Difference between present Antarctic sea ice and LGM sea ice

1
Difference between present Antarctic sea ice and
LGM sea ice
2
Ice Sheet Retreat

• Ice Sheet retreat begins about 18 -14 kyrs ago,
  and are largely gone by 6,000 years ago.

3
How thick were the ice sheets?
Two models thick (early) and thin (more
recent). Thickness determination is difficult
30 of ice is buried below level plane.
Thicknesses are now contrained by sealevel
estimates and new ice flow (cemented vs free
base) models.
4
This rebound from past glacial loading can
confuse measurements of present sea level change.
5

• How do we determine the volume of the LGM Ice
  Sheet?
• From sea level rise
• 2. Glacial moraines give lateral extent (but not
  thickness)
• 3. Rebound of the depressed continent beneath
  the ice sheet (cm/year) can be used to estimate
  thickness.

ice
70
30
continent
6
Hudson Bay paleo-beach
Post-glacial rebound. If bottom of ice sheet is
depressed (buried) below the initial level
surface, and then the ice sheet melts, the land
mass will rebound to the original height. 14C
dating of the old beach marks (as the land rose)
allow estimates of this rebound rate. 150 m/7000
yrs 2 cm/year.
7
Ice Sheet Volume
8
Sea Surface Temperature (today)
9
Sea Surface Temperature Change at LGM
10
Other Estimates of SST Change at LGM

• Alkenone content of pelagic Plankton
• d18O of CaCO3
• pelagic forams

Alkenones and d18O indicate tropical SST
decreased by 2 to 4ºC - at Last Glacial Maximum
11
ALKENONES a proxy for seawater temperatures
without needing to know ice volume!
What is an alkenone? A saturated fat used by
phytoplankton (for cell walls, interior fluid).
The degree of saturation (number of
carbon-hydrogen bonds) in these fats depends on
temperature. HIGH saturation fats become solid
at low temperatures (like lard). So extraction of
these alkenones from plankton in sediments
(specific species), and measurement of their
degree of saturation, can give the temperature
at which the formed.
e. Huxleyi everyones favorite
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What is a saturated fat?
13
transfat Not climate related
Saturated fat
Unsaturated fat
Olive oil
14
Alkenones collected from sediment cores as
function of latitude (world wide).
Previous work (Geochim. Cosmochim. Acta 62
1757-1772, 1998) showed that values for the
alkenone unsaturation index UK'37 measured in
Modern sediments throughout the open World Ocean
strongly correlate with annual mean sea-surface
temperature (SST).
15

• So by
• Picking out individual species of phytoplankton
  from sediment cores.
• extraction of alkenones (using organic solvents)
  from these skeletons
• Running the extractions in a gas chromatograph
  to get the specific fat saturated to unsaturated
  ratio
• It is possible to get sea water temperatures
  (benthic or pelagic) when the phytoplankton grew
• WITHOUT THE NEED TO KNOW ICE VOLUME.

16
ALKENONE record of SST off Santa Barbara, CA
Comparing d18O with alkenone seawater
temperatures
17
So if you do alkenone extraction and analysis
from sediments (i.e., take a series of cores
along a profile along a longitude) that are LGM
in age in the tropics, you can estimate the sea
surface temperature in the tropics at LGM
time. And that is quite small (i.e., the equator
didnt get very cold at LGM although the poles
and intermediate latitudes did).
18
Meridional temperature distribution. Remember
the Cretaceous? If the tropical SST temperature
during the LGM was only a few 0C LOWER during the
LGM than the present, and the poles (Antarctic,
Greenland) were 10 to 200 C lower, how would the
LGM curve correspond to the graph below?
N Pole
S Pole
19

• DUST times of large continental ice sheets
  produced abundant dust!
• Glacial periods were drier, colder air
  temperatures implies reduced moisture and reduced
  rainfall (but not everywhere i.e., southwest
  U.S.). Less vegetation cover.
• Glacial periods had higher winds. (think
  meridional temperature gradients)
• Ice Sheets and mountain glaciers produced lots of
  rock flour fine silt.
• The higher winds, drier climate and fine silt
  combined to produce abundant DUST which was
  transported on global scale.
• Indian Ocean sediments indicate that LGM dust
  levels were 5 x higher than present in that
  area.

