Transcript Time

Time
In 1650, James Usher,
Anglican Archbishop of
Armagh, published a
treatise in which he
calculated the time to
creation to be nightfall on
the day preceding
October 23, 4004 B.C.
In reality, it’s a bit older….
If we compress the history of our the Earth (~ 4.6 billion years) into one calendar
year, we can gain some perspective.
In the first four months of that year, the earth was very inhospitable, violent and
lifeless.
The first cell living cell developed somewhere at the ocean shoreline sometime
between may-June. In August, terrestrial plant life evolved.
Dinosaurs appeared on the scene on December 21 and became extinct by
noontime on December 26.
New Year’s Even has been a busy day. Human ancestors appeared around 8 pm.
Modern man (Homo sapiens) appeared in Africa around 11: 36 pm on December
31. Civilization, which dates back to about 30,00 years, started at 11:56 pm. The
first human arrived to North America from Asia, at 11:58 pm. The industrial
revolution took place one second before midnight. In the last seven tenths of a
second before midnight, human beings have been very busy indeed. They have
increased their population to more than 6 billion, at the stroke of midnight.
FIGURE 2.1 Geologic Time
The gathering of cosmic gases under gravity’s pull created Earth some 4.6 billion years
ago. Yet life became neither abundant nor complicated until the Cambrian period, or
slightly earlier, when the first vertebrates appeared. Source: After U.S. Geological Survey
publication, Geologic Time.
In general, the
oldest rocks in a
layer will be at
the bottom. As
you move
upward through
the strata, the
rocks (and the
fossils they
contain) become
progressively
younger.
FIGURE 2.2 Stratigraphy
Sediment settling out of water collects at the bottom of lakes. As more sediment collects,
the deeper layers are compacted by the ones above until they harden and become rock.
Animal remains become embedded in these various layers. Deeper rock forms first and is
older than rock near the surface. Logically, fossils in deeper rock are older than those
above, and their position within these rock layers gives them a chronological age relative to
older (deeper) or younger (surface) fossils.
Stratigraphy
Fossil animals occur in sedimentary rocks deposited on oceanic shorelines, one upon the
other. Subsequent cracks in the Earth’s surface, weathering, or erosion by a river open
these ancient sedimentary deposits, exposing their cache of fossils.
Index fossils can be used to match rock
strata. This helps establish ages of strata
across wide geographic areas.
Each exposure of rocks can be of a different age from other exposures. To build up an
overall sequence of fossils, various exposures can be matched where they share similar
sedimentary layers (layers of the same ages). From five sites in the southwest United
States, overlapping time intervals allow paleontologists to build a chronology of fossils
greater than that at any single site.
FIGURE 2.4 Index Fossils
After careful study at many well-dated sites, paleontologists can confirm that certain fossils
occur only at restricted time horizons (in specific rock layers). These distinctive index
fossils are diagnostic fossil species used to date rocks in new exposures. In this example,
the absence of index fossils confirms that layer B does not exist at the third location.
Perhaps rock-forming processes never reached the area during this time period, or the
layer was eroded away before layer C formed. After Longwell and Flint.
Most fossils are formed when an organism is
entrapped in soft sediment at the bottom of a lake or
ocean.
Deposits harden into sedimentary rocks.
Some fossils are more than just impressions, but
entire organisms.
Radioactive Isotope
(Parent)
Product
(Daughter)
Half-Life
(Years)
Samarium-147
Neodymium-143
106 billion
Rubidium-87
Strontium-87
48.8 billion
Rhenium-187
Osmium-187
42 billion
Lutetium-176
Hafnium-176
38 billion
Thorium-232
Lead-208
14 billion
Uranium-238
Lead-206
4.5 billion
Potassium-40
Argon-40
1.26 billion
Uranium-235
Lead-207
0.7 billion
Beryllium-10
Boron-10
1.52 million
Chlorine-36
Argon-36
300,000
Carbon-14
Nitrogen-14
5715
Uranium-234
Thorium-230
248,000
Carbon-14 dating
While alive, organisms accumulate both ordinary carbon (C12) and its unstable isotope carbon-14
(C14)into their tissues in proportion to their availability in the atmosphere. When the organism dies,
stable C12 persists, but unstable C14 decays at a constant rate and is lost slowly from the fossil. The
more time that passes, the more C14 is lost from the fossil, thereby changing the proportion of one to the
other with the passage of time. Consequently by measuring the proportion of C12 to the remaining C14,
scientists are able to calculate the geologic age of the fossil.
Radiocarbon dating is only useful for carbon-containing
materials, and has a relatively short “half-life”.
