pleistocene ice ages

Pleistocene Ice AgesPleistocene Ice Ages

A Glacial World 21,000 yrs agoA Glacial World 21,000 yrs ago

Insolation Levels Were the Same Insolation Levels Were the Same As TodayAs Today

• This seemingly contradiction can be explained– Slow response to low summer insolation– When the ice reached maximum size, summer

insolation had risen to close to today’s• Would soon melt the ice

• Insolation can’t have been a major factor in causing the glacial climate

• So, what are the factors?– The Larger ice sheets themselves provided negative

feedback– Lower greenhouse gases

Ice Ages in North AmericaIce Ages in North AmericaContinental Glaciers advanced & Receded at least 4 times during past 2 million years

During glacial advances, sea level drops exposing continental shelf

Maximum advance of last glacier 18,000 yrs ago

Sea level rises 300 ft by 8,000 yrs, resulting in present coastline

Ice Sheets in EuropeIce Sheets in Europe

• Scandinavian Ice Sheet– Northern Europe

• Barents ice Sheet– Northern Eurasian

continental shelf

Thickness of the North American Thickness of the North American Ice SheetIce Sheet

• Two Estimates– Thicker ice

• CLIMAP (Climate Mapping and Prediction) Project – an interdisciplinary effort to reconstruct past climate begun in the 1970’s

– Thinner ice from recent reconstructions

Estimating the Ice Sheet’s Estimating the Ice Sheet’s ThicknessThickness

• The weight of the ice sheet causes the underlying bedrock to subside.– 30% of its thickness– The height of the ice

sheet above the surrounding landscape is usually 70% of its total thickness.

Crustal ReboundCrustal Rebound

• As a result of a slow viscous response– Bedrock rebound occurs

long after the ice melts.

• As the land slowly rises out of the sea, a trail of fossil beach shorelines are formed.– Rate of rebound can be

estimated• 14C fossil shells dated• Measure elevations of old

shorelines above sea level.

Changes in Sea LevelChanges in Sea Level

• The thicker the ice, the greater the drop in sea level.

• CLIMAP reconstructions require at least a 129 m drop– Recent measurements from submerged coral

reefs indicate a drop of 110 m• Not enough for the thickness of the ice in the

CLIMAP estimate.

Glacial Dirt and WindsGlacial Dirt and Winds

• Ice sheets and mountain glaciers eroded huge amounts of debris and carried it to their margins

• Silt-sized loess was deposited downwind.– Mostly from the west-northwest to the east-southeast in both N. America and Europe

• Clay-sized loess– Glacial age layers in Greenland ice sheet contain 10X as much fine dust as interglacial

layers– Chemical analysis indicates that the main source was Asia, not nearby N. America.

Dust Transport at Lower LatitudesDust Transport at Lower Latitudes

• Desert margins in Arabia, the southern Sahara, and Australia– Produced larger amounts of dust during the last glacial maximum

• Probably resulted from – Greater production of debris at the source– A more turbulent atmosphere that carried dust farther and higher than

today.

Sand DunesSand Dunes

• Moving Sand Dunes Today– Africa, Arabia, and

Australia

• Last Glacial Maximum– Drier climates and

stronger winds• More extensive sand

dunes

Summary of Landscapes Features Resulting from Continental Glaciation

Unsorted(till)

Summary of Landscapes Features Resulting from Continental Glaciation

Stratified(sorted and layered)

Evidence of Glaciation in the U.S.

End Moraines in the Contiguous United States and Canada

FjiordsCoastal inlets formed by drowning of glacially carved valleys

by rising sea level

The Great LakesThe Great Lakes

Origin as Meltwater LakesOrigin as Meltwater Lakes

NY State’s Finger Lakes

Kettle Lakes

Ice Lobes in New York State

Retreat of the North American Ice SheetsRetreat of the North American Ice Sheets

• Radiocarbon dating of organic remains in moraines• Retreat began about 14,000 14C years ago• Ice disappeared completely shortly after 6000 years ago

Numbers indicate 14C-datedIce limits in 1000’s of years

Causes of Climate Change During DeglaciationCauses of Climate Change During Deglaciation

• Deglaciation 17,000 to 6,000 years ago• Rising Summer insolation• Increased CO2 Concentrations• Shrinking ice sheets had less ability to influence climate

Deglacial Rise in Sea LevelDeglacial Rise in Sea Level

• Submerged corals off Barbados– Show the deglacial history of the eustatic rise in sea level– Caused by meltwater from the ice sheets into the ocean

14C ages are younger

Which is correct?

• Rising sea level is indicated by both dating methods

• The 14C data support the Milankovitch theory - The middle of the deglaciation occurred near the insolation maximum 10,000 yrs ago.

Deglacial Deglacial 1414C Dates Are Too YoungC Dates Are Too Young

• Tree ring analysis– Dated both by:

• 14C method• Counting backward year

from modern rings to older rings

– Ages were older than the 14C analysis by 1000 years

– Same amount by which the Th/U coral ages were offset from the 14C coral ages

– 14C error is due to varying rates of 14C atom production in the past

Radiocarbon DatingRadiocarbon Dating

• Rates of cosmic bombardment were higher in the past– Earth’s magnetic field was

weaker• Other elements that are the

product of cosmic bombardment were more abundant

• Paleomagnetic data from rocks supports this

• More 14C atoms were produced– Surviving excess of 14C atoms

makes it look like less has decayed

– Seems younger

Meltwater PulsesMeltwater Pulses

• Influx of meltwater– Low δ18O values from

the Norwegian Sea and Gulf of Mexico

– Seen in CaCO3 shells of ocean planktonic foraminifera

– 17,000 to 14,500 years ago

• Rapid rise in sea level

Routes of Meltwater FlowRoutes of Meltwater Flow

• Direction of drainage changed– Southward to the Gulf

of Mexico early in the deglaciation

– Then, briefly east to the Atlantic Ocean

– Finally, north into Hudson Bay and the Arctic Ocean late in the deglaciation

Interruption on the Deglacial Warming,Interruption on the Deglacial Warming,The Younger DryasThe Younger Dryas

