Lithosphere, Earth’s crust and Aesthenosphere, molten layer on which lithosphere floats, and lithospheric plates move.

1)  Aesthenosphere from Aesthenes, Greek, - weak. Aesthenosphere, mechanically weak, deforms easily and viscously. Seismic waves have low velocity, so easily identified in seismographs. Layer suspected by 1920s, but not confirmed until Chilean earthquake in 1960.

2)  Lithosphere (rocky) crust. Density of crust varies, continents low density, oceanic high density. Deforms elastically (can recover previous shape) and through brittle failure → earthquakes. Both spheres postulated prior to plate tectonics. VG

3)  Geologic time –

a)  Herodotus, 5th century B.C., found fossil shells far inland in what are now parts of Egypt and Libya, he inferred the Mediterranean Sea had once extended much farther to the south. Few believed him, although he was correct.

b)  Relative time, principle of superposition. Material is deposited forming layers with the youngest material at the top and the oldest at the bottom. Principle holds but can be confused by lifting and tilting of the crust near plate boundaries and mountain ranges. 1785-1800 first proposed by James Hutton, Scottish Geologist, and William Smith English surveyor who realized that layers of rock underground were continuous from outcropping to outcropping and could be identified by their fossils. With the fossils providing a crude times scale → Map that Changed the World. Thus the fields of petrology (rock formation), stratigraphy (studies of layers) and paleontology (life) merged to read the tattered pages of the Earth’s crust, lithosphere, (Origin of Species, pp 316). Thus ancient seas and mountains could be uncovered and the relative ages of the events deduced.

c)  Radiometric or absolute time, determined by natural radioactive decay of various elements. 1896 Henry Becquerel, the French physicist, decay of Uranium, 1905, Rutherford--after defining the structure of the atom-- made the first clear suggestion for using radioactivity as a tool for measuring geologic time directly; 1907, Professor B. B. Boltwood, radiochemist of Yale University, published a list of geologic ages based on radioactivity. showing that geologic time would be measured in terms of hundreds-to-thousands of millions of years. These dates would be later revised with precise dating beginning in the 1950s.

d)  Parent. P = Po e-λt, def of radioactive decay. Daughter, D = Po – P = P•eλt – P = P•(eλt - 1), where λ is the decay constant given by t1/2 = ln(2)/λ. Then an age can be determined from t = 1/λ • ln(1+D/P).

e)  The following are the common elements used for aging.

Parent Isotope / Stable Daughter / Half life x 1e6
Samarium-147 / Neodynium-143 / 106,000
Ru-87 / Strontium-87 / 48,800
Th-232 / Pb-208 / 14,000
U-238 / Pb-206 / 4,500
K-40 / Ar-40 / 1,250
U-235 / Pb-207 / 704
C-14 / N-14 / 0.006

f)  Technique relies on measuring precisely D and P precisely, difficult when D or P are small.

  1. K → Ar used for all ages of rocks known. Useful only on igneous rocks, not sedimentary or fossils. Potassium is a common element found in many materials, such as micas, clay minerals, tephra, and evaporites. In these materials, the decay product 40Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallizes). Time since recrystallization is calculated by measuring the ratio of the amount of 40Ar accumulated to the amount of 40K remaining.
  2. C-14 useful for recent paleontology. Earlier sediments/fossils dated by using relative scales tied to a layer of igneous rocks.

g)  Other isotopes can be used for temperature, particularly 18O and 13C. The data are presented as isotope ratios with respect to a standard. Thus δA = rA – rAs)/rAs, where rAs is a standard ratio of two isotopes and rA is the measured ratio. Thus δ18O = (r18O – r18sO)/r18sO, where r18sO = 18O/16O = 1/498.7 for “Vienna standard mean ocean water”, and r18O = 18O/16O is the measurement. The notation %o = 0.001, similar to % = 0.01, although %o is referred to as “per mil”.

h)  The equilibrium distribution between two substances (water/vapor or water/mineral) depends on the fractionation factor, f1,2. Thus 18O/16Ov = fv,w 18O/16Ow for the difference between 18O/16O in water and vapor which are in equilibrium. In this form fv,w will be < 1 and approach 1 as temperature increases. These present powerful temperature proxies.

i)  δ18O = (r18O – r18sO)/r18sO

  1. r18sO = 18O/16O ~ 1/500 normally in sea water. As T cools the heavier isotopes are less likely to evaporate and more likely to condense. Thus over long times δ18O sea water becomes less negative (increases) and glacier ice δ18O becomes more negative (decreases).
  2. δ18O (273) = -11.7%o
  3. δ18O(290) = -10.1%o
  4. δ18O(350) = -6.0%o
  5. But this requires samples of the seawater or glaciers.
  6. Ice cores give record for last 0.6 Ma.
  7. Foraminifera (forams) shelly amoeba builds shells from CaCO3 calcium carbonate, thus capturing the δO18 present in the seawater at the time of formation. Forams appear 0.525 Ga but the best record is only last 70 Ma.
  8. Planktonic – live near surface (planktos ≡ drifting)
  9. Benthic – at the sea floor. More reflective of global climate. Temp near 0ºC.

j)  δ13C = (r13C – r13sC)/r13sC

  1. Photosynthetic life prefers 12C → shales enriched in 12C → life present. This enrichment is also transferred to heterotrophs dining on the photosynthetic organisms which can fix Carbon
  2. δ13C (outgassing volcanoes, upper mantle) = -6%o
  3. δ13C(for CO2 in atmosphere) = -8%o
  4. δ13C(fossil fuel) = -32%o
  5. δ13C(methanogenesis) = -50%o fixing carbon from CH4.

