Deglaciation

1
Deglaciation

• LGM climate controlled by
• Ice sheets and atmospheric CO2
• Deglacial world shift
• Higher insolation and CO2
• Smaller ice sheets
• As insolation increased
• Ice sheets melted
• Influenced climate much less
• CO2 had a largely secondary role in climate

2
Timing of Ice Sheet Melting

• Determined by dating organic remains formed
  during ice sheet retreat
• Although scarce, suitable samples exist
• N. American ice sheets began retreating 14,000
  14C years ago
• Gone by 6,000 14C years ago
• Area does not yield ice volume
• Thickness of ice debatable

3
Sea Level from Coral Reefs

• One meter of sea level rise 0.4 million km3 of
  ice
• Total global ice volume can be compared with
  insolation record
• Coral reefs on Barbados gave sea level history
• Know Barbados had minor subsidence
• 14C dated sea level curve supports expectations
• Rate of sea level rise maximized during maximum
  summer insolation
• Insolation record well known
• Summer insolation maximum 10,000 years ago
• Barbados corals gave a 14C dated record of sea
  level rise

4
14C Age Not True Age

• When 14C dated corals dated by Th/U
• 14C ages were too young
• Implication was that rate of 14C production from
  14N
• Greater during LGM
• More 14C present in sample
• Gives an age that is too young
• Young age confirmed by tree ring studies

5
Implications for Ice Volume

• The Th/U chronology more accurate
• Highest rates of sea level rise
• Before maximum summer insolation
• The bigger they are, the quicker they melt
• Generally consistent with Milankovitch theory
• Response time curve predict lag
• Must be other feedbacks at work

6
Rise in Sea Level Not Smooth

• Record of sea level rise is not smooth
• Rapid rise from 20K to 14K years ago
• Slow from 14-12K years
• More rapid rise after 12K years
• Rates of sea level rise changed dramatically

7
Rate of Sea Level Rise

• Rate of sea level rise slowed significantly
  between 14K and 12K years ago
• Two major pulses of freshwater influx to oceans
• Melting glaciers
• Glacier melting episodic
• Flow of meltwater to oceans episodic

8
Meltwater Pulses

• Oxygen isotopic composition of planktic
  foraminifera
• Monitor freshwater influx to ocean
• Anomalously low d18O measured in foraminifera
• Norwegian Sea
• Barents Sea ice sheet
• Gulf of Mexico
• Laurentide ice sheet via the Mississippi River

9
Meltwater Pulses Additional Evidence

• Ice-rafted debris in non-fossiliferous sediments
  west of Ireland
• Suggest large influx of fresh water into North
  Sea
• Sourced by massive release of ice bergs
• Sediments deposited between 17K and 14.5K years
  ago
• Coincide with first major meltwater pulse
• Calving ice bergs would accelerate ice sheet
  melting

10
Younger Dryas

• Mid-deglacial pause in ice melting
• Accompanied by brief climate cooling
• Particularly in subpolar N. Atlantic Ocean
• Pollen records in Europe and Scotland indicate
• Cold-tolerant tundra (including the Arctic plant
  Dryas)
• Displaced early growth of forests
• Evidence of Younger Dryas also found in N.
  Atlantic sediments

11
Younger Dryas

• Southward re-advance of polar water in the N.
  Atlantic evident in faunal assemblages
• Reversal towards Artic vegetation in Europe
• Cold-tolerant insects in England (7C)

12
Younger Dryas

• Recorded in Greenland ice core
• Ice sheet accumulation rates changed abruptly
• Ice accumulation slow during LGM and Younger
  Dryas
• Large changes in windblown dust
• As indicated by Ca content in cores
• Younger Dryas was cold, dry and windy climate

13
Causes of Younger Dryas

• Broecker called upon change in NADW formation
• Meltwater diverted from Gulf of Mexico to N.
  Atlantic
• Pulse of low-salinity meltwater cut off NADW
  formation
• Cut off heat transfer to subpolar Atlantic from
  tropics

