ATM103 Climate change and weather Module 1 discussed the nature of science, the distinctions between weather and climate, and the Earth system. Module 2 di

ATM103 Climate change and weather Module 1 discussed the nature of science, the distinctions between weather and climate, and the Earth system. Module 2 discussed how we are able to study Earth’s past climate using a wide range of techniques.In your initial thread, pleaseDescribe a concept or technique from either module that interested you the most, and why. Pose a question about material in either module that you are confused about or you would like to learn more about.In your three replies to posts by other students, either comment on and elaborate on the posts you find interesting, or take a shot at answering questions by other students. Decoding the Past –
Reading Nature’s Clues with Science
Part 1
Lecture 4 – ATM 103
Review of Previous Lecture
• Earth’s climate can change if any of the following change:
• Energy/radiation from the Sun
• Composition of the atmosphere
• Refectivity of the planet (albedo)
• Earth system is all connected.
• Matter and energy are transported through cycles,
processes, and events through Earth system reservoirs.
• Our resource consumption and waste disposal choices are
currently exceeding the Earth’s ability to replenish them
by 70%.
• Current Lecture: How do we know about Earth’s past
How do we know about
climate in the past?
• Historical records of
• We have a variety of
historical records of
temperature and
precipitation for
some regions.
• Evidence of
prolonged periods
when climate was
diferent in some
• “Little Ice Age”
• “Medieval Warm
The Hunters in the Snow by Pieter
Brueghel the Elder, 1565
Historical Evidence of Climate
in the Past 2000 years
• 1960s–1970s – Historical
evidence (temperature
records) of warm conditions
from 1000–1200 AD
(“Medieval Warm Epoch”)
followed by cooling trend
(“Little Ice Age”)
• Medieval Warm Epoch led to
Viking expansion and
settlements in Greenland.
• Little Ice Age resulted in cooler
temperatures in Europe.
• But probably not
representative of changes
in Earth’s global climate.

