Changes in tilt over time

1
Changes in tilt over time

• Current tilt of Earth 23.5?
• Tilt has varied between 22.2 ?- 24.5?

2
Eccentricity

• Shape of the Earths orbit
• E has varied between 0.005 (more circular) to
  0.0607 (more squished)
• Shape of the orbit is currently nearly circular
  (0.0167)

3
Precession PLUS eccentricity

• Today June solstice occurs near aphelion (most
  distant from the sun)
• 11,500 years ago, June solstice occurred at
  perihelion (closest to sun)
• esinw precessional index (eccentricityprecession
  )

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• Using these three orbital variations,
  Milankovitch produced a mathematical model that
  calculated latitudinal differences in insolation
  and the corresponding surface temperature for
  600,000 years prior to the year 1800.
• He then attempted to correlate these changes with
  the growth and retreat of the Ice Ages.
• He chose summer insolation at 65 degrees North as
  the most important latitude and season to model,
  reasoning that great ice sheets grew near this
  latitude and that cooler summers might reduce
  summer snowmelt, leading to a positive annual
  snow budget and ice sheet growth.
• FOR ABOUT 50 YEARS THIS THEORY WAS LARGELY IGNORED

5
       Milankovitch theory Milankovitch
proposed that low summer insolation is the
critical factor that that allows ice sheets to
grow. WHY?
Changes in insolation calculated using the
Milankovitch theory.
6
How to grow a glacier

• Northern Hemisphere summers are coolest when
• 1) Earth is farthest from the Sun due to
  precession and greatest orbital eccentricity
• 2) Tilt is at a minimum (less of high latitudes
  tilted towards the sun during the summer)
• snow can then accumulate on and cover broad
  areas of northern America and Europe. 
• At present, only precession is in the glacial
  mode, with tilt and eccentricity not favorable to
  glaciation

7
cycles change DISTRIBUTION of insolation over
seasons at different latitudes -up to 20 at
high latitudevery little change in TOTAL
insolation received by Earth
figure shows changes in solar radiation received
on the day of the June solstice compared to today
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Why do glaciers form?
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Why do glaciers form?

• Glaciers form when the amount of snow falling in
  the winter is greater than the amount that melts
  in the summer

11
How do glaciers form?

• Pore spaces around snow crystals disappear
• Snow becomes denser
• burial and pressure leads to increase in crystal
  size

12
Accumulation and ablation
13
Glacier mass balance
14
Ice sheet growth lags behind solar forcing

• Even under the most favorable conditions for ice
  sheet growth, it takes thousands of years for ice
  volumes to reach a maximum
• The rate of ice volume increase is greatest
  during coldest times, but the ice sheets dont
  reach their maximum size until thousands of years
  later

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Does solstice correspond with the peak (or
minimum) in temperature?
London, England - 52 North, 1 East
Fairbanks, USA - 65 North, 148 West
16
Ice sheet growth lags behind solar forcing

• At the 41,000 yr cycle, ice sheets lag insolation
  by 10,000 years
• At the 23,000 yr cycle, ice sheets lag insolation
  by 6,000 years

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So is this what really happened?

• Where should we look for evidence?
• Ocean sediments
• Records of ICE RAFTED DEBRIS
• records of changes in 18O
• What does the 18O record mean again?
• HIGH 18O in ocean means
• 1) more 16O trapped in ice
• 2) cooler ocean temperatures

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Shackleton and Emiliani

• continuous delta 18O for the last 2.75 Myr
• Took cores of ocean sediments
• Produced records of changes in 18O from shells of
  foraminifera

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Shackleton and Emiliani

• Two types of variations
• Numerous cycles
• Gradual trend towards positive 18O (cooling)
• Prior to 2.75 Myrno IRD and low 18O
  valueswarmer
• 2.75 Myr-0.9 Myr 41,000 year cycle (50 cycles!)
• 0.9 Myr-present 100,000 yr cycle, abrupt
  melting

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The mystery of the 100,000 year cycle
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The mystery of the 100,000 year cycle

• Tilt (41,000 yr) and precession (23,000 and
  19,000 yr cycle) should control summer insolation
  
• Eccentricity (100,000 year cycle) should only act
  to moderate precession
• But over last 0.9 Myr, 100,000 year cycle is
  dominant

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Poor Milankovitch. . .

