Back into the Icehouse: The Last 55 Million Years Global Climate Change Since 55 Myr Ago

Evidence From Ice and Vegetation Southern Hemisphere –No evidence for ice on Antarctica until 25 Mya. Ice rafted debris in continental margin coastal sediments Size of ice has increased towards the present –Large growth »13 Myr ago »7 7 Myr ago –Lower Middle Latitudes Earliest evidence in Andes –Between 7 and 4 Myr ago

Antarctica Today An ice sheet as much as 4 km thick covers more than 97% of the continent Around the margins mountains protrude through the thinner ice

Fossil Evidence Nothofagus –Type of beech tree –Found in Antarctica prior to 40 Myr ago –Disappeared with polar climate A modern beech forest at the southern tip of South America

Lichens The only vegetation found in Antarctic today –Summer meltwater ponds in coastal valleys

Ice in the Northern Hemisphere First developed on Greenland –Between 7 and 3 Myr ago 1 st glacial evidence in Alaska –High coastal mountains –About 5 Myr ago

North American and Eurasian Ice Sheets Appeared 2.7 Myr ago Formed and melted in repeated cycles –Maximum size increased after 0.9 Myr ago –Even though ice sheets developed in N. America 30 million years later than in the southern hemisphere Part of the same overall cooling trend

North Polar Regions Canadian Arctic 60 Myr ago (at 80 o N) –Palm-like vegetation –Ancestors of modern alligators

North Polar Regions Cold conditions developed –Conifer forests of spruce and larch by 20 Myr ago

Modern Tundra Has developed in only the last few million years

Tundra Scrubby, grass-like or shrub-like vegetation Grows on permafrost –Thawed layers lying above deeper frozen ground

Permafrost Siberia Alaska

Solifluction During the warm season the surface melts and slides downslope over the frozen layer –Creates a hummocky appears

Shapes of Tree Leaves Can be used to reconstruct climate –Smooth-edged leaves are found in tropics –Jagged-edged leaves grow in colder climates Cause is unknown Leaf-margin evidence in western N. America shows ongoing cooling over 55 Myr

Cooling in Western N. America Inferred from Leaf-Margin Evidence Cooling trend in middle latitudes during the last 55 Myr. Punctuated by small short-lived warmings

Oxygen Isotope Data

Periodic Table of the Elements

Recall that... Atomic Number –Number of protons in the nucleus of an atom –All isotopes of an element have the same atomic number

Atomic Weight Also called atomic mass. Given in atomic mass units (amu) –Standard Mass is determined using a neutral atom of carbon-12 –It is set at exactly 12 mass units (12 u) –1 atomic mass unit (amu) = 1/12 of this mass which is 1.66 X g. Also considers the fractional abundance of each isotope. Example using chlorine: –Of the two isotopes of Cl, approximately: 75% (75.53%) are the lighter isotope chlorine-35 and 25% (24.47%) are chlorine-37 –Finding the atomic mass ( amu x.7553) + ( amu x.2447) = amu

Two Climatically Important Isotopes of Oxygen O O Usually written as O O 16 18

Isotopes of Oxygen Provide information with regard to temperature –Oxygen trapped in ice indicates temperature at the time the oxygen was trapped –Oxygen trapped in shells is an indicator of water temperature

Ocean Water and Foraminifera Foraminifera are marine Protists that secret shells composed of calcite (CaCO 3 ). Both isotopes of oxygen exist in seawater although 18 O accounts for only about 0.2%. –Both isotopes of oxygen are found in the shells of forams. –The 18 O/ 16 O ratio in the shells provides information as to the seawater temperature at the time the forams lived.

Two Kinds of Foraminifera Live in Climatically Important Parts of the World’s Oceans

Planktonic Foraminifera Live in the upper 100 m of the ocean Shells contain oxygen taken from waters near the surface

Benthic Foraminifera Live on the seafloor and within upper layers of sediment Shells contain oxygen from deep water

Symbols Used... Parts per thousand is abbreviated as: Infinitesimally small change is abbreviated using the lower case Greek letter delta. /oo o δ

18 O/ 16 0 Measurements Individual measurements of 18 O/ 16 0 ratios –Reported as departures in parts per thousand from a laboratory standard. –Large amounts of 18 O compared to 16 O 18 O – enriched (positive δ 18 O values) –Or 16 O – depleted –Small amounts of 18 O compared to 16 O More negative δ 18 O values Referred to as 18 O – depleted –Or 16 O – enriched

Water Temperature and δ 18 O Values in Foraminifera Shells increases decreasesAs temperature increases the δ 18 O decreases –Each 4.2 o increase, δ 18 O decreases 1 / oo Modern Sub-Tropical Oceans (21 o C) –Planktonic Foraminifer have δ 18 O values of -1 / oo Benthic Foraminifer in cold water (2 o C) have δ 18 O value of / oo o o o

