chapter 13 climate during and since the last deglaciation david apostalon, ines cobeljic, zak...

Chapter 13

Climate During and Since the Last Deglaciation

David Apostalon, Ines Cobeljic, Zak Mohamoud, Jeremy Shaw, Merhawi Zerai

Chapter Overview• History & Causes of

Deglaciation– Milankovitch Theory– Climate records– Deglacial two-step– The Younger Dryas– Deglacial feedbacks

• Effects of Deglaciation– Glacial lakes & floods– Sea level rise– Tropical monsoons– Changes in vegetation– Seasonal temperature

variations• Current & Future Climate

– Insolation predictions– Another glaciation?– Agricultural humans

History of Deglaciation

• What is deglaciation?– 2.75 million years of Ice Age– Shifts between glacial and interglacial periods

• ABLATION: rate of ice loss


• Causes of ablation– Solar radiation– Conduction of heat by air

or rain

– Calving


• Milankovitch Theory– Proposed by Milutin Milanković– Summer insolation controls glaciation

• Snow & ice accumulate during winter• But...warm summers will melt ice• So...cool summers required to maintain ice

each year– Glaciation occurs during low summer

insolation at northern latitudes– Deglaciation occurs during high summer

insolation at northern latitudes


• What is insolation?– INSOLATION: incident incoming solar radiation (W/m²)

...or...Arriving solar radiation per unit area


• Insolation affects temperature


• Insolation levels change over time• Determined by cycles of Earth’s precession, tilt, and

eccentricity


• Summer insolation maximum– Changes in Earth’s

tilt and precession caused a summer insolatin maximum at northern latitudes ~10,000 years ago

– Insolation rises... deglaciation begins


• Shifting climate forces– Glaciers melt– Insolation increases– CO₂ levels increase

from 190 to 280 ppm

– CH₄ levels double• Balance of power

shifts– Melting of glaciers

accelerates


• Coral reefs: climate records of deglaciation– We can measure the rate of deglaciation by indirect means

using coral reef data– Coral reefs grow in shallow waters– As sea levels rise, corals die– We can date ancient corals to determine ancient sea levels– Sea level changes can be converted to ice volume

• 1 meter of sea level = 400,000 km³ of ice

Sea level rises, old coral dies


• Coral reefs: climate records of deglaciation– This data gives us a rate of deglaciation by measuring changes

in ancient sea levels using radiocarbon and thorium/uranium dating.

2,000-3,500 yeardiscrepancy...?


• Why the discrepancy?– ¹⁴C ages are younger than Th/U

ages– Th/U ages are more accurate

when compared to tree ring data

• ¹⁴C ages are too young!


• ¹⁴Carbon Dating– ¹⁴C production in atmosphere

varies• Cosmic rays convert ¹⁴N into

radioactive ¹⁴C – Weaker magnetic fields =

more ¹⁴C– Based on half-life (decay) of

¹⁴C • more ¹⁴C = less time has

elapsed• But....extra ¹⁴C causes

appearance of less elapsed time

THE DEGLACIAL TIME PERIOD• North American ice sheet began to retreat

15,000 14C years ago

• Reached a midpoint ~10,000 14C years ago

• Ended ~6,000 14C years ago.• Timeline is supported by Radiocarbon

dating of material found in, under, or atop moraines deposited by the ice

• Smaller Scandinavian ice sheet began retreating at the same time, but disappeared a few thousand years earlier


TIMING OF ICE SHEET MELTING

• N. American ice sheets began retreating 14,000 14C years ago and was gone by 6,000 14C years ago.

• To convert area covered by retreating ice to ice volume (thickness x area = volume). The thickness of the ice is debatable because it can be effected by the conditions in its basal layer.


THE DEGLACIAL RISE IN SEA LEVEL• Larger glaciers = more melt

water– Generally consistent with

Milankovitch theory• Rates of sea level rise changed

dramatically– Based on coral reef data

• Deglacial Two-Step– Rapid rise from 20K to 14K years

ago– Slow from 14-12K years– More rapid rise after 12K years


• The Deglacial Two-Step pattern points to more complex accelerations and decelerations in melting rates.

• Two major influxes of freshwater into oceans due to melting glaciers.

• The rates of ice melting were at least four to five times faster during the earlier and later intervals.


• A pulse of unusually negative d18O values early in deglaciation.• measured in planktic formanifera

• The flux in d18O is result of early melting of nearby Barents ice sheet, north of Scandinavia.

• A low- d18O pulse found in cores from the Gulf of Mexico indicates a short-term increase in meltwater down the Mississippi River from N. American ice sheet.

AN EARLY RAPID MELTING OF ICE SHEET!



Younger Dryas

• Climate records of Younger Dryas event– All three graphs have correlating time

The Younger Dryas• Northern Hemisphere

unexpectedly returned to near-glacial conditions.• Interruption in general deglacial

warming

• An Artic plant called Dryas reached Europe• Reversal toward Artic vegetation• Evidence comes from pollen

records


• Last Glacial Maximum• Polar water reached southward across the North Atlantic (45 N°)• southern margin defined by polar front, area of fast transition to more

temperate waters to south

• 15,000 years ago (early deglaciation)• polar front shifted northwest to point close to eastern Canada• warm water began to flow northward along European coast to moderate

climate• moderate climate allowed trees to advance northward from prior full-glacial

spot in far-southern Europe



• 13,000 years ago - 11,700 years ago (Younger Dryas period)• polar front suddenly shifts

back to the south almost reaches glacial position

• North Atlantic Ocean cools, and Artic vegetation (Dryas) return to northern Europe

• 11,700 years ago polar front quickly retreats north, and forest begin final push into north-central Europe

Why the Younger Dryas oscillation?