Historical temperature
Black line – land and ocean surface temperatures
o Land temperatures from over 4000 land stations
o Ocean surface temperature measurements from ships and
buoys since 1853
• Brown line – land temperatures only (mainly Europe and North
• Orange line – 4 European land stations only
warmer than normal
colder than normal
Land and ocean surface
Although it seems like a long time, 150–300 years worth of temperature
records is not long enough to study Earth’s past climate.
But, can we go back in time
We need to look for climate proxies.
• Climate proxy – a preserved physical
record of past climate
• Allows scientists to reconstruct Earth’s
climate before direct temperature records
were available
• Examples include tree rings, ice cores,
corals, and lake and ocean sediments
(next lecture)
Extending the historical record
back to 700 AD with proxies…
• General
agreement in
• Relatively cool
period before
• Relatively warm
period around
1000 AD
• Lots of
among proxy
• Last 100 years
has been period
of rapid warming
warmer than normal
colder than normal
How about even further
back in time (pre-historic
• We must decode climate clues left by nature
• Geologic evidence – reading the rocks for information about
past climates
• Fossil evidence – ancient life forms tell us about past
• Geomorphological evidence – reading the surface of the
land for clues to past climates
• Chemical and physical evidence – the chemical fngerprints
of past climates
• Before we can understand this climate
evidence, we need to understand
• Some basics about Earth’s history
• How we can date the evidence
• How we determine climate from proxies
Geologic and
Features  Climate
Clifs of D提over, England, made of chalk produced
by phytoplankton at the ocean surface
Yeager Rock, a 440-ton “glacial erratic” on the
Waterville Plateau, Washington, ice-rafted to its
present location 13,000 years ago inside the
4,000-foot-thick Cordilleran Glacier.
Glacial till made of sand, gravel, and rock
carried by ice at West Tarbert, UK
Long Island formed mainly by glacial moraines left
from the Ice Ages
U-shaped valley cut by glacial action, Yosemite, CA
This 6-foot long, 52-million-year-old palm
frond was found near Fossil Butte National
Monument (Wyoming) and suggests a
subtropical climate.
Petrifed Forest National Park provides evidence of
a thick forest of tall trees in what is now Arizona
200 million years ago.
Geologic Time Scale
• Geologists have
pieced together
the history of
the Earth.
• Earth is about
4.6 billion years
old, based on
• The history of the
Earth in distant
past is divided
into units based
on major events.
Dating Techniques
• Relative Dating
• Based on interpreting time
sequence of geologic strata to
determine their age relative to
one another
• Principle of Superposition – the
oldest strata are on the bottom
• Principle of Horizontality sediments are originally deposited in
horizontal layers
• Principle of Lateral Continuity sedimentary layers extend laterally
(across the surface)
• By analogy, determining whether
person A is older or younger than
person B doesn’t tell you the age
of either person. We need a way to
determine the actual age.
Example of Lateral Continuity –
Colorado River
Same layers seen in adjacent geologic features
New York State Fossil Eurypterus Remipes
This “sea scorpion” was a predator in
Silurian Seas (415–430 million years
ago) covering North America.
• Fossils – preserved remains or traces of
animals, plants, and other organisms from the
remote past
• Fossils occur in rock formations, particularly in
sedimentary strata
• Mineral rich water flls the buried body of the
organism, replacing the tissue with minerals
• Principle of Faunal Succession – fossil
organisms follow one another in a defnite
order, so we can determine time periods
through fossils
• Index fossils are particularly useful in dating
Index Fossils through Time
Using Index Fossils to Date
By identifying a known-age index fossil in a rock, we
can identify the rock’s approximate age.
Amplexograptus, an index fossil, from the Ordovician
Radiometric Dating
• Allows us to determine the age
of rocks, geological features,
and even biologic materials,
due to the radioactive decay of
certain isotopes of elements
• Radioactivity – radiation
emitted from an unstable atom
• First discovered by Antoine Henri
Becquerel (1896)
• Becquerel, Marie Curie, and Pierre
Curie received the Nobel Prize for
this discovery in 1903
• Radiometric dating frst
developed by Bertram Boltwood
in 1907 for uranium, applied to
dating the age of rocks
Antoine Henri
Becquerel (1852-1908)
Marie Curie (18671934)
Bertram Boltwood (1870-1927)
Pierre Curie (1859-1906)
• All matter is composed of
atoms of individual elements.
• Atoms are composed of
neutrons, protons, and
• Protons have a positive electric
• Electrons have a negative electric
• Neutrons are a proton and electron
combined, and have no electric
• If an atom does not have a charge,
the number of protons and
electrons is the same.
• Atomic number is the number
of protons.
• Atomic mass number is the
number of protons + neutrons.
The classical orbit representation of the
ground state of a Helium atom shows the
nucleus of two protons and two neutrons
orbited by two electrons.
• Matter is composed of
elements, which exist alone
or in combination with other
elements in the form of
• An element is a chemical
substance with a specifc set
of properties dependent on
the details of its atomic
Oxygen (O), the most
abundant element on
Silicon (Si), the
second most
abundant element on
Carbon (C), an essential
element for life on Earth
Nitrogen (N), the most
abundant element in the
atmosphere – 78% of air is
Elements are distinguished by their atomic
number (the number of protons).
Periodic table of elements
• Elements can have multiple isotopes.
• An isotope is an element with same number of protons, but
a diferent number of neutrons.
• Because the mass of the isotope is diferent, the physical,
chemical, and biological behavior of isotopes is slightly
• 91 naturally occurring elements have isotopes.
• Some isotopes are stable, meaning that they do not break
apart by radioactive decay.
• Others are unstable, meaning they break apart through
radioactive decay.
Hydrogen isotopes: protium (H-1), deuterium (H-2),
Radioactive decay
• Stable isotopes typically have the
same number of protons and
• When the number of additional
neutrons of an isotope is too large, or
the nucleus itself is too large (atomic
numbers >83), the nucleus becomes
unstable and the atom is radioactive.
• Unstable isotopes (radioisotopes)
lose energy by emission of particles of
ionizing radiation by two main
• Alpha decay – nucleus emits an alpha
particle (2 protons + 2 neutrons)
• Beta decay – nucleus emits an
electron and other particles, increasing
number of protons by 1
• Through this decay process, unstable
atoms can change from one element
to another element.
Radioactive decay of an unstable isotope of
lead (Pb-212), to Bismuth (Bi-212), to
Thalium (Tl-208) or Polonium (Po-212), and a
diferent isotope of lead (Pb-208). D提ecay
occurs through emission of alpha (⍺) and
• Radioisotopes decay over time
(milliseconds to hundreds of
millions of years) to another
isotope of an element.
• Radioisotopes continue to decay
into other daughter
radioisotopes until they become a
stable isotope that is not
• The half-life of a parent
radioisotope is the amount of time
needed for ½ of its atoms to
transform to a daughter
• If the daughter is not radioactive, it
will not decay further.
• If the daughter is radioactive, it will
decay into another daughter
Radioactive decay of uranium-235 into
daughter radioisotopes.
The age of a sample can be determined from the
relative abundance of the parent radioactive isotope to
its daughter isotope.
If there are equal amounts of the
parent and daughter, then the age
of the sample is one half-life.
If there is 25% of the parent and
75% of the daughter, then the age
of the sample is two half-lives.
etc …
etc …
Radiometric Dating
Click on the image below to watch a YouTube video.

