The Last Glacial Maximum,

1

• The Last Glacial Maximum,
• which was 18,000 radiocarbon (14C) years ago or
  21,000 calendar years ago.

2
Continental-size ice sheets as far south as 37N
in North America (Seattle is 4730N) and 48N in
Scandanavia. In the mid-west, the Laurentide IS
grew almost as far south as St Louis, MO. Today
ice sheets cover 10 of land and 3 of total
earths surface. LGM ice sheets covered 25 of
land and 7 of total earths surface. World was
4C cooler during LGM than present. NOTE this
is about the same temperature change currently
predicted for anthropogenic global warming.
North Atlantic was 8C cooler. Polar regions
were cooler during LGM (about 8C) than
present. Minimum temperature (maximum Laurentide
ice sheet extent) was at 21,000 calendar years
BP. Sea level was about 120 meters lower than
present.
3
NOTE insolation values at LGM were about the
same as present, so insolation alone could not
explain the maximum in ice sheet growth.
4
The two North American ice sheets contained about
55 of the excess ice (over present time)
during the LGM, but they were composed of two
different sheets. The Laurentide sheet in the
east, and the Cordilleran ice sheet in the west.
And they grew and decayed at different
times. I.e., the Laurentide IS started melting at
21,000 calYrs, while the Cordilleran IS kept
growing until 17,000 calYrs BP.
5
Eurasian Ice Sheet
Barents Sea had grounded ice sheet at LGM.
6
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.
7

• 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
8
This rebound from past glacial loading can
confuse measurements of present sea level change.
9
Excess ice (at LGM). North American 55
Scandinavian and Barents ice sheets 22.
Antarctica and Greenland expansion of present
ice 23.
10
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.
11
Dating of glacial moraines Evidence for maximum
extension.
12

• 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.
• 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.
13
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
14
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,
it is likely to be through ocean circulation
and dust (as a nutrient supply).
15
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.
16
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.
17
Ventilation of the oceans 14C dating of DIC
(dissolved inorganic carbon)
14C age dating means - when was the DIC 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.
18
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
19
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.
20
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.