• Died in 1958didnt live to see ocean sediment
  records
• But what would he have thought about the 100,000
  year cycle?

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Possible explanations for the 100,000 year cycle

• 1) Non-linear response of climate system to
  eccentricity forcing
• Amplification of 100,000 yr forcing
• 2) 100,000 year period is INHERENT time
  constraint of climate system with big ice sheets
  (slow response time of big ice sheets)
• Mid-Pleistocene transition before 1 million
  years ago, dominant cycle was 41,000 yr and ice
  sheets were smaller in volume
• After 1 Ma, dominant cycle is 100,000 yr and ice
  sheets are bigger.
• Maybe if ice sheets get big enough, they cant
  respond to 41 kyr cycle.

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Why are 100,000 yr glacial cycles asymmetric?

• Slow ice buildup for 80-90,000 yrs, modulated by
  obliquity (tilt) and precession (wobble) followed
  by rapid collapse
• Climate forcing doesnt have asymmetric pattern.
  BUT glacial terminations occur when 65 oN
  insolation reaches maximum
• Complication climate records from around the
  globe are synchronous
• Evidence snowlines from Alaska to S. America
  all lowered in LGM and glacial terminations all
  occur at 15,000 yrs ago.

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Why does the climate of the N. Hemisphere control
the climate of the S. Hemisphere?

• What can cross the equator?
• OCEAN changes
• Sea level change lower sea level, more land for
  ice to grow on Antarctica
• Circulation reduction in NADW formation occurs
  during glaciations (fig 11-20)
• ATMOSPHERIC changes
• Lower CO2 during glaciations can cool whole
  planet
• Glacial CO2190 ppm, interglacial280 ppm
• Which came first?
• Lower H2O during glaciations, decreased
  evaporation, decreased water vapor, less heat
  transfer to polesglobal cooling

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Ice cores and atmospheric gasses
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The mighty Vostok record

• Records extend back 400,000 yr. .
• . . Newsflash! Records now extend back 600,000 yr!

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The mighty Vostok record

• Does ice volume correlate with CO2?
• Which came first?

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How does gas get trapped in ice?

• Pressure causes recrystalization. This blocks
  off air passages
• Pressure is depth dependant, not time dependant
• Is air younger or older than surrounding snow?
• YOUNGER. Depending on accumulation, can be
  gt2,000 years younger

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It is a miracle that curiosity survives formal
education. Albert Einstein
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Reminderwhy do we care?
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Changes in Methane (CH4) concentrations over
glacial cycles

• Cyclic variations of methane
• 23,000 year cycles
• What does this correspond with?
• PRECESSION
• Variations in CH4 suggest changes in TROPICS
• Wet/dry cycles
• What causes changes in wet/dry cycles in tropics?
  
• MONSOONS
• Increased insolation in June, increased monsoon
  strength, increased methane

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Changes in CO2 concentrations over glacial cycles

• Change from 190 ppm to 280 ppm (90 ppm, 30
  change)
• Changes in ice volume correlate with CO2. But
  which came first?
• If CO2 was driver, ice volume should lag CO2 by
  several thousand years
• Ice volume doesnt laginstead, ice volume lags
  insolation

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Changes in CO2 concentrations over glacial cycles

• Orbital scale changes in ice volume change CO2
• How?

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What drives changes in CO2 over time?