4)  Age of Earth – cannot be determined by rocks on Earth since all have been recycled through molten core, thus losing their original information.

a)  The oldest rocks: 1) the Acasta GneissComplex near Great Slave Lake, Canada --4.03 Ga, 2) the Isua Supracrustal rocks in West Greenland -- 3.7 to 3.8 Ga, 3) Swaziland (3.4-3.5 billion years), and 4) Western Australia (3.4-3.6 billion years). These rocks are from lave deposited in shallow water not from Earth’s crust. Zircon (ZrSiO4 → δ18O is available → Temperature) crystals with ages of 4.3-4.4 Ga found in younger sediments, but rock origin of zircon not known.

b)  The best age for the Earth (4.54 Ga) is based on old, presumed single-stage, leads coupled with the Pb ratios in troilite (FeS) from iron meteorites, specifically the Canyon Diablo meteorite. Claire Patterson, 7 years into mid 1950s to get the final number due to contamination of laboratory samples from atmospheric lead from leaded gasoline. Long campaign by Patterson led to clean air act, 1970 and banning leaded gasoline in 1986. Mineral grains (zircon) with U-Pb agesof 4.4 Ga have recently been reported from sedimentary rocks in west-central Australia. The assumption is then that the meteorite has not been altered since its formation at the dawn of the solar system and the Earth also dates from that time. Oldest Moon rocks date from this same period, although younger ones also found.

5)  Geologic periods on Earth into deep time (1.3) VG – Geologic time

a)  4.5-3.8 Ga – obscure, T 200-500 K from volcanism and heat from interior. Early “solar abundance” atmosphere lost. Zircon crystals 4.4 Ga show evidence of contact with water, so a crust was forming at this time and would have interrupted the flow of heat from interior. Heavy bombardment also causes warm T.

b)  3.8 Ga, oldest rocks – Greenland, end of Hadean, open water, organic carbon, prokaryote – stromatolites? Other explanations w/o life possible. Not a snowball earth.

c)  2.6 Ga – proterozoic, cherts (H2O and SiO2) → δ18O → chert paleothermometer. Record indicates T = 70 → 30 C (350→300 K), also glaciation, first independent evidence of life and undisputed evidence of cyanobacteria and other prokaryotes (cell has no nucleus). Diamictites – material transported by floating ice.

d)  2.3 Ga – global glaciationprobably. Rocks impacted by glaciers formed at 12º latitude based on ferrous crystal orientation. Big O2 event.

e)  1.8 - 0.8 Ga no evidence of glaciation. Eukaryotes appear. Greenhouse gases changing. Oxygenation begins towards end of proterozoic. This requires burying the organic carbon, otherwise it would be reverted to the atmosphere through the decay by bacteria of the organic carbon. Climatic variations small (“big yawn”). Evidence of O2 from participation of isotopes of sulfur, 32, 33, 34, 36S in various reactions. There is mass independent fractionation only in absence of O2.

f)  Snowball earth at 0.7 Ga, then climate moderated between ice free and some ice present, but never reverted to a snowball Earth after the 0.7 Ga event in the neoproterozoic.

g)  0.7 end of neoproterozoic. , multicellular (metazoans) life appears, medusa, 0.5 Ga Cambrian explosion all phylums now in existence make their entry at this time, while others appear and become extinct. What precipitated the Cambrian explosion? Still unsolved. Flow of carbon – land and marine carbonates, dissolved organic and inorganic carbon in sea water, atmospheric CO2. Mass extinction at end of Permian, 96% of all marine and 70% of all terrestrial animals extinct.

h)  Last 70 Ma ice free for 35 Ma – glaciation then rebound. Paleo/Eocene thermal maximum. Extinction of benthic forams (water too warm). Hothouse from CO2 released from clathrate, CH4. 13C severely depleted. Bolide impact at the Cretaceous/Tertiary (K/T) boundary, 65 Ma → end of dinosaurs.