Loess deposits (wind driven silt) from the LGM
time. Large, thick loess deposits exist in
Eastern Washington and are responsible for
fertile wheat fields there.
20
Desert dust source regions today Arrows are
prevailing winds today during LGM, these areas
produced even MORE dust than they are today.
Note dust from the Sahara desert is blown out
into the south Atlantic, providing IRON that
fertilizes upper ocean productivity
21
Regions with abundant sand dunes during Top
today. Bottom LGM time. Dust levels in
Antarctica were 10 x those today, as estimated
from the ice cores. If surface ocean biology
(diatoms vs coccolithophores) plays a role in the
transition from glacial to interglacial
periods By INCREASING the biological PUMP it is
likely to be through ocean circulation and dust
(as a nutrient supply).
22
Climate change near the North American Ice
Sheets Near the edge of the ice sheets, the
climate was much wetter than present with Lake
Bonneville (covered 40 of the State of Utah)
being an example. Lake Bonneville existed about
15K years ago, and drained catastrophically into
the Columbia River when the natural dam in the
north failed.
This drainage may (or may not) have produced
changes in the ocean circulation in the NE
Pacific Ocean along with Lake Missoula
floods. In contrast to the SW, the Pacific NW was
colder and drier, and many areas were desert.
23
Jet stream in modern times note it is just north
of Seattle.
Jet stream (from numerical models) during Last
Glacial Maximum note that it is considerably
farther south.
24
Cross-section of bottom water formation in the
North Atlantic.
The N - S transfer of water via ocean circulation
is responsible for significant transfer of solar
heat, from the equator (high input zone) to high
latitudes. Any change in this circulation
pattern results in a change in climate in the
temperate and polar regions
25
Ventilation of the oceans 14C dating of DIC
(dissolved inorganic carbon)
14C age dating of seawater means - when was the
Dissolved Inorganic Carbon in the water last in
equilibrium with the atmosphere?
Generalized circulation of the oceans (now)
Deep-water formed in the N. Atlantic (zero 14C
age). By the time it gets to the NE Pacific (as
bottom water), about 1500 years have passed.
26
14C dating of DIC (dissolved inorganic carbon)
Dissolved inorganic carbon in seawater. HCO3
(1777 mmol/kg) and CO3 (225 mmol/kg). So mostly
bicarbonate. Some of these carbon atoms are the
isotope 14C, which is formed in the atmosphere
from cosmic ray bombardment, and decays which
with a half life (50 gone) of 5,730 years.
Probably can measure out to 6 half-lives, or
30,000 years. DIC can be obtained from water
samples taken from the top and bottom of the
water column. By carefully measuring the 14C
age of this DIC and comparing surface and
benthic values, it is possible to get an estimate
of the number of years since that bottom water
was exposed on the surface.
27
Ocean convection cell if convection is FAST,
then surface and deep water will have (about) the
same values of 14C. If the convection cell is
slow (stopped), surface and bottom water will
have very different 14C values.
14C
Young 14C
Old (decayed) 14C
28
By measuring the 14C values in sediments in
benthic and pelagic forams as a function of
sediment age, it is possible to estimate the
vigor of ocean circulation at different times
(i.e., during the present, and during the
LGM). In the Pacific, the age difference (benthic
vs pelagic) is similar to today In the
equatorial Atlantic, the age difference was about
twice (675 years) the modern value of 350
years. This means bottom water circulation in the
Atlantic at LGM was slower than today i.e, less
bottom water formation at high latitudes.
This means that during the LGM, less thermal
energy was transferred from the equatorial
Atlantic to the north Atlantic with strong
implications for climate in the northern
hemisphere. The area around Paris, for instance,
became arctic tundra.
29
Deep Water Formation Present vs LGM
30
Possible Impact of Reduced NADW Formation Rates
on Air Temperatures
31
Increased Global Aridity at LGM

• Ice Core Record of Dust
• - increased dust at LGM due either to increased
  strength of winds or dust source (aridity)

32
Vegetation Changes
NOW
Use pollen records from several lakes to
reconstruct regional vegetation distribution
during LGM.
LGM
33
Atmospheric Gases during LGM
CO2 was 180 ppm (vs 280 ppm at warm
interglacials) CH4 was 350 ppb (vs 700 ppb at
interglacials)
34
Summary Climate Conditions during LGM

• Insolation rates about the same as today.
• Colder ( -4 º C globally and -10 ºC near the
  poles (maybe colder) and -2 to -3 ºC in
  tropics).
• Ice Sheet volume was twice today.
• Sea Level lower by 125m.
• Drier and dustier (globally).
• Reduced atmospheric CO2 and CH4 levels
• Vegetation more arctic like (tundra, steppe).
• Deep Ocean circulation more sluggish.

35
Sea Level Rise
Use 14C and 230Th/238U to date the age of a
sequence of submerged corals that lived close to
the sea surface. The rate of sea level rise
has pulses. (14C ages are too young by up to 3K
yrs.)