14
Critics of Broecker

• Meltwater pulses to N. Atlantic
• Occurred when global rates of ice melting were a
  factor of 5 lower
• With such low rates of meltwater influx
• How could such a small diversion cause such a big
  change in climate?
• Mechanisms causing cooling hotly debated
• Cooling appears global (e.g., greenhouse gases)
• Signal could be transferred quickly from N.
  hemisphere ice sheets

15
Testing Broeckers Model

• If thermohaline overturn in N. Atlantic slowed
• Decrease northward heat transport
• Warm tropical Atlantic
• If greenhouse gas reduction
• Produced Younger Dryas cooling
• Expect synchronous global cooling
• SST measurements in tropical Atlantic
• Help sort out mechanism
• Greenland ice core records clearly document N.
  hemisphere, high latitude cooling
• Due to heat released from N. Atlantic
• N. Atlantic cooled

16
Synchronous or Asynchronous Cooling?

• Oxygen isotope records from GRIP and Byrd ice
  cores
• Temperature differences between N. and S.
  hemispheres
• Suggest asynchronous cooling
• Changes in rates of NADW formation
• Yet terrestrial climate records suggest
  synchronous cooling
• Changes in greenhouse gas concentrations
• Oceanic or atmospheric control

17
Temperature Records

• SST based on alkenone unsaturation in core taken
  near Grenada
• Tropical western N. Atlantic (12N)
• d18O from coexisting planktic foraminifer
• During Younger Dryas
• Alkenone SST increase
• GRIP temperature decreases
• Asynchronous cooling

18
Mechanisms of Change

• Compare SST with benthic Cd/Ca
• Cd/Ca record from Bermuda Rise
• Cd indicator of phosphate
• Barometer of changes in the source of deep water
• Low nutrient N. Atlantic deep water
• High nutrient Antarctic sources
• Cd/Ca maximum in younger Dryas indicates less
  NADW
• Slowdown in NADW formation from injection of
  freshwater
• Cooled N. Atlantic and Greenland
• Less heat transferred N. from tropics so tropics
  warmed

19
Support from 10Be

• Variations in production rates of atmospheric
  10Be and 14C
• Linked to solar activity and Earths magnetic
  field
• Concentration of atmospheric 14C also affected by
  removal of radiocarbon
• Changes in global carbon cycle
• Muscheler et al. (2000 Nature, 408567-570) used
  1OBe to constrain production rates of 14C during
  Younger Dryas
• Residual variation due to carbon cycle
• Consistent with lower ventilation rates
• Therefore a reduction in deep water formation
  during Younger Dryas

20
Warm Eastern Cold Western Atlantic

• SST records off northwest Africa show cooling
  during younger Dryas
• Implies southward advection of cold water
• Along Canary Current
• Meltwater shut down NADW formation
• Reduced NADW formation caused
• Tropical western and southern Atlantic warmed
• Eastern and northern Atlantic cooled
• Results predict asynchronous N. and S. hemisphere
  temperatures in ice cores
• On short times scales
• Consistent with Cuffy and Vineux (2001)

21
Paradox Freshwater Influx?

• If rate of NADW formation is reduced by meltwater
  pulse during Younger Dryas
• Why was the pre-Younger Dryas climate and
  presumably NADW formation seemingly unaffected
• Large documented meltwater pulses?
• Paradox seemingly resolved if large pulses
  originated from Antarctic ice sheet
• Recent modeling (Clark et al. 2002 Nature,
  415863-869)
• Thermohaline circulation sensitive to small
  changes in hydrologic cycle (0.1 Sv)
• Why no thermohaline circulation in Pacific?
• Surface waters are too fresh to sink

22
Huh? Antarctic Ice Sheet Melting

• Large ice sheets melted early and melted fast
• d18O data from Norwegian sea imply early melting
  of Barents ice sheet
• Sediments in N. Sea and d18O data in Gulf of
  Mexico imply melting of Laurentide ice sheet
• Indicate N. hemisphere ice sheet melting
• Some evidence exists for early deglacial warming
  in Antarctica
• Suggest that this acted as a trigger for melting
  ice sheets in north polar regions

