PRECAMBRIAN (CRYPTOZOIC)

Precambrian Subdivisions

• Hadean (4.6 - 4.0 Ga)

  • Defined to be from the Earth's formation to the oldest rocks. (The oldest known rock is the Acasta gneiss of NW Canada, dating to 4.03 billion years ago. The oldest known Earth materials are 4.3 to 4.4 billion-year-old zircons from ancient metamorphosed sediments in the Jack Hills, Australia.)

  • A time when the Earth differentiated into core, mantle, and iron-rich crust as heavier elements generally sank and lighter ones rose. Due to chemical affinities (elements prefer the company of some elements but not others), some heavier elements rose and lighter ones sank.

  • A time of intense bombardment from debris left over from the formation of the solar system. The moon is thought to have formed as a result of a collision between the Earth and a Mars-sized body.

  • Not much is known about the Hadean atmosphere. It was likely highly variable due to the numerous collisions with asteroids. Probably water was added to the Earth by these collisions.

  • Alternative view: Controversial evidence from the Jack Hills zircons suggests the Earth was at least occasionally (say, between massive collisions) capable of forming continental crust with surface water present. This evidence has to do with the oxygen isotope content of the zircons as well as their inferred temperature of formation (cooler than expected). Any continental crust present had to be rare, however (to satisfy hafnium isotope data in those same zircons, if you must know.)

• Archean Eon (4.0 Ga - 2.5 Ga)

  • A time when stable continents did not exist. The Earth was too hot for modern-style plate tectonics, and the crust was very mobile with abundant volcanism.

  • There was, however, liquid water on the surface as indicated by sedimentary deposits.

  • The atmosphere contained very little oxygen but did contain a large amount of greenhouse gas - possibly carbon dioxide but more likely methane.

• Proterozoic Eon (2.5 Ga - 543 Ma)

  • A time of stable continents but only single-celled life (until the very end of the eon). The Earth had cooled down enough that a more modern form of plate tectonics was probably active.

  • Oxygen generally increased in the atmosphere.

Introduction to Precambrian Rocks

• Structure of the Continental Crust

  • craton - a large stable area of a continent, consisting of a shield and a platform.

  • shield - that part of a craton where Precambrian crystalline (igneous and/or metamorphic) rocks are exposed to the surface.

  • platform - that part of a craton where Precambrian crystalline rocks are covered by younger sediments.

  • Canadian shield - exposed Precambrian rocks in northeastern North America.

• Studying Archean Rocks

  • Archean rocks are usually highly altered by folding, faulting, and metamorphism, making them difficult to study.

  • Archean rocks lack distinctive fossils to aid in correlation and establishing the relative times of events.

  • They are often buried under younger sediments and so are either inaccessible or accessible only with great effort.

  • Radiometric dating is the major tool in establishing Archean chronology.

• Studying Proterozoic Rocks

  • Proterozoic rocks are generally not as altered as Archean rocks.

  • Lack of distinctive fossils still makes radiometric dating the main technique in establishing the order of events.

• Origin of Continental Crust (hence the origin of granitic crustal rocks)

  • original crust - probably ultramafic and mafic due to its origin in the ultramafic mantle. The formation of a crust high in silicon and oxygen (silica) - basically granitic in composition - as we have today, took time.

  • partial melting - a process whereby magma higher in silica can arise from mafic rock. Since minerals high in silica have a lower melting temperature, a relatively silica-rich melt can form between mafic crystals when a mafic rock first begins to melt. If that melt is somehow squeezed out, it can collect to form a more silica-rich magma. Some granites have been shown to have been formed in this manner.

  • melting of sediment - where deeply buried sediment is "recycled" back into the crust by being melted and mobilized as an intrusive magma. Some granites have been shown to have formed this way.

  • partial melting with subsequent incorporation of preexisting crustal material into the magma - a sort of a combination of the previous two processes. There is recent evidence this may be the origin of most granitic rock.

  • crystal settling - a process whereby a mafic magma can give rise to a magma higher in silica. Since minerals high in iron and magnesium have higher melting temperatures, they tend to crystallize first in a magma chamber. If these minerals then sink to the floor of the chamber they will leave behind a magma richer in silica. This process is thought to be of minor importance compared to partial melting and melting of preexisting crustal rocks.

Archean Rocks

• Types of Archean Rocks and Their Origin

  • granite-gneiss complexes - consist mainly of schists, gneisses, and igneous rocks with metamophosed sedimentary rocks.

  • greenstone belts - consist of metamorphosed basalts, often extruded underwater as evidenced by pillow-lava forms, overlain by clastic sedimentary rocks. They are called "greenstone" belts because of the greenish hue many of the rocks have due to the green metamorphic mineral chlorite.

  • origin of granite-gneiss complexes - They probably represent the earliest continental material formed by intense moutain-building episodes (orogenies).

  • origin of greenstone belts - They appear to be formed from sediments and volcanics that were compressed, folded, faulted, and intruded by plutonic igneous rock.

  • the belt/complex story - What these two types of rocks may be telling us is that there was lots of island-arc activity in the Archean: The greenstone belts originated as basins (by extension of the crust) associated with island arcs and were filled with volcanic rocks and sediments. Later the arcs and basins were compressed during orogenies to form the granite-gneiss and greenstone belts. This was a type of rapid plate tectonics not seen in today's cooler Earth.