• Wally Broecker (geochemist)– lower N. Atlantic surface

density prevented formation of deep water

– criticized because the factor of global melting rate was slowing during Younger Dryas

• The cause of the Younger Dryas remains a mystery


Feedbacks

• Positive feedbacks accelerated loss of ice

• Between 17,000 and 14,000 years ago spikes in sea level rise caused by ice melting

• Increases in concentrations of greenhouse gases• Ice sheets melted and CO₂ and

methane levels increased at nearly the same time• Increases in these

greenhouse gases caused warming and more melting of ice


Proglacial Lakes

proglacial lakes develop in bedrock depressions left by melting ice sheets.

over time lakes move north behind the ice sheets,while the land farther south rebounds toward its undepressed elevation.

Effects of Deglaciation

Lake Agassiz

Largest pro-glacial lake innorth America.

covered more than 200,000km2, at depths of 100 m or

more.


A) Missoula flood deposits.

B) ripples in the land, too large to be seen on the ground.


flooding of coastlines

Large-scale change on the earths surface by deglacial rise of sea

level.


Stronger summer monsoons near 10,000 years ago due to earth's configuration.

summer insolation were 8% higher than today in N. Hemisphere

Kutzbach Orbital Monsoon

model simulations supported by geological observations. Lake levels higher in:

- Arabia-North Africa - Southeastern Asia

Increased insolation produced Monsoons


Timing 14C dates for lake deposits in N.

Africa

Match the 10K insolation maximum

When corrected for greater 14C production


• Upwelling in the Arabian Sea – Strong Monsoon winds blowing across Somalia and eastern Arabia– Enhanced Coastal upwelling– altering the planktic foraminifera Species

Upwelling in Arabian Sea happened 9,000 years ago


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


Shifts in Vegetation

Strong summer insolation led to a northward shift from glaciers.

Oaks (warm vegetation) transition from southeast US to Mid Atlantic State

Spruce (Cold Vegetation) transition from Central US to Northeastern Canada


Models versus Observed data, Spruce and Oak

• No Analog Vegetation – No record of such vegetation in modern times– Caused by difference change in temperature and environmental variables– Specific spruce tree can’t be found at Midwest US– From Individual pollen record


Vegetation-moisture feedback

High summer insolation peaked at about 9,000 years ago. Wetter soils and increased vegetation provided positive feedback, bringing more moisture farther into the continent of Africa.


insolation reduced monsoons

decreased summer insolation expected to weaken summer monsoons

Lake levels in N. Africa match well expected patterns.


Peak Warmth• with atmospheric CO2 levels steady and high,

glacial ice largely melted.

• summer insolation and vegetation changed– changes affected temperatures.

• insolation 5 percent higher warmed higher latitudes.

• displacement of high-albedo tundra by low-albedo spruce caused positive feedback.


Cooling Down

6,000 Years earth tilt and precession motion move Northern Hemisphere toward Aphelion position

5% decrease overall

Tundra move southward replacing forest


Evidence for cooling summer Insolation

• Ice cap record from ice cores shows summer melting before 5,000 years ago

• Greenland diatoms from sea-ice were less before 5,000 years ago

• Glacier margin reappear 3,500 despite glacial margin significantly melted before 3,500 years

• Greatly affect diatom species that’s sensitive to temperature (great change in Norway’s southwest coast)


Current and Future Climate

Current and Future Climate

Where will we be 10,000 years from now, climatically speaking? Let’s begin by looking exclusively at astronomical indications (via Milankovic Theory): The future according to axial tilt

Dominant effect on higher latitudesDiminishing tilt with time => reduced seasonality => reduced insolation at the higher latitudes

The future according to general precession

Dominant effect on low latitudes (and the global average)Northern summer solstice will take place at perihelion 10,000 yrs from now; implies greater insolation at the northern tropics and stronger monsoons

Current and Future ClimateThe current interglacial period: is global cooling long overdue? All records of the past indicate that glaciation should be well underway by nowFuture models indicate only further heating over the next 50,000 yrsAgricultural humans: the turning point?

Our video presentation of this last leg – Placing these projections in the context of changing axial tilt, precession, eccentricity, and so on! Please see:http://www.youtube.com/watch?v=oRdyNn1tB-E&list=PLmen0eQI4-Lof_GS8Amc9sysdRMrRYzps

http://www.youtube.com/watch?v=oRdyNn1tB-E&list=PLmen0eQI4-Lof_GS8Amc9sysdRMrRYzps

http://www.youtube.com/watch?v=oRdyNn1tB-E&list=PLmen0eQI4-Lof_GS8Amc9sysdRMrRYzps

chapter 13 climate during and since the last deglaciation david apostalon, ines cobeljic, zak...

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