Precambrian Earth and Life History—The Eoarchean and Archean Ch 8 Precambrian Time Span How many years Are represented By the Precambrian? How many years are Represented by the Archean Eon? What per cent of time Is represented by the Eoarchean (Hadean)? What per cent of time Is represented by the Entire Precambrian? Time check If a 24-hour clock represented all 4.6 billion years of geologic time the Precambrian would be slightly more than 21 hours long, It constitutes about 88% of all geologic time How big is a billion? 1. If you received $1,000 a day, seven days a week, how long would it take to reach a billion? 2. If you received $1 every second for your entire life, and you just reached $1 billion dollars, when were you born? 3. Scientists believe the earth to be approximately 4.5 billion years old. If someone had stashed $155 under a rock each year since the Big Bang or Creation, whichever is your cup of tea, we’d have about $700 billion just in time to solve the current financial crisis! Precambrian No rocks are known for the first 640 million years of geologic time The oldest known rocks on Earth are 3.96 billion years old Updated: http://www.sciencedaily.com/releases/2008 /09/080925144624.htm Ancient rocks such as the 3.9 billion year old Acasta Gneiss in Canada are metamorphic. What does that reveal about earth’s crust? What do detrital zircons reveal about earth’s crust? Rocks of the Precambrian Why is it difficult to interpret the geologic past of the Precambrian period? buried deep beneath younger rocks altered by metamorphism and deformed fossils are rare and not much help The Eoarchean: What is known? Except for meteorites no rocks of Hadean age are present on Earth, however we do know some events that took place during this period Earth was accreted Differentiation occurred, creating a core and mantle and at least some crust Earth beautiful Earth…. about 4.6 billion years ago • Shortly after accretion, Earth was a rapidly rotating, hot, barren, waterless planet – – – – bombarded by comets and meteorites There were no continents, intense cosmic radiation widespread volcanism Earth’s earliest crust formed Eoarchean crust was probably thin, unstable and made up of ultramafic rock rock with comparatively little silica Less than 40% SiO2 – ultramafic 40% - 60% SiO2 – mafic 60% to 70% SiO2 – intermediate Greater than 70% -- felsic This ultramafic crust was disrupted by upwelling basaltic magma at ridges and consumed at subduction zones Eoarchean continental crust may have formed by evolution of silica-rich material Sialic crust contains considerable silicon, oxygen and aluminum as in present day continental crust Only sialic-rich crust, because of its lower density, is immune to destruction by subduction Crustal Evolution A second stage in crustal evolution began as Earth’s production of radiogenic heat decreased Subduction and partial melting of earlier-formed basaltic crust resulted in the origin of andesitic island arcs Partial melting of lower crustal andesites, in turn, yielded silica-rich granitic magmas that were emplaced in the andesitic arcs Dynamic Processes During the Eoarchean, various dynamic systems similar to ones we see today, became operative, Once Earth differentiated into core, mantle and crust, internal heat caused interactions among plates they diverged, converged and slid past each other Continents began to grow by accretion along convergent plate boundaries Density of Continental Crust ~ 2.7g/cm3 Density of Oceanic Crust ~ 3.0 g/cm3 Continental Foundations: Precambrian Cratons Precambrian shields found on every continent Broad platforms of buried Precambrian rocks that underlie much of each continent Distribution of Precambrian Rocks • Areas of exposed Precambrian rocks constitute the shields • Platforms consist of buried Precambrian rocks Shields and adjoining platforms make up cratons Canadian Shield The craton in North America is the Canadian shield Occupies most of northeastern Canada, a large part of Greenland, parts of the Lake Superior region in Minnesota, Wisconsin, Michigan, and the Adirondack Mountains of New York Canadian Shield Rocks • Gneiss, a metamorphic rock, Georgian Bay Ontario, Canada Canadian Shield Rocks • Basalt (dark, volcanic) and granite (light, plutonic) on the Chippewa River, Ontario Archean Rocks The most common Archean Rock associations are granite-gneiss complexes The rocks vary from granite (felsic) to peridotite (ultramafic) to