The radioactive decay of certain
elements are useful for
determining the age of rocks.
• All uranium isotopes – a trace
element in Earth’s crust

U (uranium) 
Pb (lead)
• Half-life: 4.5 billion years

• Half-life of 703 million years

Th (thorium)
• Half-life of 80,000 years
Pitchblende is one of the many
naturally occurring minerals that
contains uranium.
• 3 naturally occurring potassium
(K) isotopes – 7th most
abundant element in Earth’s
dating allows us
to get actual
• K (potassium)  Ar (argon)
ages of rocks.
• Half life of 1.3 billion years
Time Scale
dating has
allowed us to
ages for the
time scale.
Rocks and Climate
• Radiometric dating reveals the age of
• Index fossils of known age can also be used
to identify the age of rocks.
• What can we learn about Earth’s prehistoric climate from rocks?
• Geologic evidence
• Geomorphological evidence
• Fossil evidence
Reading the Rocks
Sedimentary rocks
can record evidence
of ancient
• Ripple marks
indicate past shallow
Dinosaurs left
their footprints on
the beaches of
shallow seas
Uplifted strata at D提inosaur Ridge, CO (Cretaceous)
Ripples on beach in Wales, 2001
Reading the Rocks
• Desiccation cracks
indicate past arid
This strata
experienced a
period of drought
Rocks from NW Scotland, showing
ancient mud cracks (Pre-Cambrian)
Thick layer of mud develops deep cracks during dry periods
Reading the Rocks
• Burrow marks
indicate specifc
types of life.
t could
support life
that could
Trace left by a burrowing organism
from the late Precambrian
Creatures that disturb the soil (Meysman, Middelburg and Heip, 200
Reading the Rocks
• Coal forms in rock strata
over hundreds of millions
of years.
• Dead tropical or
temperate forest plants
accumulate in swampy
areas, are buried, heated,
and compressed into coal
• Coal provides evidence for
ancient tropical or
temperate forests.
Reading the Rocks
Oil shale
• Forms from marine
clay deposits rich in
organic remains from
thriving marine
• Presence indicates an
ancient ecosystem
with high productivity
in a region isolated
from rapid ocean
Reading the Rocks
• Makes up ~10% of the
volume of all sedimentary
rocks on Earth
• Composed of calcite and
aragonite, which are forms of
calcium carbonate (CaCO3)
• Many deposits consist of skeletal
fragments of marine organisms
(coral or foraminifera)
• Also deposited by chemical
precipitation (cave formations)
• Presence indicates a past
warm, tropical environment
with “plenty” of atmospheric
The Burren landscape in Western Ireland
(Aran Islands and much of County Clare) is
composed of thick limestone deposits from
the middle Carboniferous (~350 millions
years ago).
Reading the Rocks
Chalk deposits
• Composed of calcium
carbonate (CaCO3)
• Remains of marine
phytoplankton called
• Presence indicates “plenty”
of atmospheric CO2
• During the Cretaceous,
atmospheric CO2 was much
greater (>1000 parts per
million) than today (~400 parts
per million).
• Increase Earth’s albedo
through release of aerosols at
the ocean surface, promoting
cloud formation, and an
increase in ocean refectivity
The Clifs of D提over, on the coast of England,
are composed of ~110 m (350 ft) of chalk
deposits from the Cretaceous
Phytoplankton bloom in the Barents Sea
increases the refectivity of the ocean
Reading the Rocks
• Composed of calcite
(CaCO3), halite (salt),
gypsum, and other minerals
• Form when water evaporates
and leaves dissolved salts as
mineral deposits
• Presence indicates past hot,
dry conditions that led to the
evaporation of lakes and
• e.g., Great Salt Lake, Dead Sea,
and Bonneville Salt Flats
Great Salt Lake, Utah
Reading the Rocks
Glaciers and Ice
• Form characteristic
features in the landscape
due to the erosion by the
glacier or ice sheet of the
surface below it
• Moraines – glacially
accumulated debris
• Till – thick deposits of rocks,
sand, silt, and clay
• Tillite – rock composed of
metamorphosed glacial till
• Dropstones/Glacial
Erratics – large boulders
dropped by glaciers
Glacial Erratic
U-shaped valley
Reading the Rocks
Glaciers and Ice Sheets
• The past 2 million+ years of Earth’s history have
featured repeated periods of glacial advance and
• Glacial periods – when the landscape is dominated by ice,
temperatures are cooler, and sea level is lower (water
trapped in ice)
• Interglacial periods – when glaciers have retreated,
temperatures are warmer, and sea level is higher
Ice extent during last glacial period (left) compared to the present (right)
Fossils as Environmental
The fossil record provides a
record of ancient environments
• Fossil clam shells in limestone
• Past presence of shallow sea
• Fossil palm fronds
Modern brachiopods (Phillipines)
• Past presence of a temperate to
tropical climate
• Fossil corals
• Type of coral sensitive to water
Shale slab containing brachiopods
from the Pennsylvanian, Kansas.
Review: Decoding the Past Part I
• Direct temperature measurements on Earth only
go back a few hundred years at most
• Nature has left clues of our past climate that we
can study by studying climate proxies, which allow
us to study Earth’s climate much further back in
• We can use multiple techniques to study Earth’s
• Relative dating
• Fossil dating
• Radiometric dating
• Earth is about 4.6 billion years old
• We can link types of rocks and fossils with the
climate they existed in, and how climate may have
changed in the past.

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