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ESS 203 - Glaciers and Global Change
Outline for today• Volunteer for today’s highlights on Friday ________• Highlights from last Friday’s class – Baxter Sprouse
Today• Wrap up ice cores and paleo-atmosphere• Past climate from ocean sediments
Friday• Revisit peer review
Wednesday February 19, 2020.
Assignment for Friday Feb 21Group Term project updatesPlease turn in a 1-page report from your group including: • A paragraph outlining the topic for your group presentation and
paper. • Complete reference citations to your popular press source or
sources.• Complete reference citations for at least 2 reliable sources that
you have identified (e.g. peer-reviewed articles or government reports) that served as sources, or that cover the same topic closely.
• Tell me who in your group (if anybody) intends to do the optional W (Writing) credit.
Earth’s Atmosphere in the Past
How could we learn about changes in composition of Earth’s atmosphere in the past?
Collect air in bottles, save for future scientists.• Doesn’t help us to look back very far. Our ancestors
were not planning far enough ahead.
Find bubbles of air trapped in naturally sealed containers.• Containers must not react chemically with the air.• Containers must be really well sealed.• Guess where we can look…
What can we Measure in the Air Bubbles?Carbon dioxide CO2 • (rock weathering, plant growth & decay,
ocean uptake) • Did CO2 lead or follow temperature changes
in pre-industrial times?
d18O of oxygen gas O2(a proxy for plant respiration)
Methane CH4 • (vegetation decay, swamps, termites)• Why would it change in an Ice Age?
• Respiration removes the lighter (hence more reactive) 16O in preference to 18O, increasing the relative amount of 18O in the atmosphere.
Character of Climate Records
Composition of the atmosphere can also change with time.
e.g. CO2 and CH4 (methane) are rising today.• Would you expect the composition of atmosphere
to differ from place to place at any one time?• Why or why not?
We are not surprised if climate changes (e.g. temperature, precipitation, windiness, etc) are a bit different at different places on Earth.
e.g. remember Lake Bonneville in Utah in the Ice Age
Synchronization of Climate RecordsSo if you measure gases in one ice core, you know the
whole story of Earth�s atmosphere. Yet scientists keep measuring gas from bubbles from
more ice cores … • Why?• Does this suggest another use of gas measurements in
ice cores?
Gas-based North-South Correlation
Blunier and Brook 2000 Science.
Line up Greenland and Antarctic cores with CH4
Then compare temperature records
Ice age - Gas age Difference
• The air being trapped today is part of today’s atmosphere
• The ice that traps today’s air is already old (older than the air)
This offset in age is called the Ice age/gas age difference
Air is modern
Ice is already old
What’s the Catch?
• Can be hundreds to thousands of years.• Can be different in Glacial and
Interglacial times.An area of active work today is figuring out how to calculate the ice age/gas age difference with uncertainty as small as possible in order to understand the timing of climate change between Northern and Southern hemispheres, and to figure out the drivers (CO2, or temperature).
We may know age of ice, and we want to date the gas.We may know age of gas, and we want to date the ice.
So we have to be able to figure out this difference.
Air is modern
Ice is already old
Gas-based North-South Temperature Correlation
What’s the relationship?• Both North and South see the big events.• Antarctica warms first.• Climate changes were faster and larger in Greenland.
Blunier and Brook 2000 Science.
Greenhouse Gases and Ice-Age Temperature
CH4
CO2
T
Vostok ice core
What have we learned?
Ice cores tell us about temperature, precipitation, and atmosphere composition over the past 500,000 years and more
• Some climate changes can be very fast.• North and South see the same big picture of warming and
cooling.• Patterns differ on centennial and millennial time frames.• Greenhouse gases change along with temperature. But • do they drive climate,• do they respond to climate • or both? It’s a complicated world.
Questions about Glacial Epochs
• How do we know there was more than one past �glacial period�?
• How do we know how many former glacial periods there were?
• How do we know when these glacial periods took place?
• What caused these glacial periods?
In glacial periods, H2O in big ice sheets was depleted in 18O. Where was the “missing 18O?
Calcium (Ca), and dissolved carbon dioxide (CO2).
http://www.ucl.ac.uk/GeolSci/micropal/foram.html
~0.4 mm
Foraminifera: �bugs� living in the oceansMarine microorganisms, foraminifera (�forams�), make shells of limestone (CaCO3) using water (H2O), dissolved
Planktonic forams:• live in near-surface
waters.Benthic forams:• live in �deep� water
(<4000 m).
• Foram shells contain an isotopic record of the environment in which they live.
• O in ocean water H2O is made up of 16O and 18O.• When they die, forams sink to the ocean floor and their
shells are preserved.• Over time, layers of sediment pile up on top of one
another.• These layers of foram shells contain an environmental
record of the ocean over time (much as layers in an ice core retain a memory of temperature in the clouds).
• But what, exactly, does 18O in those shells record?
Sea-floor sediments
• What does this �record� consist of and how would one get it?
Curious Scientists wonder about Climate Record in ocean sediments
• Why was it difficult to figure out how many glacial periods there had been, based on terrestrial (�land�) records?
• Considering the answer to the above question, why would the sea floor be a good place to look for a better record?
• Why would it help to know some biology when studying the sea-floor record?