• 1) Physical oceanographic changes
• 2) chemical oceanographic changes
• 3) terrestrial carbon reserves
• Biomass
• Soil carbon

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Physical oceans and carbon storage (chapter 11)

• 1) temperature
• If you decrease temp, increase amt of dissolved
  gasses9 ppm per 1 oC
• 2) salinity
• if you increase salinity, you decrease the amt of
  CO2 you can dissolve
• Salinity was higher at LGM
• Net result, increased salinity plus decreased
  temp STORAGE of 11 ppm CO2
• BUT, we need to account for 79 ppm (190 ppm LGM,
  280 ppm pre-industrial)

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Changes in carbon storage during the LGM
41
Terrestrial reservoirs

• At LGM, 25 LESS vegetation reservoir
• Boreal forests covered w/ ice
• South of ice sheets were steppes
• Drier rainforests
• SO. . .where did extra CO2 go?

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Changes in carbon storage during the LGM
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Changes in carbon storage durign the LGM

• Reduced carbon in atmosphere, veg and soils and
  surface ocean
• Since surface ocean exchanges rapidly with air,
  surface ocean is in equilibrium with atmosphere
• Where did the rest of the carbon go? Deep ocean

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Carbon storage in the deep oceans

• How do you get carbon into the deep ocean?
• 1) Carbon pump hypothesis
• Increase productivity in surface ocean (more dead
  organic material sinks to deep ocean)
• What do you need to increase productivity?
• Nutrients and sunlight

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Carbon storage in the deep oceans

• How do you get carbon into the deep ocean?
• 1) Carbon pump hypothesis
• Increase productivity in surface ocean (more dead
  organic material sinks to deep ocean)
• CO2 H20 ? CH2O O2
• What do you need to increase productivity?
• Nutrients and sunlight

46
Carbon storage in the deep oceans

• How do you increase nutrients?
• Increase upwelling of nutrient-rich deep water
• Increase nutrient content (wind?)

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Carbon storage in the deep oceans

• Changes in ocean chemistry
• At surface, carbonate is supersaturated (CaCO3)
• At depth (4-5 km), water is undersaturated w/
  respect to CaCO3 and ocean will dissolve CaCO3
  (corrosive)
• Hypothesis at LGM, more corrosive deep water
  dissolved more carbonate and produced CO3-2 .
  When this water upwells, the CO3-2 combines with
  CO2 to produce HCO3- (bicarbonate)
• End result you decrease CO2 in atmosphere

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Summarydo we know where the carbon went?

• We lost carbon from the atmosphere
• 280 ppm (pre-industrial) -190 ppm (LGM)90 ppm
  CO2 180 gigatons
• We lost carbon from the vegetation and soil
• 25 reduction or 530 gigatons
• Carbon is likely stored in the deep ocean. .
  .research is ongoing

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The Last Glacial Maximum
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LGM

• 21,000 years ago
• Icy! Cold! Windy! Dry!
• But. . insolation levels nearly identical to
  today
• Why? Remember that ice sheets respond slowly

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LGM

• Low CO2 levels
• 150-160 meter drop in sea level
• Submerged moraines. . But isostatic rebound of
  crust. Makes estimates difficult

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Maximum extent of ice at last glaciation
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CLIMAP reconstructing the LGM
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Ice sheets at the LGM
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Retreat of the Laurentide Ice Sheet
Animation by P. Bartlein and J. Shinker
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Cordilleran ice sheet
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Other types of sediment records from the LGM

• DUST
• Aka?
• LOESS!
• Sources of loess
• DRY climate in general
• Ice sheets, glacial outwash, pluvial lakes

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Glacial outwash
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Dust in the Wind

• Winds can blow fine-grained (silt and clay-sized)
  material great distances
• Evidence from around the globe indicates that
  more debris was blowing around during the LGM

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Dust in the Wind
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Changes in Vegetation at the LGM

• How were these maps constructed? What is the
  data?
• Pollen records, models, midden records, etc.
• Vegetation indicates cooler, dryer grass-covered
  steppes, tundra, few forests

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Methods of reconstructing Quaternary flora and
fauna

• Pollen records from lakes
• Packrat midden records
• 28,000 years old! Capitol Reef
• Faunal and archeological records
• Food caches, tools, bones, charcoal, paintings. .
  