• A pause in the melting accompanied by a brief climatic oscillation

• First evidence– Pollen in Europe

• As the ice sheet melted back to the north scattered trees reoccupied northern Europe

• Near 15,000 years ago– Cold-tolerant herb tundra reinvaded the landscape, displacing

trees toward southern Europe– An Arctic plant called Dryas arrived

– Cause (?)• Possible change in meltwater flow from the North American

ice sheet– Delivered a pulse of low salinity water that rearranged the basic

circulation of the North Atlantic

The Younger Dryas Cold ReversalThe Younger Dryas Cold Reversal

• Readvance of polar water in the North Atlantic (A)

• Reversal toward Arctic vegetation in Europe (B)

• Return to cooler temperatures– Indicated by fossil

insect populations in Britain

A Recently Published TheoryA Recently Published Theory• Widespread extinctions

of herbivorous megafauna in North & South America– Between 13,000 and

12,000 years ago– Methane loss of about

9.6 trillion grams per year

– Explains between 12.5 and 100 percent of the methane decrease

– May have triggered Earth’s global cooling

Deglacial Lakes, Floods, Deglacial Lakes, Floods, and Sea Level Riseand Sea Level Rise

Proglacial LakesProglacial Lakes• As ice sheets melt back

– Land directly in front remains depressed for some time

– Meltwater pours into these depressions, forming proglacial lakes.

• Large volumes of meltwater each summer often

– Cut new channels– Overflow into other

lakes and into rivers, carrying water away from the ice sheets

Glacial Lake AgassizGlacial Lake Agassiz

• In western Canada • Existed for several thousand

years• Flooded a total of more than

500,000 km2

• Smaller areas were flooded as the glacier retreated, eventually resulting in Lake Agassiz– Covered more than 200,000 km2

– Depth of 100 meters or more

Channeled ScablandsChanneled Scablands

• Massive erosion in east-central Washington State• Ancient lava flows eroded

– Into deep canyons– Dry channels abruptly plunge over steep cliffs into larger channels– Suggests violent action of water on an immense scale– Huge boulders and displaced gravel and sand lie in channels

A Flood of Immense ProportionsA Flood of Immense Proportions

• In a few days– Carried a volume of water equivalent to all of Earth’s

rivers today– Flowed wildly over the landscape

• Gouged and eroded the lower terrain• Left the higher areas untouched• Eventually flowed down the Columbia River and into the

Pacific Ocean

– Source: Likely proglacial Lake Missoula– Cause

• Possibly a volcano beneath the ice margin caused rapid melting and the sudden release of the enormous torrent of water

Scablands TopographyScablands Topography

Deglacial Flooding of CoastlinesDeglacial Flooding of Coastlines

• Many regions were exposed during low sea level during the glacial maximum

• Submerged during deglaciation (returned 44 million km3 of meltwater to the oceans)

Retreat of the Wisconsinan Glacier

• Stages D, E, and F occurred 11,000 to 12,000 years ago• The final stage was approximately 11,000 years ago

Long Island Glaciation

Continental Glaciers Retreat

forming Long Island of Today

• Lakes form as ice sheet melts

• Sea Level Rises

• LI Sound fills with water from Ocean

• LI takes modern form

(from Newsday “Long Island Our Story”

Major Landforms of Long Island

Long Island Moraines

Topography of Long IslandTopography of Long Island

North Shore: Irregular Coast & Hilly

South Shore: Straight Coast & FlatDigital Elevation Model: 2X Vertical Exaggeration (Bennington, 2003)

Data from NYSDEC & Cornell University

Glacial Features of LIGlacial Features of LI

Lake Ronkonkoma

Ronkonkoma Moraine

Harbor Hill Moraine

Kames

Outwash Plain

Glacial TillGlacial Till

• Unsorted Sediments were deposited directly by ice.

• Round shape of many boulders indicates that they were also shaped by running water

Eroding moraine at Montauk

Boulders at Orient

Glacial Features of Smithtown & Central LIGlacial Features of Smithtown & Central LI

Outwash Plain

Ronkonkoma Moraine

Harbor Hill Moraine

Lake Ronkonkoma

Harbor Hill Moraine

Till of Harbor Hill Moraine overlying outwash deposits

(RTE 25 A, Setauket)

Closeup view of Till of Harbor Hill Moraine consisting of unsorted silt, sand, pebbles, and cobbles

Kame Deltas indicate large pro-glacial lakes as continental glacier melted

Kame DeltasOutwash Plain

Harbor Hill Moraine

Note: Kame Deltas overlie Ronkonkoma Moraine

What does this indicate about the relative age of the Deltas?

Ronkonkoma Moraine

Glacial Features of Huntington Area

Outwash Plain

Outwash Channels

Harbor Hills Moraine

Delta Deposits

Delta Deposits at Caumsett

• The sediments in the cliffs formed when streams of water carrying sand and gravel gushed from the glacier and entered the lake. The faster the water the larger size sand grains, pebbles or boulders it can carry. Upon reaching the still water, the streams slowed and dropped their sediments

forming a delta.

Weathering breaks rocks down & Erosion wears away land & transports sediments

Frost Action split this rock

Erosion of bluffs at Caumsett, forming fan

Rivers Erode the Land

Nissequogue River

Carman’s River

Ronkonkoma Moraine

Smithtown Harbor

Carman’s River

Carman’s River cuts through the Ronkonkoma Moraine.