2Glaciation on Continents – The Polar Position Hypothesis Two Key Testable PredictionsWhen continents are near the poles they should have ice sheetsIf no continents are near the poles no ice sheets should appear on EarthDoes not consider world-wide climate changesOnly considers positions of the continents
3Seafloor Spreading Has Moved Continents During the past 500 Myr continents have changed position betweenWarm low latitudesColder higher latitudesIf latitude alone is the controlling factor, these movements should have produced predictable glaciations
5South Pole Positions Correlate to Periods of Glaciation Changes in the position of the poleSlow movement of Gondwana across a stationary pole430 Myr agoS. Pole position consistent with glaciation in the Sahara
6South Pole Positions Correlate to Periods of Glaciation From 325 to 240 Myr agoGondwana continues to move across the South PoleA huge region on the southern continent was glaciated
7South Pole Positions Correlate to Periods of Glaciation Gondwana’s glaciation ended about 240 Myr agoIt moved away from the pole and merged with northern continents forming Pangaea
8The Polar Positions Hypothesis: Some inconsistencies The first southern glaciation (430 Myr ago)Brief in terms of geologic time1 to 10 Myr in durationThe slow motion of Gondwana across the South Pole doesn’t easily explain a brief period of glaciation
9Lack of Ice Sheets on Land over the South Pole Land existed at the South Pole for almost 100 Myr without glaciationThis argues against the hypothesis being the only requirement for large-scale glaciations.
10Lack 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 developedAntarctica 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.
11Pangaea’s ClimateExtended from high northern latitudes to high southern latitudesAlmost symmetrical about the equatorWedge-shaped tropical seaway indented the continent from the eastRepresented almost 1/3 of Earth’s surface. It spanned:180o of longitude at it’s northern and southern limits, both near 70o latitude¼ of Earth’s circumference at the equator
12Climate Models Input . . . Sea Level Topography Rock evidence indicates S.L. comparable to today’sTopographyTo minimize errors caused by incorrect guess as to the distribution of mountainsInterior land represented as a low-elevation plateau with a uniform height of 1000 m and gradually sloping towards the sea along the continental margins
13Climate Models Input . . . Higher CO2 level than today Compensates for a weaker Sun (about 1%)This is because geologic evidence indicates a warmer EarthAbsence of polar iceFossil vegetationPalm-like trees at latitudes as high as 40o were not killed by hard freezes on PangaeaIndicates that the hard freeze limit was at a higher latitude than today’s limit of 30o to 40o
14Precipitation on Pangaea Arid low latitudes, especially in the continental interiorLarge land area under the dry, descending portion of the Hadley CellLarge expanse of land in the tropicsTrade winds lose moisture by the time they reach the continental interior
15Supported by Evaporite Deposits Mesozoic RiftingOpens the AtlanticEvaporites inshallow basinsSalts precipitated in lakes or in coastalmargin basinsLimited exchanges of water with the oceanRequires an arid climateMore evaporates precipitated during the later phases of Pangaea than during any time in the last several hundred million years
16Temperatures on Pangaea Patterns switch back and forthbetween hemispheres withchanges in the seasons.Continental interiorSeason extremes of heating in summer and cooling in winterMay 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
17Monsoons on PangaeaStrong reversal between summer and winter monsoon circulationsWinter Hemisphere has highpressure over the interior ofthe continent- Weak insolation and highradiative cooling- Air sinks building highpressure- Air flows out towards theoceanSummer Hemisphere hasstrong solar heating- Air rises and a strong lowpressure cell develops.- Causes a net inflow of humidair
18Monson Circulation and Seasonal Precipitation Eastern margins from 0o to 45o latitudeWinds reverse directions between seasonsExtremely wet summersDry winters
19Geologic Evidence – Red Beds Permian – U.K.L. Permian, TriassicPalo Duro Canyon,TXTriassic - CASedimentary rocks stained red by oxidationWet season provides the moistureRust forms in the dry season or intervalRed beds are widespread on Pangaea and is consistent with the model of high seasonal changes in moisture
20Effect of Pangaea’s Breakup on Climate Northern Hemisphere continents moved farther northwardHigh latitude ocean water displacedSteeper global temperature gradient resulted
21Change in Oceanic Circulation A single ocean (Panthalassa) with a single continentSimple patternSeparate continentsMore complex circulationAffects atmospheric circulationWarm an cold currentsConveyer
22The BLAG Spreading Rate Hypothesis Also known as the Spreading Rate HypothesisProposes that climate changes in the last several hundred million years:Caused mostly by changes in the rate of CO2 input to the atmosphereCO2 input driven by plate tectonic processesNamed using initials of its authorsRobert BernaAntonio LasagaAnd . . .Robert Garrels