various sedimentary rocks all of which have been metamorphosed Greenstone belts are subordinate in quantity but are important in unraveling Archean tectonism Greenstone Belts An ideal greenstone belt has 3 major rock units volcanic rocks are most common in the lower and middle units the upper units are mostly sedimentary The belts typically have synclinal structure Most were intruded by granitic magma and cut by thrust faults Low-grade metamorphism makes many of the igneous rocks greenish (chlorite) Greenstone Belt Volcanics Abundant pillow lavas in greenstone belts indicate that much of the volcanism was under water Pyroclastic materials probably erupted where large volcanic centers built above sea level Pillow lavas in Ispheming greenstone at Marquette, Michigan Ultramafic Lava Flows The most interesting rocks in greenstone belts cooled from ultramafic lava flows Ultramafic magma has less than 40% silica requires near surface magma temperatures of more than 1600°C—250°C hotter than any recent flows During Earth’s early history, radiogenic heating was higher and the mantle was as much as 300 °C hotter than it is now This allowed ultramafic magma to reach the surface Sedimentary Rocks of Greenstone Belts Sedimentary rocks are found throughout the greenstone belts Mostly found in the upper unit Many of these rocks are successions of graywacke a sandstone with abundant clay and rock fragments and argillite a slightly metamorphosed mudrock Sedimentary Rocks of Greenstone Belts Small-scale cross-bedding and graded bedding indicate an origin as turbidity current deposits Quartz sandstone and shale, indicate delta, tidal-flat, barrier-island and shallow marine deposition Relationship of Greenstone Belts to Granite-Gneiss Complexes Two adjacent greenstone belts showing synclinal structure They are underlain by granite-gneiss complexes and intruded by granite Canadian Greenstone Belts In North America, most greenstone belts (dark green) occur in the Superior and Slave cratons of the Canadian shield Evolution of Greenstone Belts Models for the formation of greenstone belts involve Archean plate movement In one model, plates formed volcanic arcs by subduction the greenstone belts formed in back-arc marginal basins Evolution of Greenstone Belts According to this model, volcanism and sediment deposition took place as the basins opened Evolution of Greenstone Belts Then during closure, the rocks were compressed, deformed, cut by faults, and intruded by rising magma The Sea of Japan is a modern example of a back-arc basin Archean Plate Tectonics Plates must have moved faster residual heat from Earth’s origin more radiogenic heat magma was generated more rapidly Archean Plate Tectonics continental accretion: Continents grew quickly as plates collided with island arcs and other plates Also, ultramafic extrusive igneous rocks were more common due to the higher temperatures Southern Superior Craton Evolution Geologic map • Plate tectonic model for evolution of the southern Superior craton • North-south cross section • Greenstone belts (dark green) • Granite-gneiss complexes (light green Atmosphere and Hydrosphere Earth’s early atmosphere and hydrosphere were quite different than they are now They also played an important role in the development of the biosphere Today’s atmosphere is mostly nitrogen (N2) abundant free oxygen (O2) oxygen not combined with other elements such as in carbon dioxide (CO2) water vapor (H2O) ozone (O3) which is common enough in the upper atmosphere to block most of the Sun’s ultraviolet radiation Present-day Atmosphere Nonvariable gases Nitrogen N2 78.08% Oxygen O2 20.95 Argon Ar 0.93 Neon Ne 0.002 Others 0.001 in percentage by volume Variable gases Water vapor H2O 0.1 to .4 Carbon dioxide CO2 0.034 Ozone O3 0.0006 Other gases Trace Particulates normally trace Earth’s Very Early Atmosphere Earth’s very early atmosphere was probably composed of hydrogen and helium, the most abundant gases in the universe What are two reasons that Earth’s early atmosphere was most likely lost to space? Outgassing Once a core-generated magnetic field protected Earth, gases released during volcanism began to accumulate Called outgassing Water vapor is the most common volcanic gas today also emitted carbon dioxide sulfur dioxide Hydrogen Sulfide carbon monoxide Hydrogen Chlorine nitrogen Early Precambrian Atmosphere Eoarchean volcanoes probably emitted the same gases, and thus an atmosphere developed but one