post 11

Course announcements Remaining content: [all course lecture recordings will be posted]  4 lectures: 4.6 billion years of Earth history  2 lectures: Resources & human impact  No class on Fri Dec 4

all UVic classes cancelled 11:30-12:30 to mark National Day of Remembrance and Action on Violence against Women

 Review session Mon Dec 7, time TBD (recording will be posted)

Remaining assessment:  Q9: 9 pm Nov 23; Q10: 9 pm Dec 7; Q11, Q12: 1 pm Dec 9  Quizzes: lowest score dropped – make the last few count!  Discussion contributions: 1 pm Dec 9  Final exam: 2 pm Wed Dec 9

Course Experience Survey – your feedback is VALUED!

Stratigraphy and Geologic Time

Why is it important to document Earth history? How do we know that one rock is older than another

(relative age)? Principles.. Fossil record..

How do we know the age of the Earth, and how has our understanding changed over time?

How can we use radioactivity to determine the (absolute) age of a rock?

 How was the Geologic Timescale put together?

[Text: 8.1-8.6]

The Geologic Time Scale - Based on rock sequences in Europe correlated worldwide; includes use of fossils, radiometric dates

- Period divisions mark changes or loss in fauna

www.washingtonpost.com

Faunal successions (coming and going of organisms) divide up periods of time Many divided by extinction (species loss), others by species emergence

Image: Edwards, L.E. & Pojeta, J. Jr. (1994): Fossils, rocks and time, U.S. Gov. Printing Office 1998-675-105, 24p.

Eons  Eras  Periods

Hadean 4000

541

2.6

Download latest geological time scale here (March 2020)

485 444 419 459 323 299 252 201 145

4550

Earle, S. (2019): Online text Fig. 8.1.2 & 8.1.3

Stratigraphy and Geologic Time

 Deposition and the rock record  Biology (fossils) in the rock record  Relative vs. absolute time  Relative ages from principles (e.g., X-cutting relations)  Dating (ages) using radioactivity in minerals and

organic matter  Time - is of immense magnitude  Precise chronology of the ‘tempo’ of earth events and

evolution  Applied in geology and archeology

The Precambrian

Image: http://www.physci.mc.maricopa.edu/d43/glg/Study_Aids/geotime/time_l_fx1.jpg

541 4000Ma

4600 Ma

How old are Earth’s oldest known minerals, dated using U-Pb isotopes?

Cambrian

Proterozoic

Archean

Hadean

Image: http://www.physci.mc.maricopa.edu/d43/glg/Study_Aids/geotime/time_l_fx1.jpg

541 4000Ma

4600 Ma

Early Earth: The First 2 Billion Years

 Where are Precambrian rocks found today?

 What were conditions like on early Earth?

 How did the Moon form?

 Evidence for liquid water?

 Evidence for continents?

 The origin of life on Earth?

[Text: 8.1, 22.4]

Earle, S. (2016): Online text Fig. 8.3

P R E C A M B R I A N

40 00

M a

54 1

M a

How do we know about Earth in the Precambrian?

 Precambrian rocks are poorly exposed

 eroded or metamorphosed or deeply buried beneath younger rocks

 Fossils are seldom found in Precambrian rocks

 Long time ago - Does “Uniformitarianism” apply?

 87% of Earth history is poorly known – a fragmented record

Earle, S. (2016): Online text Fig. 8.3

P R E C A M B R I A N

First 2 billion years Hadean: 4.6-4.0 Ga: no rocks preserved Archean: 4.0-2.5 Ga

40 00

M a

54 1

M a

Great Precambrian Events

Image: http://geo.msu.edu/extra/geogmich/Precambrian.html~4600

4000

2500

1600

1000 541 Ma

4.55 to 4.0 Ga - The “Hadean” (Hades: hell like - but was it?)