FIGURE 2.5 Radiometric Dating
(a) Sand flows regularly from one state (upper portion) to another (lower portion) in an hourglass. The more sand in the bottom, the more
time has passed. By comparing the amount of sand in the bottom with that remaining in the top and by knowing the rate of flow, we can
calculate the amount of time that has elapsed since the flow in an hourglass was initiated. Similarly, knowing the rate of transformation and
the ratios of product to original isotope, we can calculate the time that has passed for the radioactive material in rock to be transformed into
its more stable product. (b) Half-life. It is convenient to visualize the rate of radioactive decay in terms of half-life, the amount of time it
takes an unstable isotope to lose half its original material. Shown in this graph are successive half-lives. The amount remaining in each
interval is half the amount present during the preceding interval. (c) A radioactive material undergoes decay, or loss of mass, at a regular
rate that is unaffected by most external influences, such as heat and pressure. When new rock is formed, traces of radioactive materials are
captured within the new rock and held along with the product into which it is transformed over the subsequent course of time. By measuring
the ratio of product to remaining isotope, paleontologists can date the rock and thus date the fossils it contains.
How do fossils form?
Pleisiosaur fossil
This Mesozoic reptile, although not formally part of the dinosaurs, was their contemporary.
It was adapted to aquatic life.
Plant fossils
A favorable splitting of this rock yields a view of the pressed plant fossil held within
(bottom) and its impression on the other face of the rock (above).
Dinosaur footprint
At the time, this footprint of a dinosaur pressed into soft mud and became preserved in the
now hardened rock.
There are
many other
types of trace
fossils. The
study of such
fossils is called
“ichnology”.
Insect in amber
This mosquito was caught, then imprisoned in stick tree sap that subsequently hardened
into this amber, preserving the insect within.
Martian microbes?
Meteorites from Mars survived passage through Earth’s atmosphere and fell in areas of the
poles. Microscopic squiggles resemble microfossils from early Earth history, suggesting a
similar early history for life on Mars. However, such microfossils could also be produced by
simple chemical reactions rather than represent the remains of small, living organisms.
FIGURE 2.6 Geological Time Intervals
The Earth’s history, from its beginnings 4.6 billion years ago, is divided into four major
eons of unequal length—Hadean, Archean, Proterozoic, and Phanerozoic. Each eon is
divided into periods, and periods into epochs. Only epochs of the Cenozoic are listed in this
figure.
FIGURE 2.7 Fossil Eggs
This clutch of dinosaur eggs from about 70 million years ago is thought to be from a
Segnosaur, an enigmatic carnivorous (or omnivorous) species about which we know little.
These eggs, found in China, were laid together in pits or holes in the ground that may have
been lined with plant material, which did not fossilize. Each egg is about 6 cm in diameter.
Photograph kindly supplied by Lowell Carhart, Carhart Chinese Antiques.
FIGURE 2.8 Fossil Ichthyosaur
Small skeletons are seen within the adult’s body and next to it. This may be a fossilized
birth, with one young already born (outside), one in the birth canal, and several more still
in the uterus. Such special preservations suggest the reproductive pattern and livebirth
process in this species.
FIGURE 2.9 Archaeopteryx
The original feathers have long since disintegrated, but their impressions left in the
surrounding rock confirm that the associated bones are those of a bird.
FIGURE 2.10 Fossil Dig in Wyoming
(a) Partiallyexposed dinosaur bones. The work crew prepares the site and notes the
location of each excavated part. (b) This Triceratops femur is wrapped in a plastic jacket to
prevent disintegration or damage during transport back to the museum. Photos courtesy of
Dr. David Taylor, Executive Director, NW Museum of Natural History, Portland, Oregon.
FIGURE 2.11 Restoration of a Fossil
(a) This skeleton of the extinct short-faced bear, Arctodus simus, is positioned in a likely
posture in life. (b) Scars on the bones from muscular attachments and knowledge of
general muscle anatomy from living bears allow paleontologists to restore muscles and
create the basic body shape. (c) Hair added to the surface completes the picture and gives
us an idea of what this bear might have looked like in its Alaskan habitat 20,000 years ago.
FIGURE 2.12 Making Fossils
The remains of extinct animals that persist have escaped the appetites of scavengers,
decomposers, and later tectonic shifting of the Earth’s crustal plates in which they reside.
Most surviving fossils are of dead animals that quickly became covered by water and
escaped the notice of marauding scavengers. As more and more silt is deposited over time,
the fossil becomes even more deeply buried in soil compacted into hardened rock. For the
fossil held in the rock to be exposed, the Earth must open either by fracture or by the
knifing action of a river.