lacking free oxygen and an ozone layer 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 Evidence for an Oxygen-Free Atmosphere Some of the evidence for this conclusion comes from detrital deposits containing minerals that oxidize rapidly in the presence of oxygen pyrite (FeS2) uraninite (UO2) Oxidized iron becomes increasingly common in Proterozoic rocks Introduction of Free Oxygen Two processes account for introducing free oxygen into the atmosphere, 1. Photochemical dissociation involves ultraviolet radiation in the upper atmosphere The radiation breaks up water molecules and releases 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 organism that practiced photosynthesis Photosynthesis Photosynthesis is a metabolic process in which carbon dioxide and water combine into organic molecules and oxygen is released as a waste product CO2 + H2O ==> organic compounds + O2 “autotrophic food” 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 Earth’s Surface Waters Outgassing was responsible for the early atmosphere and also for Earth’s surface water the hydrosphere Some but probably not much of our surface water was derived from icy comets At some point during the Eoarchean, the Earth had cooled sufficiently so that the abundant volcanic water vapor condensed and began to accumulate in oceans Oceans were present by Early Archean times Ocean water The volume and geographic extent of the Early Archean oceans cannot be determined Volcanoes still erupt and release water vapor Is the volume of ocean water still increasing? Much of volcanic water vapor today is recycled surface water First Organisms Today, Earth’s biosphere consists of millions of species found in the “five kingdoms” bacteria, fungi, protists, plants, and animals only bacteria are found in Archean rocks We have fossils from Archean rocks 3.3 to 3.5 billion years old (ancient stromatolites) Carbon isotope ratios in rocks in Greenland that are 3.85 billion years old convince some investigators that life was present then But first, What Is Life? Minimally, a living organism must reproduce and practice some kind of metabolism Reproduction insures the long-term survival of a group of organisms whereas metabolism such as photosynthesis, for instance insures the short-term survival of an individual The distinction between living and nonliving things is not always easy Are viruses living? When in a host cell they behave like living organisms but outside they neither reproduce nor metabolize What Is Life? Comparatively simple organic (carbon based) molecules known as microspheres form spontaneously show greater organizational complexity than inorganic objects such as rocks can even grow and divide in a somewhat organism-like fashion but their processes are more like random chemical reactions, so they are not living How Did Life First Originate? To originate by natural processes, life must have passed through a prebiotic stage it showed signs of living organisms but was not truly living In 1924 A.I. Oparin postulated that life originated when Earth’s atmosphere had little or no free oxygen Oxygen is damaging to Earth’s most primitive living organisms Some types of bacteria must live where free oxygen is not present How Did Life First Originate? With little or no oxygen in the early atmosphere and no ozone layer to block ultraviolet radiation, life could have come into existence from nonliving matter The origin of life has 2 requirements a source of appropriate elements for organic molecules energy sources to promote chemical reactions Elements of Life All organisms are composed mostly of carbon (C) hydrogen (H) nitrogen (N) oxygen (O) All of which were present in Earth’s early atmosphere as: Carbon dioxide (CO2) water vapor (H2O) nitrogen (N2) and possibly methane (CH4) and ammonia (NH3) Basic Building Blocks of Life Energy from lightning ultraviolet radiation probably promoted chemical reactions during which C, H, N and O combined to form monomers comparatively simple organic molecules such as amino acids Monomers are the basic building blocks of more complex organic molecules Experiment on the Origin of Life During the late 1950s Stanley Miller synthesized several amino acids by circulating gases approximating the early atmosphere in a closed glass vessel Polymerization The molecules of organisms are polymers proteins nucleic acids RNA-ribonucleic acid and DNA-deoxyribonucleic acid consist of monomers linked together in a specific