Oxygen Isotopes and the Hydrologic Cycle Lighter 16 O evaporates more easily from the ocean. Heavier 18 O is more easily removed from the atmosphere by precipitation –Along coastlines it quickly flows (runoff) back into the ocean 18 O +δ 18 O -δ 18 O

Isotope Fractionation Process by which water vapor in the atmosphere becomes progressively enriched in 16 O toward the higher latitudes Each cycle of evaporation and condensation: - Decreases the δ 18 O value of the water vapor by 10 o / oo in relation to the ocean water left behind

On a Growing Glacier Fractionation results in lighter 16 O being locked into glacial ice. Heavier 18 O builds up in seawater. –The δ 18 O is more positive than if no ice were present.

Evidence from Mg/Ca Ratio of magnesium to calcium in foraminifera shells Process of Mg replacing Ca –Depends on water temperature –Ratio increases 8 to 10% for each 1 o C increase in temperature –Linear relationship Mg/Ca trend is similar to δ 18 O “signal”

What if the ice sheets on Antarctica and Greenland melted over the next 10,000 years? And, how would this be recorded in the shells of planktonic and benthic foraminifera?

Melting of all Modern-Day Ice The ocean’s average δ 18 O value would shift from its present 0 o / oo value to about -1 o / oo –The result of lighter 16 O from melting ice This change would be recorded in the shells of benthic foraminifera everywhere in the world ocean

Possible Misinterpretation Recall that each 4.2 o C increase, δ 18 O decreases 1 o / oo. If in 10,000 years a future climate scientist measured foraminifera shells deposited during this interval and didn’t know about the deglaciation... –The -1 o / oo shift would be interpreted as a 4.2 o C warming of the entire world ocean and not as a change in ice volume

Changes in Ocean Temperature and in the Amount of Water in Ice Sheets Must be Combined

The Equation Used by Climate Scientists Considers... δ 18 O measured in foraminifera shells δ 18 O mean value of ocean water at the time the shells were formed The equation tells us that measured changes in the mean δ 18 O foraminifera are the result of: –Changes in the temperature of the water in which the shell formed and –Changes in the mean δ 18 O of the oceans

So, what is the equation? Don’t worry about it. We’re not going to get into the math. But, if you must find it look on page 101 of your text!

δ 18 O Trends Over the Last 55 Myr Shows global cooling δ 18 O values increase towards the present- day.

δ 18 O Trends Over the Last 55 Myr This trend could be a result of: –Cooling of the deep ocean –Growth of ice sheets on land –A combination of both factors

δ 18 O Trends Over the Last 55 Myr The volume of ice that did exist was negligible. Cooling of deep water must have been the main cause. Between 55 and 50 Myr δ 18 O values increased +1.5 o / oo resulting in cooling of over 6 o C (1.5 o / oo x 4.2 o C/ o / oo )

δ 18 O Trends Over the Last 55 Myr 35 Myr Some ice had appeared on Earth The volume of ice is unknown. The composition of the ice is also unknown.

δ 18 O Trends Over the Last 55 Myr Present Increase due to a combination of: Ice sheet growth Deep water cooling

Cooling of the Deep Ocean Between 40 Myr ago and today –Deep ocean δ 18 O values increased o / oo About 1 o / oo was due to δ 18 O deficient ice sheets Additional cooling (another 7 o C) of the deep ocean accounts for the residual 1.74 o / oo. (1.74 o / oo x 4.2 o C/ o / oo ) Total deep ocean cooling has been about 14 o C –When changes in temperature or ice volume occurred over the last 35 Myr can’t be determined –Both probably affected δ 18 O values simultaneously

More Temperate Polar Climates 55 Myr Ago Deep ocean temperatures today average 2 o C. The deep ocean has cooled by at least 14 o C in the last 55 Myr –55 Myr ago the deep ocean temperature must have been 16 o C. Assuming that deep ocean water originated at high latitudes as today –The water originating in polar climates 55 Myr ago must have been warmer than today indicating that those climates were much more temperate than they are today.

Why Did Global Climate Cool Over the Last 55 Myr?

Was it BLAG? There should be evidence of: –A slowing of global mean seafloor spreading and subduction rates –Resulting slower rates of CO 2 input into the atmosphere to cause global cooling

Changes in Spreading Rates The average rate slowed until 15 Myr ago –Consistent with cooling Increased in the last 15 Myr –But, ice appeared in the northern hemisphere in the last 15 Mry BLAG may have caused global cooling before 15 Myr ago, but it doesn’t explain the cooling since then.

