Precambrian Earth and Life History

1. Precambrian Earth and Life History

2. Precambrian Time Span • 88% of geologic time The Precambrian lasted for more than 4 billion years!

3. Precambrian • The term Precambrian is informal but widely used, referring to both time and rocks • The Precambrian: 4.6 bya to 542 mya • Oldest known rocks are 3.96 by old • Other information about pC is inferred using what we know about planet formation

4. Key Events of Precambrian time Acasta Gneiss is dated at 3.96 bya. It is near Yellowknife Lake , NWT Canada Zircons possibly a bit older in Australia

6. Hot, Barren, Waterless Early Earth • Shortly after accretion, Earth was • a rapidly rotating, hot, barren, waterless planet • bombarded by comets and meteorites • with no continents, intense cosmic radiation • and widespread volcanism • about 4.6 billion years ago

7. Oldest Rocks • Judging from the oldest known rocks on Earth, the 3.96-billion-year-old Acasta Gneiss in Canada and other rocks in Montana and Greenland some continental crust had evolved by early Archean time (3.8 bya) Sedimentary rocks in Australia contain detrital zircons (ZrSiO4) dated at 4.4 billion years old so source rocks at least that old existed • These rocks indicte that some kind of Eoarchean crust was certainly present, but its distribution is unknown

8. Early Archean Crust • Early Archean crust was probably thin and made up of ultramafic rock • igneous rock with less than 45% silica • This ultramafic crust was disrupted by upwelling mafic magma at ridges, and the first island arcs formed at subduction zones • Early Archean continental crust may have formed by collisions between island arcs as silica-rich materials were metamorphosed. • Larger groups of merged island arcs (protocontinents) grew faster by accretion along their margins

9. Origin of Continental Crust • Andesitic island arcs • form by subduction and partial melting of oceanic crust • The island arc collides with another (accretion)

10. Each continent has an ancient, relatively flat interior with very little tectonic or mountain-building activity. These large “tracts” of exposed metamorphic rocks in continental interiors are called Precambrian shields, and are some of the oldest crustal rocks. M&W, Fig. 7.1

11. Continental Foundations • Continents consist of rocks with composition similar to that of granite • Continental crust is thicker and less dense than oceanic crust • Precambrian shields consist of vast areas of exposed ancient rocks and are found on all continents • Outward from the shields are broad platforms of buried Precambrian rocks that underlie much of each continent

12. Cratons • A shield and platform make up a craton, a continent’s ancient nucleus • Along the margins of cratons, more continental crust was added as the continents took their present sizes and shapes • Both Archean and Proterozoic rocks show evidence of episodes of deformation accompanied by metamorphism, igneous activity, and mountain building • Cratons have experienced little deformation since the Precambrian

13. Canadian Shield • The exposed part of the craton in North America is the Canadian shield which occupies most of northeastern Canada a large part of Greenland parts of the Lake Superior region in Minnesota, Wisconsin, and Michigan and the Adirondack Mountains of New York • Its topography is subdued, with numerous lakes and exposed Archean and Proterozoic rocks thinly covered in places by Pleistocene glacial deposits

14. Canadian Shield Rocks • Outcrop of Archean gneiss in the Canadian Shield in Ontario, Canada

15. Archean Rocks Beyond the Shield • Archean Brahma Schist in the deeply eroded parts of the Grand Canyon, Arizona

16. Early Continents (Cratons) Archean • Archean cratons consist of regions of light-colored felsic rock (granulite gneisses) • surrounded by pods of dark-colored greenstone (chlorite-rich metamorphic rocks). • Pilbara Shield, Australia • Canadian Shield • South African Shield Mafic Greenstone Belts Felsic Islands 40km

17. Archean Plate Tectonics • Plate tectonic activity has operated since the early Proterozoic (or perhaps late Archean) • Most geologists are convinced that some kind of plate tectonic activity took place during the Archean as well but it differed in detail from today • Plates must have moved faster with more residual heat from Earth’s origin and more radiogenic heat, and magma was generated more rapidly

18. Formation of Rodinia Grenville Orogeny 1.3-1.0 bya Collisions between N. Am, S. Am, Africa and Antartica creates supercontinent Climate change!

