Precambrian Earth and Life History—The Eoarchean and Archean 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 silica-rich 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 but associations of passive continental margin sediments are widespread in Proterozoic terrains 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 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 Greenstone belts (dark green) Granite-gneiss complexes (light green Geologic map Plate tectonic model for evolution of the southern Superior craton North-south cross section

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 4.0 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 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 3.5-billion-year-old Warrawoona Group with schematic restoration shown at the right of each