3/26/2014 1 Course Business • Welcome back • Midterm #2 April 8th, mark it Today: • Finish dating methods & paleo climate • Begin primate evolution and fossil record Relative Dating Techniques • Stratigraphy: – Principle of Superposition – Stratigraphic Correlation: textbook example of Bryce to Zion to Grand Canyon • Biostratigraphy 23/26/2014 2 Relative Dating: Paleomagnetism 3 Absolute or Chronometric Dating Techniques using radioactive decay as a clock Potassium– Argon (40K-40A) Dating Dating volcanic sediments from 500,000 years to 4.5 billion years 43/26/2014 3 Argon/Argon Dating (40Ar/39Ar, an advanced measurement of traditional K-Ar to date single grains) 5 • Materials: living tissue that incorporates carbon. • Range: 200 years – 60,000 years Another important chronometric dating technique: Radiocarbon 63/26/2014 4 Radiocarbon is only a quasi-absolute technique until it can be calibrated to calendrical age 7 “Mind the Gap”: Other important chronometric dating techniques that fill in between the ranges of Ar/Ar and 14C techniques • Uranium/Thorium Dating: 238U to 230Th – range 1,000 years - 500,000 years, – Material: carbonate sediments like stalagmites and flowstone in caves, also bones & teeth. 83/26/2014 5 Mind the Gap: “Archaeologically-relevant Dating Techniques for the Next Century. Small, hot and identified by acronyms” • The Electron Trap Techniques = TL, OSL, & ESR 9 “Mind the Gap”: A Chronometric, Electron Trap Technique: Thermoluminescence (TL) Material: burnt stone tools Range: 5,000 to 500,000 years Event? The heating of the artifact, not the formation of the rock or its shaping by a hominin 103/26/2014 6 Chronometric Dating Techniques: Thermoluminscence (TL) Age = Paleodose as TL Annual dose rate 11 Questions? 123/26/2014 7 How do we know the conditions of natural selection in the past? 13 Take-Home Messages to Look For 1. Earth’s climate is influenced by large-scale cycles dictated by orbital parameters. 2. The way the large-scale cycles affect the Earth are altered by feedback systems between the Geosphere and the Hydrosphere. 3. Climate is historically contingent: small details in one variable can change the whole system! 143/26/2014 8 Initial point: “Climate” is not “Weather” 15 Setting the Stage: Global temperature drop at the end of the Miocene Epoch Bipedal apes 163/26/2014 9 The primary influence on Earth’s climate: the Sun. Solar energy is unevenly distributed over the surface of the earth because the earth is round (a globe). This amount of energy is distributed over this amount of space at the equator far from the equator 17 Variation in Solar Radiation received by the Earth: …the seasons. 183/26/2014 10 Milankovitch Cycles 1) Obliquity (tilt) of spin axis (41 ky cycle, greater the tilt, the greater the seasonality) 19 Milankovitch Cycles 2) Eccentricity of the orbit or ellipse (96 ky cycle). Weakest theoretically but dominates the periodicity of glaciations after 1 million ago. 203/26/2014 11 Milankovitch Cycles 3) Precession of the equinoxes (19/23 ky cycle) Affects interaction of the first two cycles. 21 The combined effects of tilt, eccentricity, and precession result in a very accurate “prediction” of insolation (radiation received by the planet) which is then mirrored in our best paleoclimate proxies 223/26/2014 12 Milankovitch Forcing IS necessary but NOT sufficient to cause an Ice Age; it is the feedbacks between the Geosphere, Hydrosphere, & Atmosphere which push the planet over the edge in either direction. 23 Positive feedback toward glaciation • Snow and ice reflect sunlight and thus heat away from planet = albedo. – Albedo prevents snow melting, which makes more albedo… • Orogeny = mountain-building due to plate tectonics allows for more places with high albedo. 243/26/2014 13 Questions? 25 Plate Tectonics: continental drift 263/26/2014 14 27 The Drake Passage Still Closed • Connection of South America to Antarctica before 25 mya: • Antarctica exposed to warm waters circulating down from the Equator to the polar region. 283/26/2014 15 Opening the Drake Passage 29 Separation of South America & Antarctica at 25 mya: • Allows the creation of Antarctic Circumpolar Current by Earth’s rotation. • Cuts Antarctica off from warm southward flowing currents, leaving it to freeze. • Historical contingency of Earth’s climate. Early Miocene collision: ~20 Ma 303/26/2014 16 Mediterranean Sea at 6.2 mya 31 The Messinian Salinity Crisis (6.2-5 mya) and the Death of the Mediterranean Sea 6% drop in world ocean salinity =40x volume of Mediterranean evaporated 323/26/2014 17 The Messinian Salinity Crisis (6.2-5 mya) and the Death of the Mediterranean Sea 6% drop in world ocean salinity =40x volume of Mediterranean evaporated → raised the global oceans’ freezing point REMEMBER HISTORICAL CONTINGENCY!!! 33 Setting the Stage: Global temperature drop at the end of the Miocene Epoch 343/26/2014 18 Again: why do we care about global climate change in the study of evolution? Why do we care what the local environment or habitat was like when and where a particular fossil lived? 35 SPECIATION & EXTINCTION! Figure 04.06 36 Allopatric speciation: Geographic separation (habitat islands) causes a loss of gene flow Parapatric Speciation relies upon the concept that hybrids have less Fitness, which is usually specific to habitat type Climate change affects habitat distribution and composition, which affects suitability of individuals to their niche, and thus micro- and macroevolution.3/26/2014 19 37 How do we know what we know about prehistoric global climate? The Oxygen Isotope Cycle 37 Ocean Glacier 18O 16O 18O 18O 16O 16O 16O During Glacials, light evaporated water gets “stuck” in glaciers, so ice becomes “light” with 16O Sea water becomes “heavy” with 18O “Delta-18O”or δ18O ratio recorded in sediments at bottom of the sea 383/26/2014 20 39 Calcareous shells of Foraminifera (planktonic & benthic) as proxies for sea water temperature & volume 39 40 Deep sea sediment cores: marine sludge is good for science 403/26/2014 21 41 If marine sludge is good for science …so is freezing your ass off. 41 42 423/26/2014 22 43 • We are currently in Oxygen Isotope Stage 1 (OIS 1). • Odd numbers = Interglacials (warm, less ice) • Even numbers = Glacials (cold, more ice) 43 44 Figure 01.11
View Full Document
Unlocking...