20. Snowball Earth • Rodinia: abundant basalts with easily weathered Ca feldspars. Ocean gets Ca+ + . CO2 tied up in extensive limestones. Less greenhouse effect. Atmosphere can’t trap heat – Earth gets colder • Grenville Orogeny left extensive highlands from high latitudes to equator • About 635 mya glacial deposits found in low latitudes and elevations • Huge Ice sheet reflects solar radiation “Albedo” • Some workers believe oceans froze

21. Break up of Rodinia • Hypothesis: Ice an insulator, heat builds up • Heavy volcanic activity poured CO2 into atmosphere – greenhouse effect • Warming melted snowball earth

22. Earth’s Very Early Atmosphere • Earth’s very early (Hadean-Archean) atmosphere was probably composed of hydrogen and helium • If so, it would have quickly been lost into space because Earth’s gravity is insufficient to retain them and because Earth had no magnetic field until its core formed (magnetosphere) • Without a magnetic field the solar wind would have swept away any atmospheric gases

23. Outgassing • Once a magnetosphere was present atmosphere began accumulating as a result of outgassing • Water vapor is the most common volcanic gas today but volcanoes also emit carbon dioxide, sulfur dioxide, carbon monoxide, sulfur compounds, hydrogen, chlorine and nitrogen

24. Archean Atmosphere • Archean volcanoes probably emitted the same gases, thus an atmosphere developed • It was rich in carbon dioxide, and gases reacting in this early atmosphere probably formed • ammonia (NH3) • methane (CH4) • This early atmosphere persisted throughout the Archean

25. Evidence for an Oxygen-Free Atmosphere • The atmosphere was chemically reducing • rather than an oxidizing one • Some of the evidence for this conclusion comes from detrital deposits containing minerals that oxidize rapidly in the presence of oxygen • pyrite (FeS2) • But oxidized iron becomes increasingly common in Proterozoic rocks indicating that at least some free oxygen was present then

26. Introduction of Free Oxygen • Two processes account for introducing free oxygen into the atmosphere, one or both of which began during the early Archean. 1. Photochemical dissociation involves ultraviolet radiation in the upper atmosphere • The radiation disrupts water molecules and releases their oxygen and hydrogen • This could account for 2% of present-day oxygen but with 2% oxygen, ozone forms, creating a barrier against ultraviolet radiation 2. More important were the activities of organisms that practiced photosynthesis

27. Photosynthesis • Photosynthesis is a process in which carbon dioxide and water combine into organic molecules and oxygen is released as a waste product 6CO2 + 6H2O + sunlight + chlorophyll  C6H12O6 + O2 • Even with photochemical dissociation and photosynthesis, probably no more than 1% of the free oxygen level of today was present by the end of the Archean

28. Earth’s Surface Waters • Outgassing was responsible for the early atmosphere and also for Earth’s surface water the hydrosphere • However, some—but probably not much— of our surface water was derived from icy comets • Once Earth had cooled sufficiently, the abundant volcanic water vapor condensed and began to accumulate in oceans • Oceans were present during early Archean times

29. Decreasing Heat • Ratio of radiogenic heat production in the past to the present • The width of the colored band indicates variations in ratios from different models • Heat production 4 billion years ago was 3 to 6 times as great as it is now • With less heat outgassing decreased

30. First Organisms • Today, Earth’s biosphere consists of millions of species of bacteria, archea, fungi, protistans, plants, and animals, whereas only bacteria and archea are found in Archean rocks • We have fossils from Archean rocks 3.3 to 3.5 billion years old • Chemical evidence in rocks in Greenland that are 3.85 billion years old convince some investigators that organisms were present then

31. Oldest Known Organisms • The first organisms were members of bacteria and/or archaea, both of which consist of prokaryotic cells, cells that lack an internal, membrane-bounded nucleus and other structures • Prior to the 1950s, scientists assumed that life must have had a long early historybut the fossil record offered little to support this idea • The Precambrian, once called Azoic (“without life”), seemed devoid of life

32. Oldest Known Organisms • Charles Walcott (early 1900s) described structures from the early Proterozoic Gunflint Iron Formation of Ontario, Canada that he proposed represented reefs constructed by algae • Now called stromatolites, not until 1954 were they shown to be products of organic activity Present-day stromatolites (Shark Bay, Australia)

33. Stromatolites • Present-day stromatolites form and grow as sediment grains (calcium carbonate) are trapped on sticky mats of photosynthesizing cyanobacteria. • The oldest known undisputed stromatolites are found in rocks in South Africa that are 3.0 billion years old but probable ones are also known from the Warrawoona Group in Australia which is 3.3 to 3.5 billion years old

34. At right is a layered stromatolite, produced by the activity of ancient cyanobacteria. The layers were produced as calcium carbonate precipitated over the growing mat of bacterial filaments; photosynthesis in the bacteria depleted carbon dioxide in the surrounding water, initiating the precipitation. The minerals, along with grains of sediment precipitating from the water, were then trapped within the sticky layer of mucilage that surrounds the bacterial colonies, which then continued to grow upwards through the sediment to form a new layer. As this process occured over and over again, the layers of sediment were created.

35. Earliest Organisms • The earliest organisms must have resembled tiny anaerobic bacteria meaning they required no oxygen • They must have totally depended on an external source of nutrients that is, they were heterotrophic • They all had prokaryotic cells • The earliest organisms, then, were anaerobic, heterotrophic prokaryotes

36. Fossil Prokaryotes • Photomicrographs from western Australia’s 3.3- to 3.5-billion-year-old Warrawoona Group, with schematic restoration shown at the right of each