Foraminifera: bugs living in the oceans

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Foraminifera bugs living in the oceans
Marine microfossils, foraminifera (forams),
make their shells from limestone (CaCO3) using
H2O, dissolved Ca, and dissolved CO2.
400 x 10-6 m (lt0.5 mm)
http//www.ucl.ac.uk/GeolSci/micropal/foram.html
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Sea-floor sediments

• O in ocean water H2O is made up of 16O and 18O.
• Foram shells contain a record of 18O/16O ratio of
  seawater in which they live.
• When they die, forams settle to the ocean floor
  and this record is preserved.
• Over time, layers of sediment pile up on top of
  one another.
• These layers then contain record of 18O/16O ratio
  in seawater over time (much as layers in an ice
  core retain a memory of 18O/16O in the clouds).

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Forams and sea-floor records
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More about forams

• If we measured the 18O/16O ratio on some forams,
  how would we know whether we are measuring
  changes in the isotopic content of the water or
  changes in ocean temperature?
• In the deep ocean, the temperature is always very
  near 0oC, so changes in the 18O/16O ratio in
  deep-water forams should mostly reflect changes
  in the 18O/16O ratio of seawater.
• Knowing this, we could correct the record from
  shallow-water forams, which might also contain
  some information about changes in sea-surface
  temperature.

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Forams and paleotemperature

• Biologists knew that uptake of 16O and 18O in
  forams depended on water temperature.
• More 18O was taken up if the water was colder.
• Could the 18O/16O ratio in forams be used to
  measure changes in ocean water temperature over
  time?
• But wait cant the ratio of 18O/16O in seawater
  change over time also?

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Isotopic changes in the ocean
d 0o/oo
d gt 0o/oo
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More about forams
Planktonic forams live in near-surface
waters. Benthic forams live in deep water
(lt4000 m).

• Variations in the 18O/16O ratio over time are
  similar in both planktonic and benthic forams.
• All forams record d18O composition sea-water.
• Planktonic forams also record changing
  temperature in ocean surface waters, but overlay
  is weak.
• The18O/16O signal in deep-sea sediment cores
  changes as water is alternately stored by, and
  released from, large ice sheets.

The 18O/16O signal in deep-sea sediment cores is
a relative measure of ice-sheet volume.
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Ocean in Ice Age
18O enriched relative to today (?18Ogt0)
Ocean today
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d18O record from the sea floor
Benthic planktonic
mcd meters composite core
http//www-odp.tamu.edu/publications/177_SR/chap_0
9/c9_f4.htm982709
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Oxygen isotope record in deep-sea sediment
?18O more positive gt relatively more ice on
land ?18O more negative gt relatively less ice on
land changes are relative to standard used,
which is based on modern-day sea water so
relative to present-day ice volume.
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Magnetic Stripes on Sea Floor
mid-ocean ridge
Earths magnetic field has reversed many times.
http//www.newgeology.us/presentation25.html
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Dating field reversals
http//www.newgeology.us/presentation25.html
http//www.newgeology.us/presentation25.html
http//upload.wikimedia.org/wikipedia
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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 still water, they
  can align themselves with the Earths magnetic
  field.
• Direction of magnetization can be measured in a
  sediment core.

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SPECMAP
stack of many cores
single core
http//www-odp.tamu.edu/publications/177_SR/chap_0
9/c9_f5.htm982733
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Transfer of magnetic reversal ages to sediments
100 ka
http//www-odp.tamu.edu/publications/177_SR/chap_0
9/c9_f5.htm982733
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Changes in Ice Cores and Ocean sediment Cores
agree on Timing
100 ka
http//www-odp.tamu.edu/publications/177_SR/chap_0
9/c9_f6.htm982743
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Summary of deep sea records

• Records show history of ice volume changes over
  time.
• Records show many glacial and interglacial
  periods over past 1 Ma.
• Ice cores show a similar record.
• For the last 800 ka, ice-age periodicity is 100
  ka Why?

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Where does the 100 ka climate periodicity come
from?

• Changes internal to earth system
• Changes in energy reflected to space.
• Changes in trapped energy.
• Changes in energy transport.
• Changes external to earth system
• Changes in amount of sunlight getting to Earth.

