ESS1 - Changing Climate –

2. Large-scale glaciations – Chapter 14 cont’d

http://www.jamestown-ri.info/glaciation.htm

http://www.geo.cornell.edu/mastodon/Photos/People/peopleframe.htm

Outline

Announcements – review materials on line

Evidence of conditions in the last glaciation

Successive Pleistocene Glacial/Interglacial Ages – orbital controls

Climate feedbacks amplifying orbital controls

Earth is presently in a cool period – dark bands show times when large ice sheets were present on continents.

However, during much of Earth’s history, there is no evidence for ice ages – warmer oceans, warmer conditions.

We will come back to this later.

Possible causes? Faint young sun? Changes in greenhouse gases? Changes in the positions of the continents?

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Snowball Earth?

Evidence of Glaciations in the Past Last 20 million years

Striations (grooves) in bedrock in an area that now has no iceIce thickness likely was up to a mile thick in areas like this.

Geologic evidence for Glaciation

Till deposits – poorly sorted, angular rocks that were moved by glaciers

Moraine left by retreating glacier – Mt. Cook, New ZealandRadiocarbon dating of plants that were ‘ploughed under’ gives the timing

Glacial erratic boulder – Yellowstone National Park – Boulder had to have been dropped by a melting glacier

Erratic Boulders found in ocean sediments are evidence of ice rafted debris

Ice cores from Greenland and Antarctica contain ice that is as much as 400,000- years old

Vostok

What is preserved in ice?

• Gases – trapped in bubbles in ice – past atmospheric composition

• Oxygen isotope composition – proxy for temperature (see next pages)

• Dust – dustier atmosphere

Oxygen Isotopes

Oxygen has two naturally occurring isotopes: 99.762% 16O 8 protons, 8 neutrons 0.200%   18O  8 protons, 10 neutrons When water is evaporated, the water containing the lighter

isotope (H216O) evaporates faster than water containing

the heavy isotope  (H218O).  Building a large ice sheet on

continents requires evaporation of a lot of water from the ocean, which then accumulates as ice on land.  The ice will be enriched in  H2

16O, while the water left behind will be enriched in  H2

18O.  The   18O/16O ratio of water in glacial times must therefore have been higher than in present times. Luckily it gets preserved in the shells of organisms of in the ice on continents, so we can measure it

Oxygen isotopes are used in two ways:

(1) A measure of ice volume(2) A measure of the temperature at which precipitation occurs (or the

temperature shells are made in seawater)If you know (1), you can infer (2)

Carbon dioxide concentrations have varied in concert with temperatureAmplifying feedback?

glaciationglaciationglaciationglaciation

Feedbacks that might be operating to amplify the orbital signals

Surface Temperature

Carbon dioxide in atmosphere

Amount of surface covered by ice

Positive feedback

Increased reflectance

The Last Glaciation

Last glaciation: 70,000 - 10,000 years ago

This last glaciation, which peaked about 20,000 years ago, was one in a series of glaciations that took place during the Pleistocene Epoch (2 million to 10,000 years ago)

Figure 14., Skinner et al., 1999,

Feedbacks that might be operating to amplify the orbital signals

Surface Temperature

Carbon dioxide in atmosphere

Amount of surface covered by ice

Positive feedback

Observation –But mechanism?

Greenhouse gas

The many glaciations + Miocene

What was the climate like in the Last Glacial Maximim (about 20,000 years ago)

Continental Ice Sheets expandedSea Level – 135m lower Ocean 18O higher, Salinity higher, circulation changed (no NADW formation)

TemperatureLand cooler (average 5 degrees C, of the order of what is predicted for warming from CO2) Pollen, plant distribution Dissolved gases in groundwaterSurface oceans mostly only 2 degrees cooler (Celcius) but some areas cooled more Oxygen isotopes in plankton shellsDifferent distribution of warm and cold dwelling species of plankton

Precipitation – rainforests were drier grasslandsAtmospheric Composition Lower in greenhouse gases – colder air holds less water; lower CO2 and methane

http://www.elmhurst.edu/~chm/vchembook/globalwarmA.html

The Last Glaciation: Temperature Changes

Evidence comes from

•Relationship between modern snowline of modern glacial extent to ice age glacial extent (assume same snowline relationship and infer temperature)

•Fossil pollen: vegetation implies a certain climate

•Oxygen isotopes

•Existence of permafrost-controlled landforms

•Fossils in deep sea sediments

Figure 14.1, Skinner et al., 1999

The Last Glaciation: Temperature Changes

Average SST differences only about 2 °C

Regional differences, e.g. Northern Oceans, quite large

Figure C14.1, Skinner et al., 1999

Figure 14.1, Skinner et al., 1999

The Last Glaciation: Ice Age Vegetation

Much information on ice-age vegetation comes from fossil pollen

Deposited in lakes, ponds, bogs, and protected from decomposing in the wet environment

