The Precambrian Record Precambrian Time • Timeframe: 4.6 billion to 570 million years ago) • 85% of Earth’s History!! • Simple Life starts in the oceans EARTH DURING PRECAMBRIAN TIME EARTH DURING PRECAMBRIAN TIME Precambrian Was called a Supereon The Precambrian Supereon Is divided into three Precambrian eons, the Proterozoic, the Archaean, and Hadean. There were not many fossils during Precambrian Time, because Precambrian Time had a lot of soft bodied. Important Facts Many Precambrian fossils are Trace fossils. Land (Geological) Features Important Facts Earth formed Life arose First tectonic plates arose and began to move Atmosphere became enriched in oxygen 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 Precambrian Era Simple life starts in the oceans Primarily covered by the ocean and Had a lot of volcanic activity. 4,600,000,000 years ago to 570,000,000 years ago 85% of Earth’s History Precambrian •4.6 billion years to, say, 548 or 544 million years (depending on method). •Represents 88% of all of the history of the earth. •Referred to as the Cryptozoic Eon. –“hidden life” (no more BIFs) (prokaryotes) Early Hadean Highlights 1 Earth formed about 4.6 billion years ago from coalescing interstellar dust. Earth was bombarded by large planetesimals adding to earth’s mass (adds heat) Hot spinning pre-earth mass melted, caused differentiation of materials according to density. Distinct earth layers begin to form Dense iron and nickel migrate to center (core) silicate material moves out to mantle Early Hadean Highlights 2 Huge impact from a Mars-sized planetessimal created the moon. Caused earth to spin faster. Possible Tilt change Moon controls earth’s spin and creates tidal forces. Moon’s orbit at an angle to planets around Sun Earth got most of the core – outer part molten. Earth rotates. We have magnetic field and, therefore, an atmosphere Moon Origin hypotheses -1 Speed and approach angle unlikely Moon Origin hypotheses - 2 Chemical composition of the Moon suggests that it could not have co-formed with the earth. Moon Origin hypotheses - 3 The center of Earth was melted by great pressure and intense heat. Oldest rock found in Western Australia 4.1BYA The surface of Earth cooled forming the crust An atmosphere formed as Earth cooled. primitive atmosphere forms from volcanic eruptions Did NOT contain oxygen gas Oceans form from water vapor released in eruptions & maybe comets The Earth Forms 4.5BYA Rocks in the solar system condensed to form Earth 6 BYA The Solar system formed from swirling gases and dust 4.6 BYA Precambrian Time A molten Earth forms 4.3 BYA Precambrian Time Surface cools, forming the crust Oldest rock found in Australia & Canada primitive atmosphere forms from volcanic eruptions (No Free oxygen gas). Oceans form (from water vapor released in eruptions & comets) Comparison of Atmospheres Precambrian Early Atmosphere •First earth atmosphere H He lost to solar wind. No magnetic field •Early permanent earth atmosphere mostly Nitrogen (inert) and CO2 Post-differentiation start of liquid core dynamo •Liquid water is required to remove CO2 from atmosphere. –Mars is too cold to have liquid water. –Venus is too hot to have liquid water. –So both have CO2 atmospheres. •On Earth, most of the world’s CO2 was converted to O2 by photosynthesis. •Enough by 2.0 bya to sustain life. •CO2 is locked up in life, limestones, dolomites. Mars Venus Earth Early Permanent Atmosphere • Gasses from cooling magmas formed early atmosphere mostly N2, CO2, with CH4, H2O •Early earth not conducive to modern oxygen breathing organisms: too much solar UV gets through atmosphere. • Little oxygen occurred in the atmosphere until the evolution of photosynthetic organisms (Eubacteria) 3.5 billion years ago. Fully oxygenated about 1.9 billion years ago. Where did Life Start? Oceans (nutrient rich, possibly with complex molecules) Ice (opportunity for organization) Meteorites (rich with building blocks of life) Precambrian Early Oceans from 4 bya OCEANS •Much water vapor from volcanic degassing. •Salt in oceans is derived from weathering and carried to the oceans by rivers. •Blood of most animals has chemistry of seawater. •Part of the earth’s water probably came from comets. –Comets are literally large dirty snowballs. –Provide fresh water. First continental crust At high temperatures, only Olivine and Ca-Plagioclase crystallize “Komatiite” First Komatiite partially melts, Basalt gets to surface, piles up. The stack sinks, partially melts when pressure high enough. Fractionation makes increasingly silica-rich magmas Then: Water out Density differences allow subduction of mafic rocks. Further partial melting and fractionation makes higher silica melt that won’t subduct Archean: Growth of the early continents Magmatism from Subduction Zones causes thickening 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 SNOWBALL EARTH SNOWBALL EARTH Break up of Rodinia Hypothesis: Ice an insulator, heat builds up Heavy volcanic activity poured CO2 into atmosphere – greenhouse effect Warming melted snowball earth Animals/Organisms(Biological that First Appeared Most organisms had soft bodies which looked like modern jellyfish and worms Stromatolites: layered mats formed by colonies or groups of cyanobacteria (blue-green algae thought to be the first or one of the earliest life forms on Earth.) Invertebrates (animals with no backbone) appeared near the end of the Precambrian Time 3.5 BYA Precambrian Time Oldest fossils of bacteria (prokaryotes). Simple and small Lack a nucleus Do not need oxygen to survive The Precambrian Era 4.5 billion – 570 million years ago Banded iron is believed to have formed when oxygen released by blue green algae combined with iron present in ancient ocean waters to form iron oxide precipitates that settled to the ocean floor. Stromatolites are dome shaped mineral formations built by microbes. They continue to survive today in the waters around Australia. http://www.eps.mcgill.edu http://gsc.nrcan.gc.ca/paleochron/03_e.php http://gsc.nrcan.gc.ca/paleochron/03_e.php Fossil stromatolites are one of the most common forms of fossil life identified in Precambrian rocks. 3.0 BYA Precambrian Time Cyanobacteria (blue-green algae) evolve creates energy from photosynthesis gives off oxygen gas (beginning to form the atmosphere of today) Form large stromatolites (below) 1.5 BYA Precambrian Time Eukaryotes contain a nucleus more complex, larger, single celled organisms Evolved from prokaryotes 1.5 BYA Eukaryotes (complex, larger, single celled organisms) evolve from prokaryotes (bacteria). 630 MYA Precambrian Time Multicellular organisms evolve with soft bodies Species include: jelly fish; coral stalks; segmented worms; algae WHAT CAUSED THE MASS EXTINCTION MASS EXTINCTION IS THE EVENT THAT ENDED PRECAMBRIAN TIME. * glaciation event Precambrian time were marked by a prolonged global ice age. This may have led to widespread extinctions.
