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Transcript
Chapter 8 Precambrian Earth and Life History—The Eoarchean and Archean Time check • The Precambrian lasted for more than 4 billion years! – Such a time span is almost impossible for us comprehend • 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 Precambrian Time Span Precambrian • The term Precambrian is informal term referring to both time and rocks • It includes time from Earth’s origin 4.6 billion years ago to the beginning of the Phanerozoic Eon 545 million years ago • 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 Rocks of the Precambrian • The earliest record of geologic time preserved in rocks is difficult to interpret because many Precambrian rocks have been • • • • • altered by metamorphism complexly deformed buried deep beneath younger rocks fossils are rare the few fossils present are of little use in stratigraphy • Because of this subdivisions of the Precambrian have been difficult to establish • Two eons for the Precambrian – the Archean and Proterozoic Eons of the Precambrian • The onset of the Archean Eon coincides with the age of Earth’s oldest known rocks • approximately 4 billion years old • lasted until 2.5 billion years ago (the beginning of the Proterozoic Eon) • The Eoarchean is an informal designation for the time preceding the Archean Eon • Precambrian eons have no stratotypes – the Cambrian Period, for example, which is based on the Cambrian System, a time-stratigraphic unit with a stratotype in Wales – Precambrian eons are strictly terms denoting time US Geologic Survey Terms • Archean and Proterozoic are used in our discussions of Precambrian history, but the U.S. Geological Survey (USGS) uses different terms • Precambrian W begins within the Early Archean and ends at the end of the Archean • Precambrian X corresponds to the Early Proterozoic, 2500 to 1600 million years ago • Precambrian Y, from 1600 to 800 million years ago, overlaps with the Middle and part of the Late Proterozoic • Precambrian Z is from 800 million years to the end of the Precambrian, within the Late Proterozoic The Hadean? • Except for meteorites no rocks of Eoarchean 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 Oldest Rocks • Judging from the oldest known rocks on Earth, the 3.96-billion-year-old Acasta Gneiss in Canada some continental crust had evolved by 4 billion years ago • Sedimentary rocks in Australia contain detrital zircons (ZrSiO4) dated at 4.2 billion years old • so source rocks at least that old existed • These rocks indicted that some kind of Hadean crust was certainly present, but its distribution is unknown Hadean Crust • Early Hadean crust was probably thin, unstable and made up of ultramafic rock » rock with comparatively little silica • This ultramafic crust was disrupted by upwelling basaltic magma at ridges and consumed at subduction zones • Hadean continental crust may have formed by evolution of sialic 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 silicarich granitic magmas that were emplaced in the andesitic arcs Crustal Evolution • Several sialic continental nuclei had formed by the beginning of Archean time by subduction and collisions between island arcs Dynamic Processes • During the Hadean, various dynamic systems similar to ones we see today, became operative, – not all at the same time nor in their present forms • 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 Continental Foundations • Continents consist of rocks with composition similar to that of granite • Continental crust is thicker and less dense than oceanic crust which is made up of basalt and gabbro • 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 Cratons • A shield and platform make up a craton – a continent’s ancient nucleus and its foundations • Along the margins of cratons, more continental crust was added as the continents took their present sizes and shapes • Both Archean and Proterozoic rocks are present in cratons and show evidence of episodes of deformation accompanied by – Metamorphism – igneous activity – and mountain building • Cratons have experienced little deformation since the Precambrian 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 • It’s topography is subdued, with numerous lakes and exposed Archean and Proterozoic rocks thinly covered in places by Pleistocene glacial deposits 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 Amalgamated Cratons • The Canadian shield and adjacent platform consists of numerous units or smaller cratons that were welded together along deformation belts during the Early Proterozoic – Absolute ages and structural trends help geologists differentiate among these various smaller cratons Archean Rocks • The most common Archean Rock associations are granite-gneiss complexes • The rocks vary from granite to peridotite 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 • Plate tectonic activity has operated since the Early Proterozoic or earlier • Most geologists are convinced that some kind of plate tectonics took place during the Archean as well but it differed in detail from today • Plates must have moved faster – residual heat from Earth’s origin – more radiogenic heat • magma was generated more rapidly Archean Plate Tectonics • As a result of the rapid movement of plates, continents, no doubt, grew more rapidly along their margins a process called continental accretion as plates collided with island arcs and other plates • Also, ultramafic extrusive igneous rocks were more common due to the higher temperatures Archean World Differences – We have little evidence of Archean rocks – deposited on broad, passive continental margins – Deformation belts between colliding cratons – indicate that Archean plate tectonics was active – but associations of passive continental margin sediments – are widespread in Proterozoic terrains – but the ophiolites so typical of younger convergent plate boundaries are rare, – although Late Archean ophiolites are known The Origin of Cratons • Certainly several small cratons existed by the beginning of the Archean • During the rest of that eon they amalgamated into a larger unit – during the Early Proterozoic • By the end of the Archean, 30-40% of the present volume of continental crust existed • Archean crust probably evolved similarly – to the evolution of the southern Superior craton of Canada 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 Carbon dioxide CO2 Ozone O3 Other gases • Particulates 0.1 to 4.0 0.034 0.0006 Trace 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 • If so, it would have quickly been lost into space because Earth’s gravity is insufficient to retain them. – Also because Earth had no magnetic field until its core formed the solar wind would have swept away any atmospheric gases 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 Hadean-Archean Atmosphere • Hadean 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 • 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) • 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 • 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 Hadean, 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 • Nevertheless, we can envision an early Earth with considerable volcanism and a rapid accumulation of surface waters • 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 of bacteria, fungi, protistans, plants, and animals, – only bacteria are found in Archean rocks • We have fossils from Archean rocks – 3.3 to 3.5 billion years old • Carbon isotope ratios in rocks in Greenland that are 3.85 billion years old convince some investigators that life was present then 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 evaporationand 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 – meaning 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 – meaning they lacked a cell nucleus – and lacked other internal cell structures typical of eukaryotic cells (to be discussed later in the term) 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