First 50 Myr - Earth's core/layering

 Layering requires process of differentiation

 Initial heating, partial melting: ‘magma ocean’

 Fe metal sinks to form core

 Less dense ‘silicate’ melt (Si,O, remaining other elements) forms lighter mantle & crust

Recall: Why was early Earth hot?  Heat from gravitational contraction  Accretionary heat from asteroid impacts  Radioactive decay

Image: http://higheredbcs.wiley.com/legacy/college/levin/0471697435/chap_tut/chaps/chapter08-05.html

Collisions with asteroids and planetismals

Radiogenic Heat Production (K, U, Th)

Origin of the Moon? ~4.5 Ga

Image: NASA

Giant Impact Hypothesis (the Big Splash)

(Theia)

Image by Joe Tucciarone: https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question38.html

Earth’s oldest minerals Jack Hills, Australia 4.4 billion yr-old zircons

Wilde et al. (2001): http://www.geology.wisc.edu/~valley/zi rcons/Wilde2001Nature.pdf

Photo by Michael John Cheadle: https://www.nsf.gov/news/news_images.jsp?cnt n_id=104546&org=NSF

Image: Wikimedia Commons

Isotope data  Magma in which the zircons

formed included melt from crustal material that must have interacted with liquid water

 Liquid water present during early Hadean

Hadean: Earth’s oldest minerals Archean: Earth’s oldest rocks (oldest preserved)

P R E C A M B R I A N

Archean (4.0 to 2.5 Ga)

40 00

M a

54 1

M a

Earle, S. (2016): Online text Fig. 8.3

Where are Precambrian rocks found today?

Green > 2.5 Ga Red > 1.8 Ga, Orange > 1.0 Ga Ga = giga years

Archean rocks: form cores of continents - cratons - metamorphosed granite, volcanic & sedimentary rocks in ‘belts’

Image modified from http://earthsci.org/mineral/mindep/diamond/Whlook.html

Exposure of Archean crust in North America: the Canadian shield

Earth’s Oldest Rocks

Acasta

Image modified from http://news.bbc.co.uk/2/hi/science/nature/2546019.stm

Acasta Gneiss, NWT 3.962 Ga

Oldest rock age (igneous origin)

Images: http://www.geo.titech.ac.jp/lab /ueno/research.html

Porpoise Cove, Quebec 3.8 Ga Metamorphic rocks (parent: either volcanic or sedimentary rocks that formed near surface)

https://en.wikipedia.org/wiki/Nuvvuagittuq_Greenstone_Belt

Isua, West Greenland 3.8 Ga

Deformed pillow lavas Implication?  subaqueous eruption  ‘ocean water’ existed

Image: http://www.mue.titech.ac.jp/rock/isua/title02/

modern Archean

Sub-aqueous eruptions

Image: http://www.punaridge.org/doc/factoids/eruptions /default.htm

Image: http://umanitoba.ca/science/geological_sciences/faculty/arc/pictures/pillows.jpg

Earth’s oldest soil Pilbara paleosol (fossil soil) – 3.46 Ga

Implication?

Problem - Early Earth Surface Temperature Should be Freezing (no liquid water)

4.5 Ga: Sun’s output only 70% that of today (Stellar evolution)

Too cold to maintain a liquid ocean  refuted by geologic evidence

(Sagan and Mullen, 1972)

How to explain?

‘Faint Young Sun’ paradox

Image: http://www.everythingselectric.com/faint-young-sun-paradox/

Why so few rocks older than ~3.8 Ga? Late Heavy Bombardment

 ‘Impact zones’ & impact melt rocks on the Moon  Ages peak at 3.8 Ga  Consequence for Earth’s surface nearby?  obliterated

Image: http://www.origin-life.gr.jp/3603/3603055/fig2.jpg

Why is there such a record on Moon, but not on Earth?

Image: http://seprin.info/2016/11/18/asteroid-strike-made-instant-himalayas/

Archean Plate Tectonics?

 Higher internal temperature of the Earth

 Faster plate motion?

 Many small, mobile plates

 Early crust oceanic – continents formed later

Evidence for continents in the Archean (4-2.5 Ga)?