Core samples from sediments reveal the assemblage of local vegetation

If modern analogs can be identified, then these climatological conditions existed in that region at that time

Figure 14.12, Skinner et al., 1999

Figure 14.1, Skinner et al., 1999

The Last Glaciation: Ice Age Vegetation

Figures 14.9, 14.10, Skinner et al., 1999

Figure 14.1, Skinner et al., 1999

The Last Glaciation: Ice Age Vegetation

Changing distribution of Spruce, Hemlock and Elm based on fossil pollen data

18,000 years ago to present

Darker color indicates greater relative abundance

Gives idea of how fast vegetation zones can move

Figure 14.14, Skinner et al., 1999

Paige Museum web site - http://www.tarpits.org/

….” the Pleistocene climate of Los Angeles was not drastically different from the present day. In fact, the best available evidence points to a climate similar to coastal Monterey or San Francisco, California. This climate is slightly more humid and cooler than one typically associated with the present day environment of the Los Angeles area….”

The Last Glaciation: Dusty AtmosphereMid-latitudes were windier and duster then today

Evidence from dust layers in Greenland ice cores:

•dust percentage increases significantly in the part of cores corresponding to last glaciation

•Source was central Asian deserts and valleys of meltwater streams south of N. American ice sheet

Other evidence:

•Glacial-age loess (fine, wind-borne dust produced when glacial ice pulverizes underlying bedrock into powder) deposits east (downwind) of former meltwater streams

•Thick loess deposits of central China lie east of the central Asian deserts

These deposits are separated by soils formed during interglacial time periods, when climate was more similar to today

http://whyfiles.org/shorties/100dust_china/

Chinese Loess Deposits

Loess – large deposits of dust formed during the last glaciation

The Last Glaciation: Sea and Lake Level Changes

Previously discussed sea level changes

Falling sea levels caused rivers to deepen their valleys and transport river mouth deposits to new mouths

Enlargement of existing lakes, creation of new ones

•e.g. Great Salt Lake was occupied by gigantic glacial Lake Bonneville

•300 m deeper than Great Salt Lake, covered 1/3 of present-day Utah

•Horizontal benches or terraces (Figure 14.11 in Skinner et al) mark ancient shorelines of this vast, Pleistocene lake

Glacial Lake Bonneville

http://www.ugs.state.ut.us/online/PI-39/pi39pg01.htm

Figure 14.1, Skinner et al., 1999

Successive Pleistocene Glacial and Interglacial Periods

Seafloor sediment core biota (warm vs cold); oxygen isotope ratios; calcium carbonate ratios all point to successive glacial-interglacial periods

Over last 800,000 years, duration is 100,000 years

Prior to this, glacial duration is 40,000 years

Changes in duration and magnitude of peaks points to fundamental change in Earth system

Figure 14.16, Skinner et al., 1999

Milankovitch Cycles

Small changes in Earth’s orbit around Sun (eccentricity), wobble (precision), and tilt can have large impact on climate.

Figure 14.1, Skinner et al., 1999

Glaciation Prior to Pleistocene

Oxygen isotope data show that ocean has grown colder over last 50 million years. There was no ice cap in Antarctica or Greenland before about 35 million years ago

In parallel, glaciers began moving out from Antarctic highlands and reached the sea; by 12-10 million years ago, Antarctic ice sheet had formed

From then on, mountain glaciers begin forming, but large ice sheets don’t appear in N. Hem. Until about 2.5 million years ago

The Mesozoic (248-65 million years ago) was a long interval of relatively mild land and sea temperatures, and large-scale glaciation apparently did not occur

Figure 14.17, Skinner et al., 1999

Figure 14.1, Skinner et al., 1999

The Warm Middle Cretaceous

Around 100 million years ago, climate was one of the warmest in Earth history

• Coral reefs grew much closer to the poles

• Warm water fauna were widespread

• Peat deposits formed at high latitudes

• Dinosaurs ranged north of the Artic Circle

• Sea level was 100 - 200 m higher than today, implying an absence of ice sheets

• Global average temperatures are estimated to be 6-14°C warmer

• Equator to pole temperature difference was 17-26 °C compared to 41 °C today

Figure 14.18, Skinner et al., 1999

Figure 14.1, Skinner et al., 1999

The Warm Middle CretaceousPaleoclimate computer model simulations suggest that a combination of changes in geography, ocean circulation and atmospheric composition (relative to the present) explain the differences

Arrangement of continents affects albedo (reflected solar energy) and ocean circulation, can account for about 5 °C of the warming

How about the rest of the warming? Could differences in ocean heat transport explain the high latitude warming?

Models suggest that the answer lies in CO2

• Model simulations show that an 8-fold CO2 increase can account for the warming

• geochemical reconstructions point to a ten-fold increase

Figure 14.19, Skinner et al., 1999