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The Pleistocene overkill or climate change?
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The Pleistocene overkill hypothesis

• Theory championed by Paul S. Martin of the
  University of Arizona in Tucson. 
• A definition of "Overkill" was offered by Martin
  (1984) as meaning "the human destruction of
  native fauna either by gradual attrition over
  many thousands of years, or suddenly in as little
  as a few hundred years or less". 
• His hypothesis uses the fact that extinctions
  were most numerous and sudden on continents
  humans invaded and where they had not developed
  their hunting skills.  North America, South
  America, and Australia, which were invaded by
  humans, all experienced large extinctions,
  whereas in Africa and Eurasia, where humans
  evolved their hunting techniques, fewer
  extinctions occurred.

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Blame North America megafauna extinction on
climate change, not human ancestors

• "While the initial presentation of the overkill
  hypothesis was good and productive science, it
  has now become something more akin to a
  faith-based policy statement than to a scientific
  statement about the past," Donald Grayson, a UW
  anthropology professor

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Ice sheets and temperature at the LGM

• North American ice sheets were hugeequivalent to
  Antarctic ice sheet today
• Glacial world 4 degrees C cooler
• North Atlantic 8 oC cooler

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Problems with CLIMAP

• Ice height too high (3 km)
• Assumed ice was frozen to bed, so could build up
  in height
• But most of Laurentide was not frozen to bedmust
  have been thinner
• Post glacial isostatic rebound indicates height
  was 1 km less

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Other effects of ice on climate

• Enhanced albedo
• Cold wind drainage off ice sheets
• Size of ice reacts w/ atmospheric circulation
• Jet stream (12 km high). Ice sticks up 2 km.
• Jet stream pushed SOUTH
• Results in lots of pluvial lakes in SW
• High pressure over ice, production of glacial
  anticyclone? (maybe?)

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Paleolakes in the western U.S. during the last
glaciations (pluvial lakes)
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Pluvial Lakes at the LGM
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Pluvial Lakes at the LGM

• Do pluvial lakes mean more precipitation?
• Not necessarily. Just less evaporation

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G.K. Gilbert The father of Geomorphology
B. K. Emerson and G. K. Gilbert examine a rock
specimen found in a glacial moraine. Photographed
by Edward Curtis, 1899.
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G.K. Gilbert The father of Geomorphology

• 1843 1918
• He was one of 6 original members of the USGS
• He recognized the block-fault nature of the Basin
  Range (his term)
• He studied problems caused by hydraulic gold
  mining in the Sierrassediment transport, and
  environmental impact
• He was part of the 1899 Harriman Alaska
  Expedition -- developed theories about glacial
  climate, topography and motion
• He figured out the existence of Lake Bonneville
  and the Bonneville floods

Gilbert, G. K. (1890). Lake Bonneville U. S.
Geological Survey Monograph 1, 438 pp.
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Lake level changes of Lake Bonneville
82

• Lake Bonneville represents the highest lake level
  at about 5090 feet.
• The lake reached this level about 16,000 years
  B.P.
• A catastrophic event occurred about 15,000 B.P.
  in which the natural dam at Red Rock Pass gave
  way and released massive amounts of floodwater
  into the Snake River Valley.
• The lake was lowered by 350 feet as a result of
  this single event.
• The lake again stabilized about 14,500 years B.P.
  when the erosion at Red Rock stabilized and the
  Provo level became established.  

raven.umnh.utah.edu/units/great.salt.lake/
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• Estimated that the probable peak discharge of the
  flood was 15 million cubic feet per second.
• This is to be compared with a maximum historic
  discharge in the upper Snake River of 72,000 cfs
  at Idaho Falls in June of 1894.
• The total flood volume is believed to be about
  380 cubic miles.