 Oldest rocks (Acasta Gneiss, 3.96 Ga; Isua pillow lavas, 3.8 Ga)  3.46 Ga fossil soil (paleosol): Pilbara region, Australia

Liquid oceans  warm greenhouse atmosphere

Subaerial weathering, soil formation

Continents above sea level

Early Atmosphere

 Initial H, He lost to space  Volcanic outgassing  water vapour, CO2, SO2, H2S, CH4..  Meteorites/comets  water, nitrogen

 Convection in core  magnetic field deflects solar wind, so gases/atmosphere can accumulate

 Early atmosphere dense, very hot  Mostly water vapour, CO2, nitrogen

 Very little oxygen  Earth inhospitable to most forms of life as we know it today

Archean Fossils Most Archean fossils: stromatolites and single cells Stromatolite: dome-like layered structure formed from mat-like colonies (of single- celled cyanobacteria) that trap sediment and calcium carbonate

Oldest stromatolites: 3.5 Ga (W. Australia)

Image: http://hoopermuseum.earthsci.carleton.ca/stromatolites/ARCHEAN1.htm

https://www.newscientist.com/article/2217747-fossilised-microbes-from-3-5-billion-years-ago-are-oldest-yet-found/

Modern stromatolites

(extreme environments: e.g., high-salinity Shark Bay, Australia)

Image: http://geol.queensu.ca/museum/index.ph p?option=com_content&view=article&id= 50&Itemid=57

Archean single-celled micro-organisms

 Simplest form of modern carbon-based life  Lack DNA-packaging nuclei  Only life on Earth for next 2 billion years

Infer: Photosynthesis: occurring by 3.0 Ga,

possibly as early as 3.5 Ga  oxygen

Image: http://hoopermuseum.earthsci.carleton.ca/stromatolites/ARCHEAN1.htm

Origins of Life: What is Needed?

 Carbon Hydrogen Oxygen Nitrogen Phosphorus Sulfur

 Proteins (chains of amino acids): build living materials, catalysts for reactions in organisms

 Nucleic acids (DNA, RNA)

 Organic phosphorus: transforms light/chemical fuel  energy

 Cell membrane: encloses cell components

Amino acids formed in simulated early Earth atmosphere

Miller and Urey experiment (1950's):

Formed amino acids (building blocks of life) from:

H2, CH4 (methane), NH3 (ammonia), H2O (steam) gases & sparks (simulated lightning)

Image: http://history.nasa.gov/SP-349/ch1.htm

Early Organisms

 Developed in presence of an oxygen-free atmosphere (anaerobic - no oxygen for respiration)

 No oxygen  no ozone shield (O3) against harmful ultraviolet radiation

Where to live?  Below sediment – e.g., stromatolites  Beneath the surface of rocks?  Under water?

Deep-sea hydrothermal vents  Hyperthermophiles or

microbes thrive in seawater hotter than 100oC

 Derive energy by chemosynthesis, not by photosynthesis

 Hyperthermophiles are Archaea, different from bacteria (also single-celled)

 Possible environment for origin of life

(or did life arrive on an asteroid?)

Shen and Buick, 2004

O2 –rich atmosphere led to complex life forms?

The genetic tree of life

Single-celled Single-celled Different genes

Multi-celled, complex

Summary: Hadean-Archean - long book with few pages

 Age of Solar System 4.567 Ga  Oldest detrital zircons ~ 4.4 Ga Western Australia: implication of oceans  Oldest rocks 3.96 Ga, Acasta Gneisses, NWT Canada  Oldest supracrustal volcanics and seds 3.8 Ga, Isua, Greenland  Oldest well preserved fossils 3.5 Ga, W. Australia

Image: http://elements.geosci enceworld.org/conten t/gselements/2/4/201 /F3.large.jpg

Archean… Thoughts

 Faint Young Sun…  More radiogenic

heat production  No or little ozone  Only simple life  Fragmented record

 Earth’s surface T  Plate tectonics?  Life challenges  Crust?