygen in it.
Feldspars could have been chemically modified at about the same rate as
today…..sulfides and oxides would not have been able to be as rapidly oxidized.
IV.
Photosynthesis
A. use of chlorophyll and energy from sunlight to make
carbohydrates from carbon dioxide and water.
Important point is that oxygen is released.
--animals cannot, of course,
synthesize carbohydrates---so they need FOOD!.
The process of converting carbohydrates into energy, and carbon dioxide
and water is respiration. So, the cycle
is complete.
--photosynthesis and
respiration provide the means to cycle carbon dioxide and oxygen among the
oceans, atmosphere, land surface, and biosphere.
--burial of organic material
creates an imbalance (although a fortunate one) in the overall global cycle of
carbon dioxide and oxygen production and destruction. This imbalance presumably explains the increase in the growth of
oxygen in the earth’s atmosphere.
B. Oxygen
--increase in photosynthetic
organisms—leads to increase in oxygen in atmosphere, for short and long term,
because organic material starts to become buried.
--increase in oxygen brings
about changes in weathering patterns.
Also an increase in the ozone level.
Ozone is created through the transformation of oxygen molecules in the
upper reaches of the stratosphere by solar radiation. Ozone acts as a strong filter for much of the incoming UV
radiation from sun.
--what is the evidence for
early oxygen??? No question that photosynthetic bacteria were around by about
2.0 by, probably earlier. Look at what
are called banded iron formations and the nature of paleosols. Brief explanation of each. Since about 1.5 by, strong evidence for
multicellular marine organisms. Then,
starting in the Cambrian, abundant evidence for complex marine organisms
(example is the Burgess Shale, of the Canadian rockies).
V.
Geochemical Cycles
A. Introduction
--processes that provide
feedback mechanisms to prevent exhaustion of one element or compound with time
--reservoirs—atmosphere,
hydrosphere, etc. capable of holding chemicals for a considerable period of
time. Change in concentration of some
chemical is called a flux. Attempts to
quantify fluxes allow a better understanding of exactly how the many systems of
the planet work.
--look at the calcium cycle,
for example.
B. Basically, look at where calcium resides, and how it
can be transferred from one reservoir to another.
--ocean—dissolved solids and
gases
--calcium—lots in the
ocean. 5.6 x 10 20 grams. Comes from river transport, by chemical
weathering of geologic materials.
--so, does calcium build up
with time? Nope—precipitation as
calcium carbonate and gypsum. Fine. This implies that the oceans are in pretty
much of a steady state. That means that
one cannot use the concentration of any dissolved species in ocean water as
indicative of the age of the planet, as was proposed quite some time ago.
--residence time—how long
does an atom linger (or loiter, as the case may be). This depends on the amount of material entering the reservoir and
the solubility of the material. For
calcium, the residence time is about 850,000 years. For sodium, it is about 48 million years. For iron, it is only 100 years.
-what about ocean-atmosphere
interactions? At the sea surface, gas
molecules can escape, but gases can also dissolve in the ocean from the
atmosphere.
--residence time in the
atmosphere? Generally shorter than
oceans, because the reservoir is very much smaller. Carbon dioxide has a residence time of only 10 years. Interesting. [actually, as a sidelight, the federal government in now
investing lots of bucks in the notion of atmospheric carbon sequestering—reacting
the atmosphere with ultramafic materials to such carbon from the atmosphere—will
it work?????].
C. Carbon cycle
--photosynthesis and
respiration are major aspects of this cycle.
--look at figure 24.9
--how does carbon dioxide
enter the atmosphere? Volcanism,
sedimentation of calcium carbonate (this reaction releases one molecule of
water and one molecule of calcium carbonate for each molecule of calcite
preciptated)
--metamorphism (liberation of
carbon dioxide from carbonate minerals, as they are converted to silicates)
--how does carbon dioxide
leave the atmosphere? Weathering,
burial in the crust as carbonate minerals, and burial in the crust as organic
carbon. Burial of organic carbon
results in an increase in oxygen in the atmosphere.
--problem. On the short term, we are disturbing a
generally steady state system by pumping into the atmosphere too much carbon
dioxide by the burning of fossil fuels.
D. A geochemical model for the earth
--the planet can be thought
of as a number of interconnected beakers….., with fluxes in between them…..
VI.
Climate Change
A. look at effects of volcanism on climate change
VII.
Climate Change and Mass
Extinctions
A. overview of times of major mass extinctions
--two of greatest interest
--PermoTriassic boundary
--Cretaceous-Tertiary
boundary
--mid-Tertiary, Eocene to
Oligocene transition from a “greenhouse” to an icehouse climate
--periodicity to mass
extinctions????? Raup/Sepkowski
hypothesis (Nemesis affair)
VIII.
Cretaceous/Tertiary
Boundary
A. general introduction
--possible causes of the
extinction (food, struggle for survival, atmosphere contamination, dramatic
change in climate?)
--previous thoughts
--setting the stage, mid 1970’s. work at Gubbio, Italy
--boundary associated with a
geomagnetic field reversal? Nope.
--W. Alvarez and colleagues—investigate
the concentration of rare metals (iridium, osmium, etc.) in the limestone/shale
sequence at Gubbio, to better understand depositional rates?
--alarming discover in that
the K/T boundary mud layer was associated with an unusually high concentration
of iridium.
B. A new hypothesis emerges
--explanations…..complex, but
very interesting. Bolide impact—body some
10 km in diameter arriving at a velocity of at least 75,000 km/hour could have
immense and catastrophic conseqeuces for the planet.
--IN SCIENCE, HYPOTHESES MUST
BE TESTABLE. The impact hypothesis, if
true, must permit:
discovery of additional K/T
boundary sections with very high concentrations of Ir, Os, etc.
discovery (perhaps) of the
actual impact site
discovery of associated
features with respect to the boundary layer (e.g., shocked mineral grains,
tsunami deposits?, charcoal?, impact glass, etc.)
recognition that each
specific boundary has the identical magnetic polarity and has the exact same age
(with the precision of the measurement techniques).
C. New science emerges!
--lots of boundary sites
identified and characterized
--shocked quartz and other
minerals, plenty of impact glass, etc.
--unusual depositional
features
--even the potential culprit
found (end of Yucatan peninsula, Chicxulub)
--all boundary sites
associated with reverse magnetic field polarity
--all boundary sites dated at
about 65.0 to 64.5 myr age.
D. Alternative hypotheses
--how about lots of volcanism
concentrated over a relatively short period of geologic time?
--ok, then evidence?
--most likely culprit would
the Deccan basalt plateau province, in India
--one of the largest
preserved accumulations of mafic volcanic rocks on the planet
--age is perfect—from about
65.5 to about 64.5 myr.
--origin by an immense mantle
plume/hotspot—
--this would explain a more
gradual extinction, as the paleontologists have noted through much
research.
IX.
What does the future
portend?
A. Population
B. Pollution
C. More and more influence from developing nations.
D.