Transcript Week_7_LGM_08

Last Glacial Maximum (~20K yrs ago) and
afterwards
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What was climate like during LGM?
What happened to end LGM?
How has climate varied since LGM?
What were the likely mechanisms of climate
change?
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Repeating Series of Glacial Conditions
Punctuated with Interglacial Events
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A Glacial Threshold on Earth
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Solar Insolation
• Solar insolation
during the LGM
was about the
same as today.
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Sea Level
• Sea Level lower than today by ~125m
– based on depth of submerged corals that lived
~20K yrs (14C or U/Th dated)
• Large area of continental shelves (~7% of earth)
are exposed
• Ocean is more saline (by ~1 ppt)
• d18O of seawater enriched (higher) by ~1.1 ‰
– as a result of Ice Sheet growth
• Continental Ice Sheets were about about double
the size of today.
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Ice Sheet Extent
• Laurentide, Cordilleran and Scandinavian Ice Sheets
• Areal extent at LGM reconstructed by 14C dated end
moraines (25% of land covered at LGM vs 10% today).
• Thickness of ice sheets is harder to estimate than area.
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Ice Sheet Volume
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Sea Surface Temperature (today)
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Ocean Temperatures at LGM
• CLIMAP (1970s)- use temperature
sensitivity of foram distribution in
today’s surface ocean, coupled with
paleodistribution of forams at LGM
measured in sediment cores to estimate
sea surface temperature at LGM.
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Sea Surface Temperature Change at LGM
(CLIMAP)
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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 ~2-4ºC.
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Increased Global Aridity at LGM
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10
5
2
1
-380
260
-400
240
-420
220
-440
dD (‰)
CO 2 (ppm)
Taylor Dome
nss-Ca 2+ flux
(ng/cm2/yr)
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200
180
10000
12000
14000
16000
Age (yr BP)
18000
20000
• Ice Core Record of Dust
- increased dust at LGM due either to increased strength of winds or dust source
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(aridity)
Aridity
• Soil cores indicate
more extensive desert
areas (sand dunes)
during LGM
• Also increased
deposition of wind
blown dust (loess)
during LGM
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Aridity Exceptions
• Southwest US
was wetter
during LGM as
indicated by
paleo-lake shore
elevations.
• Jet Stream
position? (El
Nino analog?)
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Pollen Record in Lakes
• Pollen records from lake
cores (14C age dated)
indicate larger extent of
tundra and arctic steppe
type vegetation during
LGM.
• Can use to qualitatively
estimate temperature and
precipitation changes.
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Vegetation Changes
Use pollen records from
several lakes to reconstruct
regional vegetation
distribution during LGM.
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Atmospheric Gases during LGM
CO2 was ~200 ppm (vs 280 ppm at interglacials)
CH4 was ~350 ppb (vs 700 ppb at interglacials)
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Deep Ocean Circulation: d13C as a Proxy
d13C and phosphate depend on respiration rates and the age of
deep water. Older deep water has lower d13C and higher 18
PO4. Today deep water in Pacific is older than in Atlantic.
d13C of Ocean during LGM
• Measure the d13C of
CaCO3 foram shells at
several different locations
buried during the LGM.
• Generally, the d13C of the
deep Atlantic was lower
than today which implies
water is older and/or
respiration rates were
higher.
• Deep ocean circulation
rates were likely slower at
LGM.
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Summary: Climate Conditions during LGM
• Insolation rates about the same as today.
• Colder (~ -4 º C globally and ~ -10 ºC near the
poles 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.
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Climate Change after the LGM
• What triggers the change?
• Was it a smooth transition from glacial to
interglacial conditions?
• What mechanisms could have caused episodes of
rapid climate change?
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Solar Insolation Changes
• Summer insolation starts
to increase at ~20K and
reaches a maximum at
~10K.
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A Glacial Threshold on Earth
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Rise in Temperature and Atmospheric Gases
• Temperature, CO2 and CH4 start increasing ~18K yrs.
• Implies changes in radiation budget (Temp), ocean circulation
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/biology/chemistry (CO2) and precipitation (CH4).
Ice Sheet Retreat
• Retreat begins
~16K and ice
sheet gone by
~6K.
• (Real age = 14C
age plus ~ 1700
yrs.)
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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.)
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Transition from Glacial to Interglacial
Conditions
• Rapid Bolling-Allerod
warming event at ~14.5
Kyrs.
• Younger-Dryas is a
period of cooling at ~12
Kyrs that lasted for ~
1000 yrs.