23
Abrupt Melting Events

• Suggest feed backs in climate system accelerated
  ice sheet melting
• Iceberg calving would increase rate of melting
• Moves ice quickly into relatively warm waters
• Ice sheets in marginal marine environments
• Susceptible to rapid melting
• Internal flow of ice sheets increased
• Ice fluxed to margins along ice streams
• Effectively thinning the ice sheet
• Lowering volume but not aerial extent of ice

24
Changes in Landscapes

• Morphological changes accompanied deglaciations
• Proglacial lakes
• Flooding
• When impounded water in proglacial lakes was
  suddenly released
• Rise in sea level
• Inundation of coastal regions
• Submerged land connections between continents
  exposed during LGM

25
Proglacial Lakes

• Proglacial lakes develop in bedrock depressions
  left by melting ice sheets
• Lake Agassiz, largest proglacial lake N. America
• 200,000 km2, 100 m deep (20,000 km3)
• Sudden release of large proglacial lakes caused
  massive floods

26
Increased Insolation Produced Monsoons

• Earths orbital configuration 10K years ago
• Summer insolation 8 higher than today
• Conducive to summer monsoon development
• Model simulations supported by geologic
  observations
• Lake levels higher in
• Arabia
• North Africa
• Southeastern Asia

27
Enhanced Upwelling in Arabian Sea

• Strong monsoon winds blowing across Somalia and
  eastern Arabia
• Enhanced coastal upwelling
• Altering the planktic foraminifera species

28
Climate Evidence

• Evidence for wet climate range from
• Large dry river valleys in deserts
• Fossil evidence includes
• Grass pollen in lake deposits
• Variety of water-loving animals (hippopotamuses,
  crocodiles, turtles, rhinoceroses, etc)

29
Timing

• 14C dates for lake deposits in N. Africa
• Match the 10K insolation maximum
• When corrected for greater 14C production

30
Intensity

• Summer insolation 8 higher but lakes 24 larger
  in volume
• Relationship not necessarily linear
• Mismatch between models and observations
• Required addition of vegetation-moisture feedback

31
Insolation Reduced Monsoons

• Decreased summer insolation expected to weakened
  summer monsoons
• Lake levels in N. Africa match well expected
  patterns
• Most lakes today much lower or dried out

32
Climate Change Over Last 10K Years

• Ice sheets melting (reduced influence)
• Atmospheric CO2 levels stable and high
• Summer insolation gradually decreasing
• Expect warmer and then cooler climate

33
Vegetation

• General gradual movement of warm-adapted biomes
  north
• Pollen records indicate spruce and oak moved
  north
• Mid-glacial produced no-analog vegetation
• Mixtures that do not exist today
• Different response of a particular type of plant
  to changing climate

34
Peak Deglacial Warmth

• With atmospheric CO2 levels steady and high
• Glacial ice largely melted
• Summer insolation and vegetation changes affected
  temperatures
• Insolation 5 higher warmed high latitudes
• Displacement of high-albedo tundra by low-albedo
  spruce caused positive feedback
• Greater warming

35
Cooling Followed Deglacial Warming

• Ample evidence for gradual cooling
• Summer insolation dropped over last 10K years
• Less frequent melting of ice caps
• More frequent sea ice off Greenland indicated by
  drop in diatoms
• Advances in ice caps on Arctic islands
• Lower Atlantic SST
• Southward shift in the boundary between spruce
  and tundra

36
Future Climate

• Over the next 10K years precession maximize at
  low latitude
• Intensify summer monsoons
• Tilt should minimize at high N. latitudes
• Help promote further glaciations
• Pattern consistent with glaciations in next few
  thousand years
• Predictions complicated by millennial-scale
  oscillations and anthropogenic greenhouse gases