sequence How did polymerization take place? Water usually causes depolymerization, however, researchers synthesized molecules known as proteinoids some of which consist of more than 200 linked amino acids when heating dehydrated concentrated amino acids Proteinoids The heated dehydrated concentrated amino acids spontaneously polymerized to form proteinoids Perhaps similar conditions for polymerization existed on early Earth,but the proteinoids needed to be protected by an outer membrane or they would break down Experiments show that proteinoids spontaneously aggregate into microspheres are bounded by cell-like membranes grow and divide much as bacteria do Proteinoid Microspheres Proteinoid microspheres produced in experiments Proteinoids grow and divide much as bacteria do Protobionts Protobionts are intermediate between inorganic chemical compounds and living organisms Because of their life-like properties the proteinoid molecules can be referred to as protobionts Monomer and Proteinoid Soup The origin-of-life experiments are interesting, but what is their relationship to early Earth? Monomers likely formed continuously and by the billions and accumulated in the early oceans into a “hot, dilute soup” (J.B.S. Haldane, British biochemist) The amino acids in the “soup” might have washed up onto a beach or perhaps cinder cones where they were concentrated by evaporation and polymerized by heat The polymers then washed back into the ocean where they reacted further Next Critical Step Not much is known about the next critical step in the origin of life the development of a reproductive mechanism The microspheres divide and may represent a protoliving system but in today’s cells, nucleic acids, either RNA or DNA, are necessary for reproduction The problem is that nucleic acids cannot replicate without protein enzymes, and the appropriate enzymes cannot be made without nucleic acids, or so it seemed until fairly recently Azoic (“without life”) Prior to the mid-1950s, scientists had little knowledge of Precambrian life They assumed that life of the Cambrian must have had a long early history but the fossil record offered little to support this idea A few enigmatic Precambrian fossils had been reported but most were dismissed as inorganic structures of one kind or another The Precambrian, once called Azoic (“without life”), seemed devoid of life Oldest Know 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 Stromatolites Different types of stromatolites include irregular mats, columns, and columns linked by mats Stromatolites Present-day stromatolites form and grow as sediment grains are trapped on sticky mats of photosynthesizing blue-green algae (cyanobacteria) they are restricted to environments where snails cannot live 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 Other Evidence of Early Life Carbon isotopes in rocks 3.85 billion years old in Greenland indicate life was perhaps present then The oldest known cyanobacteria were photosynthesizing organisms but photosynthesis is a complex metabolic process A simpler type of metabolism must have preceded it No fossils are known of these earliest organisms Earliest Organisms The earliest organisms must have resembled tiny anaerobic bacteria they required no oxygen They must have totally depended on an external source of nutrients that is, they were heterotrophic as opposed to autotrophic organisms that make their own nutrients, as in photosynthesis They all had prokaryotic cells they lacked a cell nucleus and lacked other internal cell structures typical of eukaryotic cells (to be discussed later in the term) An alternative to the ocean/land model of earliest life forms: Hydrothermal vents, high in metals and sulfides, may have contained the materials and energy (earth’s heat) to cause polymerization of monomers. Previously unknown life communities are being observed today in these volcanic vents under the oceans. Fossil Prokaryotes Photomicrographs from western Australia’s 3.3- to 3.5billion-year-old Warrawoona Group with schematic restoration shown at the right of each Iron Banded Iron formations are sedimentary rocks consisting of alternating layers of silica (chert) and iron minerals About 6% of the world’s banded iron formations were deposited during the Archean Eon Although Archean iron ores are mined in some areas they are neither as thick nor as extensive as those of the Proterozoic Eon, which constitute the world’s major source of iron