• Transition from the
LGM climate to present
interglacial climate was
not smooth.
Greenland Ice Core
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Younger-Dryas
Event:
atmospheric,
marine and
continental
proxies
Y-D climate signal is
.strongest in N. Atlantic
region.
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Rapid Temperature Change during the Y-D
Sawtooth Pattern
of Y-D
At end of Y-D, temperature in Greenland increased by 7ºC in 50 29
yrs.
Role of Proglacial
Lakes in Climate
Change
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Possible Pathways of Meltwater Flow
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Appearance of Meltwater Pulses
From sea level record
From d18O-CaCO3 record
Why do meltwater pulses show up in d18O-CaCO3 record?
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Meltwater Pulses as a Climate Change Trigger
• Pulse of freshwater discharge into the N.
Atlantic would reduce the formation rate of
deep water (N. Atlantic Deep Water) by
reducing salinity (density) of surface water.
• This would reduce the ocean’s transport rate
of heat via Gulf Stream to N. Atlantic region
and cause cooling in the region.
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Deep Water Formation: Present vs LGM
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Possible Impact of Reduced NADW
Formation Rates on Air Temperatures
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Millenial Scale
Temperature
Oscillations
During last glacial period
(20-100 K yrs BP) there
were lots of large and fast
temperature swings
recorded in the Greenland
ice cores and deep sea
sediments from N. Atlantic.
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Events
Heinrich Events: Ice RaftedHeinrich
Debris
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Dansgaard-Oeschger and Heinrich Events recorded in
the N. Atlantic Region
Fairly consistent ~1500 yr period between warm periods
(D-O events). Several of the coldest periods are associated
with Heinrich events (e.g., Younger-Dryas). Sawtooth
pattern of fast warming and slow cooling.
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Possible Mechanism: Salt Oscillator
Hypothesis: Changes in the
salinity of the N. Atlantic,
resulting from ice melting or
ice formation, determines the
strength of NADW formation
and, as a result, the rate of
heat transport to the N.
Atlantic region.
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Salt Oscillator: Ocean circulation has two modes
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Salt Oscillator: Explaining the time
sequence of D-O and Heinrich events
Hypothesis: The
sawtooth pattern of
temperature change is
caused by a slow
decrease in rate of
NADW formation,
until it eventually
stops, which is then
followed by rapid
return of NADW
formation after a
critical value of
salinity is reached.
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Model Simulations
Does S. Hemisphere warm when the N. Hemisphere cools?
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Antarctic warmed during Younger Dryas
• Antarctica temperature
increases during Y-D
cooling.
• Globally, atmospheric
CO2 levels increase.
• Globally, atmospheric
CH4 levels decrease.
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Interhemispheric Seesaw
• Model simulations of Heinrich events indicate that
reduced heat transport into the N. Atlantic yields less
heat loss from S. Hemisphere and thus warming.
• Models indicate that when NADW formation is
reduced, then Antarctic Bottom Water formation rate
increases, which in turn means higher ocean to
atmosphere heat transfer and warmer temperatures in
Antarctica.
• Temperature records during Y-D from Antarctic ice
cores indicate warming while Greenland cooled.
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Were D-O events and Younger-Dryas global?
Figure 1. Map showing locations where abrupt climate changes (i.e., Dansgaard-Oeschger
events) have been documented in records kept in marine sediments or polar ice (red and blue
dots). Yellow dots show those locations where the last of these events (i.e., Younger Dryas) is
recorded by major advances of mountain glaciers. While for most of the globe, these events are
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in phase, in parts of the Southern Ocean and of the Antarctic ice cap, they are clearly
antiphased.
Rapid Climate
change evidence
in Santa Barbara
Basin
• Warming events
associated with negative
d13C, which author
interprets is a result of
methane hydrate release.
• Did ocean circulation
rates change between
warming and cooling
events?
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Summary: Rapid Climate Change
• During the last 100K yrs there have been repeated oscillation
between warm (D-O) and cold (Heinrich) conditions, with
very fast temperature changes (7ºC in 50 yrs in Greenland).
• The most recent (strong) cold event occurred about 12K yrs
ago (Younger-Dryas) during the transition from LGM to
current interglacial period.
• Current hypothesis is that variations in deep water formation
rates in N. Atlantic driven by salinity, and thus poleward heat
transport, is a likely mechanism.
• These rapid climate change events are strongest in the N.
Atlantic, but some evidence that they occurred globally.
• Generally, there is an antiphasing of temperature fluctuations
between N. and S. Hemispheres.
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