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• The Provo stage of Lake Bonneville occurred from
  about 13,500-14,500 years B.P.
• The lake level at this time was about 4,470 feet
  with overflow leaving the lake through Red Rock
  Pass, Idaho.
• This outlet flowed out to the Snake and Columbia
  Rivers and ultimately reached the Pacific Ocean.

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• The Gilbert stage of the lake occurred from about
  11,000-10,000 years B.P.
• During this time, the lake rose to a level of
  4,250 feet and then began to decline. This level
  marks the culmination of historic Lake Bonneville
  and the beginning of the Great Salt Lake. This
  period shows a transition in climate from cooler,
  high precipitation to slightly warmer with less
  precipitation. 

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• The Altithermal Period occurred from about 6,000
  to 7,000 years B.P. and may have seen complete
  dessication (drying up) of the lake.
• Ancient sand dunes and buried mudcracks on the
  floor of the lake suggest that a warming climatic
  trend may have evaporated all or close to all of
  the water content of the lake basin. 

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• Little Ice Age (1400-1800 AD) may have resulted
  in higher lake levels.
• The lake level at this time may have exceeded
  another threshold at 4,217 feet in elevation
  increasing the lake level from 2,800 square miles
  to 3,700 square miles. The overflow would have
  filled the floor of the Great Salt Lake Desert
  located to the west of present Great Salt Lake. 

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Isotasy
In equilibrium, the pressures exerted by
overlying masses of rock will always be the same
at a certain depth.
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Isostatic rebound

• The mass of glaciers and ice sheets has caused
  the surface of the earth to be depressed
• Ice depresses underlying bedrock by an amount
  to 30 of the ice thickness (height of ice above
  landscape represents 70 of its total
  thickness)
• Since removal of the weight from glacial melting,
  the land surface is rebounding

www.uwgb.edu
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Isostatic rebound

• The mass of glaciers and ice sheets has caused
  the surface of the earth to be depressed
• Since removal of the weight from glacial melting,
  the land surface is rebounding

• http//www.homepage.montana.edu/geol445/hyperglac
  /isostasy1/

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Isostatic rebound and Lake Bonneville
Adams, 1999. Figure 1
pubs.usgs.gov/circ/c1050/first.htm
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Isostatic rebound and Lake Bonneville

• Tilting of Bonneville shorelines first noticed
  by. . GK Gilbert! (1890)
• Gilbert theorized the tilting was caused by the
  load of the lakefrom this he hypothesized a
  liquid substrate beneath the crust
• Bonneville shoreline is 74 meters higher in the
  center of the basin than at Red Rock pass (on the
  margin)

Adams, 1999. Figure 1
pubs.usgs.gov/circ/c1050/first.htm
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Missoula Floods
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Missoula Floods

• Harlen J. Bretz 1920-30s
• Box 14.2
• Geologists estimate that the cycle of flooding
  and reformation of the lake lasted on average of
  55 years and that the floods occurred
  approximately 40 times over the 2,000 year period
  between 15,000 and 13,000 years ago.

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• Northern Idaho covered by Cordilleran ice sheet
• Mountain glaciers in the Sawtooths, Bitteroots,
  Lost Rivers, Lemhis, Beaverheads, and Salmon
  River Mountains

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The LGM in Idaho
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OSL 2 - Maldes trench - ladder sample
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The LGM in Idaho
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The LGM in Idaho
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ELAs (Equilibrium Line Altitude) at the LGM
(Last Glacial Maximum)
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Coming out of the LGMthe Bolling-Allerod Warm
Interval

• Bolling-Allerod Period 14,500-12,900 years ago
• Caused by strengthening of NADW formation?