Forams and sea-floor records
How can we learn from them?
Biologists know that the relative uptake of 16O and 18O in forams depends on temperature of water in which they live.
• More 18O is taken up when the water is colder. •d18O in the shells could be a paleothermometer for
seawater temperature.
But the relative uptake of 18O/16O also depends even more strongly on the relative amounts of 18O and 16O in the seawater in which the forams lived.
• More 18O is taken up in the shells when the water has more 18O in it. (You are what you eat and drink.)
Interpreting d18O in foram shells
Forams and sea-floor recordsGlacial Period• Ice sheets on land are
depleted in 18O.• Remaining sea water
must be enriched in 18O.• Sea-level is low.• Forams live in 18O-rich
ocean.
Today• Not so much ice on land.• Sea-level is high again.• Forams live in ocean that
is now poorer in 18O again.
d18O in foram shells records -Ø 18O/16O ratio of seawater in which they lived.ØTemperature of sea water in which they lived.
A Dilemma
• When we measure d18O in foram shells, how do we know whether we are measuring past ice-sheet volume, or past ocean temperature?
With two types of forams we can get two types of information
Now, we know the change in the ratio 18O/16O in the seawater. And it was the same for everybody.
• Remove those sea-water isotopic changes from the record from shallow-water (planktonic) forams.
• The signal that’s left tells us about changes in Sea-Surface Temperature (SST).
In the deep ocean, temperature is always near 0oC • Changes in the 18O/16O ratio in deep-water (benthic)
forams should mostly reflect changes in the 18O/16O ratio of the seawater.
• So, measure these guys first.
http://www-odp.tamu.edu/publications/177_SR/chap_09/c9_f4.htm#982709
mcd –
meters
composite
core
A d18O record from the sea floor
BenthicPlanktonicRed – core % reflectance
Benthic and planktonic records are very similar.• So the the major signal is
ice volume on land.• Temperature signal (in
planktonic) is a subtle addition.
• High d18O large ice volume• Low d18O Small ice volume
€
δ18O =Rsample - Rstandard
Rstandard
×1000, R ≡18O16O%
& '
(
) *
• d18O more positive => relatively more ice on land
• d18O more negative => relatively less ice on land
… d shows changes are relative to the standard used, which is based on modern-day sea water … so relative to present-day ice volume.
But, how can we date the ocean sediments?(There are no annual layers to count.)
Oxygen isotope record in deep-sea sediment
10001RR
=Ostandard
sample18 ´÷÷ø
öççè
æ-dor,
Bugs High and Low
We’re foraminiferaand we live in the seaSome folks call us little bugsHow insulting can they be!?
We Planktonics live shallowand frolic in sunlightwhere SST varies …We Benthics live deep where the temperatures keep.
We are what we drink and eat; Our 18O goes up
when ice-sheets build upOur 18O goes down when ice-sheets melt.Planktonics also recall SST.
(Anon)
Earth’s Dynamo
Currents in conductive liquid iron outer core generate Earth’s magnetic field.
http://www.windows.ucar.edu/
http://www.newgeology.us/presentation25.html
mid-ocean ridge
Magnetic Stripes on Sea Floor
Earth’s magnetic field has reversed many times.What does this mean?
http://www.newgeology.us/presentation25.html
http://www.newgeology.us/presentation25.html http://upload.wikimedia.org/wikipedia
Dating field reversals
Basalt rocks on mid-ocean ridge are magnetized as they cool through about 400oC.
• Geologists can measure the direction of their magnetization.
• They can also date when the rocks cooled, using their radioactivity.
How does this help to date sediments?• Mineral grains are washed into the oceans by rivers. • They slowly settle out and become part of the ooze
on the ocean floor.• Some grains contain magnetite.• They act like tiny magnets.• As they settle slowly through deep, still ocean water,
they can align themselves with Earth’s magnetic field at the time.
• Direction of magnetization can be measured in a sediment core,
• and correlated with dates of reversals known from the lava flows on land and basalts on the sea floor.
http://www-odp.tamu.edu/publications/177_SR/chap_09/c9_f5.htm#982733
single core
�stack� of many cores from ocean basins around the world
SPECMAP
Lots of land ice
Little land ice
And now depth in the sea floor can be converted to Age.
http://www-odp.tamu.edu/publications/177_SR/chap_09/c9_f5.htm#982733
~100 ka
Transfer of magnetic
reversal ages to sediments
http://www-odp.tamu.edu/publications/177_SR/chap_09/c9_f6.htm#982743
~100 ka
Changes in Ice Cores (temperature) and Ocean sediment Cores (ice volume) agree on Timing
• Records show history of changes in d18O in seawater over time.
• This is directly related to history of changes in ice volume on land.
• Records show many glacial and interglacial periods over the last ~1 Ma.
• Ice cores show a record with similar ups and downs, but for temperatures in polar regions.
• For the last ~800 ka, ice-age periodicity is ~100 ka… Why?
• Stay tuned for Lab in a couple of weeks …
Summary of deep sea records
The other way that glaciers tell us about changing climate
Without studying the detailed geochemistry inside the ice sheets, we can still learn about past climate change.• How’s that?• By seeing how much glaciers grew or shrank …
