Plate Tectonics and Climate

Glaciation on Continents – The Polar Position Hypothesis Two Key Testable Predictions When continents are near the poles they should have ice sheets If no continents are near the poles no ice sheets should appear on Earth Does not consider world-wide climate changes Only considers positions of the continents

Seafloor Spreading Has Moved Continents During the past 500 Myr continents have changed position between Warm low latitudes Colder higher latitudes If latitude alone is the controlling factor, these movements should have produced predictable glaciations

Three “Icehouse Eras” During the Last 500 Myrs

South Pole Positions Correlate to Periods of Glaciation Changes in the position of the pole Slow movement of Gondwana across a stationary pole 430 Myr ago S. Pole position consistent with glaciation in the Sahara

South Pole Positions Correlate to Periods of Glaciation From 325 to 240 Myr ago Gondwana continues to move across the South Pole A huge region on the southern continent was glaciated

South Pole Positions Correlate to Periods of Glaciation Gondwana’s glaciation ended about 240 Myr ago It moved away from the pole and merged with northern continents forming Pangaea

The Polar Positions Hypothesis: Some inconsistencies The first southern glaciation (430 Myr ago) Brief in terms of geologic time 1 to 10 Myr in duration The slow motion of Gondwana across the South Pole doesn’t easily explain a brief period of glaciation

Lack of Ice Sheets on Land over the South Pole Land existed at the South Pole for almost 100 Myr without glaciation This argues against the hypothesis being the only requirement for large-scale glaciations.

Lack of Ice Sheets on Land over the South Pole After the breakup of Pangaea Antarctica, India, and Australia moved back over the South Pole. No ice developed Antarctica remained directly over the pole from 125 Myr ago to almost 35 Myr ago, but free from ice. Again, this argues against the hypothesis being the only requirement for large-scale glaciations.

Pangaea’s Climate Extended from high northern latitudes to high southern latitudes Almost symmetrical about the equator Wedge-shaped tropical seaway indented the continent from the east Represented almost 1/3 of Earth’s surface. It spanned: 180o of longitude at it’s northern and southern limits, both near 70o latitude ¼ of Earth’s circumference at the equator

Climate Models Input . . . Sea Level Topography Rock evidence indicates S.L. comparable to today’s Topography To minimize errors caused by incorrect guess as to the distribution of mountains Interior land represented as a low-elevation plateau with a uniform height of 1000 m and gradually sloping towards the sea along the continental margins

Climate Models Input . . . Higher CO2 level than today Compensates for a weaker Sun (about 1%) This is because geologic evidence indicates a warmer Earth Absence of polar ice Fossil vegetation Palm-like trees at latitudes as high as 40o were not killed by hard freezes on Pangaea Indicates that the hard freeze limit was at a higher latitude than today’s limit of 30o to 40o

Precipitation on Pangaea Arid low latitudes, especially in the continental interior Large land area under the dry, descending portion of the Hadley Cell Large expanse of land in the tropics Trade winds lose moisture by the time they reach the continental interior

Supported by Evaporite Deposits Mesozoic Rifting Opens the Atlantic Evaporites in shallow basins Salts precipitated in lakes or in coastal margin basins Limited exchanges of water with the ocean Requires an arid climate More evaporates precipitated during the later phases of Pangaea than during any time in the last several hundred million years

Temperatures on Pangaea Patterns switch back and forth between hemispheres with changes in the seasons. Continental interior Season extremes of heating in summer and cooling in winter May explain lack of ice sheets in high latitudes because summers were so warm that rapid summer melting prevented the build-up of snow. Freezing average daily winter temperatures extended to 40o latitude

Monsoons on Pangaea Strong reversal between summer and winter monsoon circulations Winter Hemisphere has high pressure over the interior of the continent - Weak insolation and high radiative cooling - Air sinks building high pressure - Air flows out towards the ocean Summer Hemisphere has strong solar heating - Air rises and a strong low pressure cell develops. - Causes a net inflow of humid air

Monson Circulation and Seasonal Precipitation Eastern margins from 0o to 45o latitude Winds reverse directions between seasons Extremely wet summers Dry winters

Geologic Evidence – Red Beds Permian – U.K. L. Permian, Triassic Palo Duro Canyon,TX Triassic - CA Sedimentary rocks stained red by oxidation Wet season provides the moisture Rust forms in the dry season or interval Red beds are widespread on Pangaea and is consistent with the model of high seasonal changes in moisture

Effect of Pangaea’s Breakup on Climate Northern Hemisphere continents moved farther northward High latitude ocean water displaced Steeper global temperature gradient resulted

Change in Oceanic Circulation A single ocean (Panthalassa) with a single continent Simple pattern Separate continents More complex circulation Affects atmospheric circulation Warm an cold currents Conveyer

The BLAG Spreading Rate Hypothesis Also known as the Spreading Rate Hypothesis Proposes that climate changes in the last several hundred million years: Caused mostly by changes in the rate of CO2 input to the atmosphere CO2 input driven by plate tectonic processes Named using initials of its authors Robert Berna Antonio Lasaga And . . . Robert Garrels