6)  Plate tectonics –

a)  Alfred Wegner, German geophysicist, meteorologist, trained astronomer but preferred meteorology and climatology. Balloonist, pioneered weather balloons, winter in Greenland, recovered ice core to 25 m. Died in 1931 on Greenland resupply mission (50 and a smoker).

b)  Proposed continental drift in 1912 by comparing e.g. coasts of South America to West Africa and India and Madagascar to East Africa. But he could not find a mechanism, so theory not accepted, although he showed the rocks on the continental boundaries of Africa and South America were similar and proposed that mid ocean ridges might be evidence. US geologists organize conference in opposition.

c)  1950s paleomagnetism. 1953 evidence indicated that India had been in southern hemisphere as predicted by Wegener. 1960s sea floor spreading confirmed. Alternating alignment of ferrous crystals in mid Atlantic ridge consistent with paleomagnetic shifts of N and S magnetic pole. Iron crystals in lava align with Earth’s magnetic field. Wegener recognized as the father of one of the revolutionary scientific theories of the 20th century.

d)  Earth’s magnetic field – Present south magnetic pole of Earth in north → N end of compass is attracted to S magnetic pole. Magnetic fields arise from Ampere’s law ( ) while electric fields arise from Faraday’s law ( ). It is thought there is a current in the liquid outer core spinning with the Earth. Origin of current? Motion of Earth in a magnetic field? Chicken and egg problem. Reversals of poles occur due to large disruptions in the chaotic flow in the liquid outer core, where convection, Coriolis forces play large roles.

e)  Lithosphere two types of crust, oceanic, more dense, and continental crust, less dense.

f)  Plate tectonics still in the realm of kinematics – motion can be described but driving forces still point of serious research. Three basic ideas

  1. Mantle dynamics – convection in the upper mantle
  2. Gravitational secondary forces coupled with convection
  3. Earth rotation related forces – tidal forces (initially proposed by Wegener). VG – plate tectonics

7)  Super continents

a)  Importance of continents to climate

  1. Platform for glaciers
  2. Silicate weathering → CO2 abundance in atmosphere. Abundance dependent on the extent of weathering
  3. Influence the transport of heat by the oceans as continents move.
  4. Habitat for life.

b)  Columbia (2.0 Ga) proposed 2002 – evidence in Columbia river drainage

c)  Rodinia, (motherland in Russian), 1.2-0.75 Ga, first supercontinent about which we have undisputed evidence, most mass low latitudes, NH

d)  Pangaea (0.3 Ga) Phanerozoic

  1. Gondwanaland – southernmost land mass split from Pangaea about 0.2 Ga. VG – super continents

(Venus, 97% CO2, 2% N2 and less than 1% of O2, H2O and CH4 (methane)

Earth Venus Mars

N2 0.79 2 3 x 10-4

O2 0.20 < 0.001 10-7

Ar 0.01 0.005 2 x 10-4

CO2 0.0003 64 0.009

H2O ~ 0.02 ~ 0.01 ~10-6

Total 1.00 90 0.01

8)  Impact on life

a)  Origin of life –

  1. Ambiguous evidence
  2. 3.46 Ga old chert in NW Australia - single celled organisms –cyanobacteria?
  3. Older rocks in NW Australia (3.51 bya),
  4. 3.0 bya South Africa \
  5. Oldest known rocks, 3.85 bya banded iron outcrops on the island of Akilia - SW coast of Greenland.
  6. All have been used by paleontologists to stake claims to evidence of life.
  7. Some bacterium like structures.
  8. Others Greenland samples (3.85 bya) from isotopic 12C / 13C ratios. In all samples 12C enhanced, but there are non-biological pathways, to the same ratios → numerous challenges.
  9. Although intriguing structures all cases alternate explanations, not involving life, are also consistent with the observations. The timing of the origin of life is open.
  1. Unambiguous evidence in:
  1. 1954 cherts of the Gunflint formation in NW Ontario fossils of 2 Ga bacteria from an iron rich sea. Fossil record revealed cyanobacteria and iron metabolizing bacteria, difficult to find today. There were large atmospheric changes occurring about 2 Ga as the cyanobacteria production of oxygen was beginning to cause iron in the oceans to precipitate → iron metabolizing and anaerobic bacteria driven underground.
  1. Even older fossils can be identified by a biomarker molecule, hopane a 5 ring organic, found in a petroleum like residue representing the remains of marine algae. Such droplets of residue were found in 2.7 Ga shale, the Pilbara Craton in W. Australia. The primary job was to rule out any sort of contamination and to establish that these 2.7 byo rocks had never experienced temperatures that could have destroyed the hopane, i.e. temperatures had to remain below ~300°C. This done the results were published in 1999 (Summons).
  1. A second paper at the same time announced results from nearby 2.5 Ga remains which contained a hopane variant indicative of cyanobacteria. Cyanobacteria, dominant organism on the planet for a period exceeding by a lot even the reign of the dinosaurs.

b)  Cyanobacteria

  1. thrive in coastal habitats, characteristic of the geologic formations in which they have been discovered, particularly chert, created in coastal environments as silica is precipitated.
  2. Tidal flat cyanobacteria must survive desiccation (low tide), then osmotic trials when submerged. → cyanobacteria secrete an extracellular envelope to protect cells inside.