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Changes internal to Earth

• (1) Changes in energy reflected back to space
• Changes in vegetation
• Changes in cloud cover
• Changes in snow, land and sea ice cover
• (2) Changes in trapped energy
• Changes in greenhouse gasses (CO2, H2O, CH4)

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Changes internal to Earth

• (3) Changes in transport of energy
• Changes in topography
• mountain building, ice sheets
• Changes in ocean circulation
• short term sea/land ice, climate feedbacks
• long term sea floor spreading, new ocean basins
• Changes in heat from Earth (geothermal flux)
• How likely is this?

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Changes external to Earth

• Changes in the amount of sunlight getting to
  Earth
• Changes in solar output over time.
• Changes in blocking of sunlight between Earth
  and sun.
• Changes in distance between Earth and sun.
• Changes in timing and distribution of Earths
  seasonal exposure to sunlight.

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Changes in orbital parameters control changes in
sunlight reaching Earth

• Changes in obliquity of Earths axis relative
  to ecliptic plane
• 41,000 years
• Changes in eccentricity of Earths orbit around
  sun
• 19,000 and 23,000 years
• Precession of equinoxes (wobble of Earths axis
  in time)
• 100,000 and 400,000 years

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Changes in obliquity

http//earthobservatory.nasa.gov/Library/Giants/Mi
lankovitch/milankovitch_2.html
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Changes in Eccentricity
http//earthobservatory.nasa.gov/Library/Giants/Mi
lankovitch/milankovitch_2.html
e 0
e 0.5
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Precession of Axis
http//earthobservatory.nasa.gov/Library/Giants/Mi
lankovitch/milankovitch_2.html
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Sunlight Variations at 65oN
http//en.wikipedia.org/wiki/ImageMilankovitch_Va
riations.png
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(applet by Derek Fox)
Summer insolation (W m-2) 60 degrees North 1 Ma
to present day Check it out http//cs.union.edu
/Archives/SeniorProjects/CS.2005/foxd/hw/finalproj
/applet/
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Question for Curious Scientists

• Why have we only been concerned about changes in
  insolation in the northern hemisphere?
• Where is ice stored in the form of glaciers
• on land or on ocean?
• Where has most of the earths land mass been
  located for the last few million years
• northern or southern hemisphere?

To be stored for long periods of time before
being released back to the ocean, ice must be on
land.
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How can insolation changes trigger an ice age?
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How can insolation changes trigger an ice age?
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How can insolation changes trigger an ice age?

• Suppose insolation at some latitude, e.g. 60oN,
  is reduced by orbital variations.
• 60oN now gets the sunlight that the Earth used to
  get farther north (e.g., 65oN).
• As we found in lab, the elevation of the snowline
  (and glacier ELA) decreases as we move farther
  north.
• Glaciers and seasonal snow cover exist at lower
  elevations than they can at lower latitudes.
• Lowering insolation has the same effect as moving
  poleward.

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How can insolation changes trigger an ice age?

• During periods of lower insolation, snowlines are
  lower, and glaciers can exist at lower latitudes.
• Implications
• More of Earths surface is covered with
  reflective snow and ice.
• The amount of energy retained is further reduced
  because of higher planetary albedo.
• Cooling is further enhanced (positive feedback).

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More on changes in orbital parameters
(Milankovitch cycles)
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Obliquity cycle 41 ka

• Changes in seasonality
• Summer/winter temperature contrast is
• large if obliquity is at a high angle (hot
  summers, cold winters)
• small if obliquity is at a low angle (cool
  summers, warmer winters)

http//deschutes.gso.uri.edu/rutherfo/milankovitc
h.gif
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Eccentricity Cycle 100 ka

• Changes in earth-sun distance
• Larger eccentricity (more elliptical) results in
  a larger earth-sun distance at aphelion (point
  farthest from sun).
• Larger distance equals less radiation received at
  surface.

http//deschutes.gso.uri.edu/rutherfo/milankovitc
h.gif
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Precession of the Equinoxes 19 ka and 23 ka
Changes where on eccentricity cycle summer/winter
falls In the upper panel, N.H. winter occurs far
from sun. In the lower panel, N.H. summer occurs
far from sun. enhances or diminishes the effect
of obliquity-cycle-induced seasonality
Which is better for growing and keeping glaciers?
http//deschutes.gso.uri.edu/rutherfo/milankovitc
h.gif
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http//www.homepage.montana.edu/geol445/hyperglac
/time1/milankov.htm