23CO2 Released into the Atmosphere by Plate Tectonics Most CO2 is releasedAt Mid Ocean ridgesBy Subduction Volcanoes
24CO2 Released into the Atmosphere by Plate Tectonics A smaller input of CO2 is released at hot spotsMost are not associated with plate boundaries
25Distribution of Hot Spots Identified by volcanic activity and structural uplift within the last few million years
26Rate of Seafloor Movement Controls Delivery of CO2 from Rocks into the Air Rates of plate motion presently varies from plate to plateSouth Pacific spreads up to 10X faster than the Mid-Atlantic Ridge
27Age of the SeafloorMagnetic data shows widely varying rates over millions of yearsContinue to change
28Fast Spreading Larger releases of CO2 to the ocean Results in faster subductionLarger volumes of carbon-bearing sediment and rock melt
29Increased CO2 Causes an Initial Shift Towards a Greenhouse Climate Activates increased chemical weatheringcombined effect of temperature, precipitation, and vegetationCO2 drawn out of atmosphere at a faster rateNegative Feedback
30Slow Plate MovementSlow CO2 input results in cooling
31A Colder Icehouse Climate Decreased chemical weatheringDecreased removal of CO2 (greater amount remains in the atmosphereReduces the rate of coolingNegative Feedback
32Carbon Cycling in the BLAG Hypothesis Carbon cycles continuously between rock reservoir and the atmosphere
33Removal of Carbon from the Atmosphere Carbon from chemical weatheringEnds up in shells of marine lifeForms sediments when marine organisms die
34Return of Carbon to the Atmosphere SuductionSome sediment is scraped off, eroded and redepositedMost is taken into Earth’s interiorDoesn’t reach the mantleReturned to the atmosphere by volcanism
35Does Data Support BLAG?Data does seem to support the BLAG Hypothesis
36The Uplift Weathering Hypothesis Asserts that chemical weathering is:The active driver of climate changeNot just a negative feedback to BLAG
37Available Surfaces Affect the Rate of Chemical Weathering BLAG views chemical weathering as responding to three climate factors:TemperaturePrecipitationVegetationThe Uplift Weathering Hypothesis considers availability of fresh rock and mineral surfaces to be weatheredThis exposure can override the combined effects of BLAG’s three factors
38Rock Exposure and the Rate of Weathering As rocks an minerals physically disintegrate, the total surface area of the particles increases
39Increased 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.
40Uplift and WeatheringTectonics 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.
41Factors Increasing Weathering Rates in Uplifting Areas Steep SlopesErosional processes are unusually activeHigher frequency of earthquakes in young mountain regions along plate boundariesDislodge debris and further weaken bedrock
42Factors Increasing Weathering Rates in Uplifting Areas Steep SlopesErosional pocesses called Mass Wasting are unusually activeRock slides and fallsLandslidesFlows of water saturated debrisRemoval of overlying debris exposes fresh bedrock
43Mass Wasting or Mass Movement is . . . the movement in whichbedrock,rock debris,or soilmoves downslope in bulk, or as a mss, because of the pull of gravity.Examples
45TalusAn apron of fallen rock fragments that accumulates at the base of a cliff.
46Yosemite Valley Rockfall, 1999 Two 80,000 ton slabs of an overhang broke offSlid a short distance over steep rock and then flew 500 meters, launched as if from a ski jumpThey shattered upon impact and created a huge dust cloud.
47Debris 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 partremained mostly intact asit moved in blocks.Debris in the lower portionflowed with rotationalsliding.Earthflow andSlumping
48EarthflowEarthflow in Santa Tecia, El Salvador, January 13, 2001
49Factors Increasing Weathering Rates in Uplifting Areas Steep SlopesMountain GlaciationPulverizes underlying bedrockCarries sediment to lower elevationsIncreases regional rates of chemical weathering
50Factors Increasing Weathering Rates in Uplifting Areas Steep SlopesHeavy precipitation generated onHigh but narrow mountain beltsIntercept moisture carried by tropical easterlies and mid-latitude westerliesLarge plateaus create their only monsoonal circulation (e.g., Tibetan Plateau) by pulling moisture from adjacent oceans
51Tectonic Uplift Ocean-continent convergence Subduction occurs relatively steadily over timeTotal amount of high mountain terrain on Earth remains constant through time- Locations and heights of individual ranges may vary
52Tectonic UpliftContinent-continent collision – the Himalayas and Tibetan Plateau
53Active Tectonic Uplift Cools Climate Uplift accelerates chemical weatheringDraws CO2 out of the atmosphereCools climateGreenhouse ConditionsSlower upliftLess chemical weatheringMore CO2 in atmosphere
54Does Data Support the Uplift Weathering Hypothesis? Data does seem to support the hypothesis.