Was it the Uplift Weathering Hypothesis? Its three main predictions must be verified: –High elevation terrain must be more common today than in the past 55 Myr. –This high terrain must be causing unusual amounts of rock fragmentation This creates more surface area for greater rates of weathering. –There must be unusually high rates of chemical weathering.

Prediction 1 Extensive High Terrain

Earth’s High Topography Brown, Blue, white: Areas more than 1 km elevation Highest bedrock surfaces are the Tibetan Plateau and mountains of S. Asia, Andes of S. America, Rocky Mts. and CO Plateau of N. America, and volcanic plateaus of eastern and southern Africa

The Tibetan Plateau Strong evidence for the amount high terrain today being greater than in the past Formed by the collision of India with Asia about 55 Myr ago

The Tibetan Plateau Uplift has occurred since the collision No other continental collisions occurred from 100 to 65 Myr ago No such massive plateaus existed then or for the preceding 150 Myr

The Himalayan Mountains

Most Other High Elevation Regions Ocean-Continent Convergence (Subduction)

Andes Mountains The modern Andes and the central Altiplano Plateau result from subduction The western Andes started forming with subduction 100 Myr ago The Altiplano and eastern Andes were both created in the last 55 Myr from eastward expansion which has increased the total mass of the high Andes.

Uplift in Western N. America Subduction has occurred along western North America. New evidence indicates uplift in a large region in the Rocky Mountain area was offset by lower of high terrain farther west, near Nevada. Other scientists emphasize recent uplift, within the starting about 20 Myr ago.

High Topography in the American West

Sierra Nevada Mountains

High Topography in the American West

Basin and Range Mostly in Nevada Ranges bordered by normal faults Sierra Nevada at the western margin has risen along the faults more than 3000 m above the valley to the east

High Topography in the American West

Rocky Mountains

High Topography in the American West

Colorado Plateau Arizona (Grand Canyon) Utah

Low Plateaus in Eastern and Southern Africa 1 km high plateaus from deep-seated heating that causes broad upward doming and of lava flows. Seems to have been built in the last 30 Myr.

Summary The Massive Tibetan Plateau makes modern topography unusual compared to much of geology history. –This is consistent with the weathering hypothesis. Regions of high youthful terrain also exist along subducting margins. –It’s not confirmed that these are higher than similar features formed during earlier intervals.

Prediction 2 Unusual Physical Weathering

Sediments From Rivers Deposited in Oceans Provide best evidence of terrestrial erosion Today the largest mass of young sediment is found south of the Himalaya Mts. In the Indian Ocean Rate of sediment influx has increased tenfold in the last 40 Myr

Cause of High Sediment Influx Southern Himalayan margin of the Tibetan Plateau is has very steep terrain The large size of the Tibetan Plateau creates its own weather, including the powerful Asian Monsoons.

Rates of Physical Weathering Definitive global rates can’t be determined from ocean sediments. –Much of eroded terrestrial sediments are consumed by subduction –Some sediments are reworked after deposition Skews compilations of deposition rates towards younger ages

Summary Rapid deposition of huge amounts of Himalayan sediment –Supports the hypothesis that physical weathering is stronger today on a global basis than in the past This conclusion is unproved due to: –Loss of sediment by subduction into ocean trenches –Assuming subduction is occurring a similar rates as in the past, erosion rates are probably higher because of the extra sediment from the eroding plateau.

Prediction 2 Unusual Chemical Weathering

Quantifying Rates of Chemical Weathering Amount of ions dissolved and transported by rivers is measured on a regional basis. –Reflects the amount of weathering in each river’s watershed. Measurement Difficulty –Human interference with the natural weathering process –Ions from hydrolysis of silicate rocks and rapid carbonation of carbonate rocks are difficult to distinguish Only hydrolysis affects the atmospheric CO 2 balance –It’s difficult to study enough rivers to accurately estimate the global weathering rate. Too many rivers contribute to the global total

Inferring Chemical Weathering Rates: Suspended Sediments in Rivers Tibetan-Himalayan complex –Largest concentrations of suspended load –Very high elevations compared to previous intervals

Inferring Chemical Weathering Rates: Suspended Sediments in Rivers Steep slopes with unusually large exposure of fresh rocks –Receive some of the most intense rainfall on Earth, especially the Tibetan Plateau which itself generates monsoon rains

Summary It is inferred that this combination of favorable factors should promote unusually rapid chemical weathering with increased CO 2 removal from the atmosphere. This is only inference and is not proof.