CO2 Released into the Atmosphere by Plate Tectonics Most CO2 is released At Mid Ocean ridges By Subduction Volcanoes

CO2 Released into the Atmosphere by Plate Tectonics A smaller input of CO2 is released at hot spots Most are not associated with plate boundaries

Distribution of Hot Spots Identified by volcanic activity and structural uplift within the last few million years

Rate of Seafloor Movement Controls Delivery of CO2 from Rocks into the Air Rates of plate motion presently varies from plate to plate South Pacific spreads up to 10X faster than the Mid-Atlantic Ridge

Age of the Seafloor Magnetic data shows widely varying rates over millions of years Continue to change

Fast Spreading Larger releases of CO2 to the ocean Results in faster subduction Larger volumes of carbon-bearing sediment and rock melt

Increased CO2 Causes an Initial Shift Towards a Greenhouse Climate Activates increased chemical weathering combined effect of temperature, precipitation, and vegetation CO2 drawn out of atmosphere at a faster rate Negative Feedback

Slow Plate Movement Slow CO2 input results in cooling

A Colder Icehouse Climate Decreased chemical weathering Decreased removal of CO2 (greater amount remains in the atmosphere Reduces the rate of cooling Negative Feedback

Carbon Cycling in the BLAG Hypothesis Carbon cycles continuously between rock reservoir and the atmosphere

Removal of Carbon from the Atmosphere Carbon from chemical weathering Ends up in shells of marine life Forms sediments when marine organisms die

Return of Carbon to the Atmosphere Suduction Some sediment is scraped off, eroded and redeposited Most is taken into Earth’s interior Doesn’t reach the mantle Returned to the atmosphere by volcanism

Does Data Support BLAG? Data does seem to support the BLAG Hypothesis

The Uplift Weathering Hypothesis Asserts that chemical weathering is: The active driver of climate change Not just a negative feedback to BLAG

Available Surfaces Affect the Rate of Chemical Weathering BLAG views chemical weathering as responding to three climate factors: Temperature Precipitation Vegetation The Uplift Weathering Hypothesis considers availability of fresh rock and mineral surfaces to be weathered This exposure can override the combined effects of BLAG’s three factors

Rock Exposure and the Rate of Weathering As rocks an minerals physically disintegrate, the total surface area of the particles increases

Increased Surface Area Results in a Faster Weathering Rate The proportional increase of weathering far exceeds the estimated result from changes in temperature, precipitation, and vegetation.

Uplift and Weathering Tectonics results in the uplifting of Earth’s crust and the formation of mountains at many plate boundaries. In regions of uplift exposure of freshly fragmented rock is enhanced.

Factors Increasing Weathering Rates in Uplifting Areas Steep Slopes Erosional processes are unusually active Higher frequency of earthquakes in young mountain regions along plate boundaries Dislodge debris and further weaken bedrock

Factors Increasing Weathering Rates in Uplifting Areas Steep Slopes Erosional pocesses called Mass Wasting are unusually active Rock slides and falls Landslides Flows of water saturated debris Removal of overlying debris exposes fresh bedrock

Mass Wasting or Mass Movement is . . . the movement in which bedrock, rock debris, or soil moves downslope in bulk, or as a mss, because of the pull of gravity. Examples

Rockfalls

Talus An apron of fallen rock fragments that accumulates at the base of a cliff.

Yosemite Valley Rockfall, 1999 Two 80,000 ton slabs of an overhang broke off Slid a short distance over steep rock and then flew 500 meters, launched as if from a ski jump They shattered upon impact and created a huge dust cloud.

Debris Slide A coherent mass of debris moving along a surface Rotational debris slide (slump) if the movement is along a curved surface. La Concita, CA (1995) Debris in the upper part remained mostly intact as it moved in blocks. Debris in the lower portion flowed with rotational sliding. Earthflow and Slumping

Earthflow Earthflow in Santa Tecia, El Salvador, January 13, 2001

Factors Increasing Weathering Rates in Uplifting Areas Steep Slopes Mountain Glaciation Pulverizes underlying bedrock Carries sediment to lower elevations Increases regional rates of chemical weathering

Factors Increasing Weathering Rates in Uplifting Areas Steep Slopes Heavy precipitation generated on High but narrow mountain belts Intercept moisture carried by tropical easterlies and mid-latitude westerlies Large plateaus create their only monsoonal circulation (e.g., Tibetan Plateau) by pulling moisture from adjacent oceans

Tectonic Uplift Ocean-continent convergence Subduction occurs relatively steadily over time Total amount of high mountain terrain on Earth remains constant through time - Locations and heights of individual ranges may vary

Tectonic Uplift Continent-continent collision – the Himalayas and Tibetan Plateau

Active Tectonic Uplift Cools Climate Uplift accelerates chemical weathering Draws CO2 out of the atmosphere Cools climate Greenhouse Conditions Slower uplift Less chemical weathering More CO2 in atmosphere

Does Data Support the Uplift Weathering Hypothesis? Data does seem to support the hypothesis.