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Coming out of the LGMthe Bolling-Allerod Warm
Interval
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Climate after the LGM

• Deglaciation
• Increased insolation
• Max Solar radiation (long intense summers) 100
  th yr cycle
• - precession (23th yrs)
• - eccentricity (100 th yrs)
• - perihelion (summer)
• 15,000 - 13,000 yrs ago
• The Bolling-Allerod Warming
• Younger Dryas 12,900 - 11,500 yrs ago
• - pause in warming
• - cold period
• - less snow accumulation
• - some glacial advancement
• - slowed Global melting
• Continued Warming

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Quaternary Climate change the last 600,000 years
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The Younger Dryas

• 12,900 - 11,500 yrs ago
• Pause in warming
• Intense Cooling of North Atlantic, near full
  glacial cold

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Younger Dryas Cold Spell
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Glacial retreat and meltwater
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North American Ice Sheet Retreat
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Glacial Lake Agassiz
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Meltwater Pulses Recorded as pulses of higher
concentrations of d18O values in planktonic
shells
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Ocean thermohaline circulation Broeckers
conveyor belt
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The Younger Dryas

• Intense Cooling of the North Atlantic, near full
  glacial cold
• It is a popular belief that fresh water flooded
  the northern Atlantic and altered the
  thermohaline ocean circulation
• Shut Down of the Conveyor Belt

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Evidence of the Younger Dryas

• 1st evidence in European pollen records
• Name derived from an Artic plant called Dryas
• Changes in ocean surface temps of N. Atlantic by
• about 7 degrees C
• Ice Cores from Greenland
• Lower accumulation rates
• Cooler temps in England evident in insect fossils
• Stopped Ice Sheet retreat
• Scandinavia shows glacial advancement
• Slowed Global melting

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Evidence of the Younger Dryas
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The Younger Dryas as a freak catastrophic event

• Uncommon event in glacial termination
• Methane levels dropped from 680-460 ppb during
  the Younger Dryas
• Antartica ice cores show no evidence of Younger
  Dryas type events (return to cooling) following
  shortly after a period of deglaciation

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Quaternary Climate change Younger Dryas and
Bolling-Allerod
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End of the Younger Dryas

• Abrupt ending
• Decades
• Return to warming conditions
• Rapid melting
• Kick start the conveyor belt
• Remember increased ( maximum) solar radiation _at_
  100 th yrs
• Had a simultaneous affect on large global regions

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Lake Agassiz Hypothesis

• Teller suggests that a catastrophic flood of
  Glacial Lake Agassiz provided a surge of fresh
  water to the N. Atlantic
• This abrupt decrease in salinity shut down the
  conveyor belt

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N. American Ice Sheet Drainage Pattern
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N. American Ice Sheet Drainage
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Lake Agassiz Hypothesis

• Melt water from the N. American Ice Sheet was
  diverted from the Gulf of Mexico
• -evident in a decrease of d18O-deficient glacial
  melt-water (using planktonic shells)
• (this means more d18O and less d16O) Decrease
  in d18O in the St. Lawrence Valley _at_ this time
• No sedimentary evidence of Eastern Drainage
• Glaciation dominated the land where previously
  proposed drainage patterns exist

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Arguments against the Lake Agassiz Trigger

• Lack of evidence of a flood drainage channel
• -Later, smaller floods created well defined
  canyons
• Perhaps the flood used the same channel/s as the
  post Younger Dryas floods
• As a result of calving, numerous icebergs could
  have supplied the fresh water surge
• -an abundance of ice rafted debris from this
  time suggests a fleet of icebergs

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More Posssibilities

• The water from Lake Agassiz may have escaped
  under the ice, without radiocarbon traces.
• An equatorial temperature disturbance may have
  triggered a change in the wind pattern over the
  Atlantic, permitting ice to form. This may have
  resulted in the conveyor shut down.

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What Caused the Younger Dryas?

• Perhaps YOU can discover the true cause, or at
  least make an educated guess!!!

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Global Warming

• Increased fresh water flow to N. Atlantic
• What does this mean?
• Perhaps another conveyor belt shut down?
• Small changes can produce catastrophic events and
  larger changes in the climate

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Future climates are dependent on how we live
now!!!