Was it the Ocean Heat Transport Hypothesis? Changes in Oceanic Gateways

Oceanic Gateways Narrow passages linking major ocean basins –Changes in their configuration alters amount of seawater exchanged between oceans Affects the heat and salt carried by seawater O-GCMs –Ocean General Circulation Models –Used to study two major gateway changes during the last 55 Myr

Opening of Ocean Circulation Around Antarctica Drake’s Passage

65 Million Years Ago Flow of ocean around Antarctica –Impeded by: Land connection with South America Barrier of northward-projecting Australian continent Warm ocean currents diverted poleward from lower latitudes –Brought enough heat to prevent glaciation

Opening of Drake’s Passage Separation of land masses opened land barriers Strong eastward flow around Antractica –No warm poleward flow of heat –Continent cooled and glaciation began Problems with the hypotheses: Drake’s passage opened 10 Myr later than first appearance of Antarctic ice and 10 Myr earlier than the interval of intensified Antarctic glaciation O-GCMs have shown that the Antarctic climate would not have changed with an open or closed passage.

Closing of the Central American Seaway Links Two Events

Closing of the Central American Seaway Closing of the deep ocean passage separating North America and South America –During last 10 Myr –Created the Central American part of the Cordilleran mountain chain –Final closure occurred before 4 Myr ago

First large-scale North American glaciation about 2.7 Mry Ago

Closing of the Isthmus of Panama Warm, salty water –Previously driven out of the tropical Atlantic and into the eastern Pacific by the trade winds –Redirected towards into the Gulf Stream and towards the high latitudes

Closing of the Isthmus of Panama Reduced the extent of sea ice due to increased salinity More moisture available to nearby landmasses –Increased evaporation Triggered growth of ice sheets as increased moisture precipitated

Contradictions by O-GCMs While GCMs confirm the prediction –of redirection of warm, salty water northward in the Atlantic by the closing of the Isthmus They indicate that the warmer water would have –Transferred a large amount of heat to the atmosphere, warming the regions where ice sheets eventually formed This would have increased summer melting, opposing conditions needed for glaciation

Comments on the Gateway Hypotheses Differing assumptions reflect disagreements among scientists as to how oceans affect the mass balance of ice sheets Neither set of model experiments supports the hypotheses but –GCMs are still at an early stage of development Whether or not gateway changes affect climate on a global scale, they definitely alter production and flow of deep and bottom ocean water.

Brief Tectonic-Scale Climate Changes Volcanic Aerosols Burial of Organic Carbon

Aerosols from Volcanic Eruptions

Locations of Earth’s Volcanoes Earth’s active volcanoes mainly occur in regions of convergent plate boundaries

Plate Boundary Map

Volcanic Aerosols Short-term climate cooling due to the blocking of insolation

Volcanic Explosions and Cooling Large volcanic eruptions emit sulfate aerosols into the stratosphere Climate is cooled for a few years after the eruption Equatorial volcanoes cause a greater cooling effect –Sulfates can be carried into both hemispheres

Mt. Pinatubo – 1991 Eruption

Cooling After the Eruption

Long-Term Cooling by Volcanoes? Probably not... Cooling by volcanic sulfates would eventually be overwhelmed by warming effects of: –Volcanic emissions of CO 2 (BLAG Hypothesis) Remain in atmosphere longer than sulfates Effects should dominate over intervals longer than a century

Burial of Organic Carbon Cooler climate due to less CO 2 in the atmosphere

Organic Carbon Potential to affect climate relatively rapidly Large amounts can be quickly buried in the sedimentary record. –This would result in rapid reductions in atmospheric CO 2 levels.

Burial of carbon can be increased by:

Increase in Oceanic Upwelling Stronger winds result in greater upwelling –Could be caused by long- term climate cooling Increased carbon production (plankton) Carbon is buried in coastal sediment when plankton die, reducing atmospheric CO 2 levels. Referred to as the Monterey Hypothesis –Large increase in δ 18 O values 13 Myr ago Followed an interval when carbon-rich sediments were deposited in shallow waters around the margins of the Pacific Ocean, including the Monterey coast of CA.

Greater Supply of Carbon-Rich Sediment Increased carbon burial in the oceans –Greater supply of carbon eroded from older terrestrial sedimentary rocks (e.g., in the Himalayas) From run-off which eventually becomes buried in sediment

Wetter Climates on Continental Margins Flat topography –Favors formation of swampy/marshy conditions Organic matter is formed in swamps and is deposited out into sediment.

Uplift Hypothesis Summary Positive v. Negative Feedback

Negative Feedback Increased chemical weather resulting from increased uplift Results in lower CO 2 and global cooling

Negative Feedback Results in lower temperatures, precipitation, and vegetation causing a decrease in chemical weathering

Negative Feedback Reduction in chemical weathering reduces CO 2 removal from the atmosphere –This reduces the initial cooling due to uplift

Positive Feedback Uplift causes increased weathering resulting in global cooling –Increased glaciers on Earth –Cause increased rock grinding (weathering and fragmentation) –More fresh materials increases weathering rates Removes even more CO 2 from the atmosphere Increases global cooling