  • absence of continental shelf rocks - Notable by their absence are Archean rocks that indicate a stable continental shelf environment. This tends to confirm the view that plate tectonics was different in the Archean.

  • absence of ophiolite suites - An ophiolite suite is a series of rocks that appears to be a slice of oceanic crust associated with converging plate boundaries. They are common after the Archean but rare during it. (Two proposed Archean ophilite suites - one in China and another in Greenland - are controversial.) Again the inference is of a different type of plate tectonics in the Archean.

  • Slave Protocontinent and Plate Tectonics - The Slave craton contains the Acasta gniess, the oldest rocks known. Evidence exists that it was the nucleus of a larger protocontinent that included fragments now found in Wyoming, Zimbabwe, and other locations. Some think this continent amounted to the Earth's first supercontinent, called "Kenorland", and its breakup at the end of the Archean was the beginning of modern plate tectonics.

Proterozoic Rocks

• Differences between Archean and Proterozoic Rocks

  • There are fewer granite-gneiss and greenstone belts.

  • Ophiolites become common.

  • Proterozoic rocks exhibit less metamorphism than Archean rocks.

  • Abundant passive continental margin rocks appear, such as sandstones, shales, and carbonates.

• What Differences between Archean and Proterozoic Rocks Mean

  • Stable continental land masses have formed.

  • Modern-like plate tectonics processes are operating.

• Growth of Continents

  • The first continental masses probably formed by the consolidation of island-arc systems.

  • continental accretion - the theory that continents grew larger gradually over time by material being added to their margins.

  • evidence of continental accretion

    • Continental rocks often get older as you go toward the center of the continent.

    • Isotopic (Nb/Th) evidence supports gradual crustal growth.

    • Continents consist of orogens (also called structural provinces) - usually elongated regions of rocks that date to about the same age. These structural provinces are thought to be additions to the continent by means of orogenies on the continental margins.

  • Laurentia

    • Laurentia was a continental mass consisting of North America (except Mexico and Central America), Greenland, and parts of what is now northwestern Europe.

    • Laurentia grew during the Proterozoic by the addition of structural provinces ("orogens") to its margin.

    • The last orogen added to Laurentia was the Grenville orogen whose rocks (approx. 1 billion years old) are found in Greenland and NW Europe as well as North America

  • Proterozoic Supercontinents

    • A supercontinent consists of several continental pieces joined together into a single land mass.

    • Controversial: As previously mentioned a supercontinent called Kenorland may have existed at the end of the Archean.

    • Also controversial: A supercontinent, called Hudsonland may have existed 1.8 to 1.3 Ga. This supercontinent was proposed based on paleomagnetic evidence.

    • Rodinia is the oldest fairly well-established supercontinent. It existed 1.3 to 1.0 Ga and broke up around 800 Ma.

    • The continental fragments of Rodinia seem to have reassembled around 650 Ma, forming the supercontinent Pannotia, which broke up around 550 Ma.

Evolution of the Atmosphere

• Primary Atmosphere

  • Of uncertain nature, it contained gases given off by volcanic activity (volcanic outgassing) and possibly gases left over from the solar nebula out of which the solar system formed.

  • It was subject to disruption or even destruction by large impacts between the Earth and debris left over from solar system formation.

• Secondary Atmosphere

  • More is known about this atmosphere since we have data from ancient soils.

  • Consisted primarily of volcanic gases that interacted chemically under the influence of ultraviolet light ("photochemistry")

  • Likely constituents include water, carbon dioxide, ammonia, nitrogen, and probably methane from methane-producing microorganisms.

  • Methane could have been the major "greenhouse gas" that kept the Earth from freezing in spite of the lower energy output of the early sun (the so-called faint-sun problem).

  • It may have resembled the atmosphere of Saturn's moon, Titan, which has a nitrogen-methane atmosphere.

  • Little or no oxygen was present in the Archean as indicated by:

    • Banded-iron formations (BIF):

      • composed of alternating iron oxide (hematite) and chert layers deposited in seawater

      • Iron can only be dissolved in seawater if the ocean is devoid of oxygen.

      • The iron oxide was precipitated from iron-rich ocean water by the addition of oxygen, probably produced by seasonal blooms of photosynthetic microbes.

    • Unoxidized minerals in Archean soils (pyrite and uranite)

    • Sulfur isotope ratios in pyrite that could only exist in the absence of oxygen.

    • The lack of oxygen was important for the development of life because oxygen is chemically very reactive and early organic matter could not exist in an oxygen environment.

  • Oxygen generally increased in the Proterozoic as indicated by:

    • the disappearance of the BIFs

    • the appearance of oxidized sediments (redbeds)

    • the appearance of oxidized minerals in soils

    • change in sulfur isotope ratios indicating the presence of oxygen

• Modern Atmosphere

  • Composed primarily of nitrogen, oxygen, water vapor, argon, carbon dioxide, and smaller amounts of other gases.

  • nitrogen from the breakup of ammonia and other nitrogen-bearing gases by ultraviolet light

  • oxygen from photosynthesis

  • carbon dioxide dissolves in seawater and is taken out of the atmosphere; becomes (mainly) carbonate rocks

Proterozoic Glaciation

There were two episodes of glaciation in the Proterozoic, one in the early Proterozoic and one toward the end of the Proterozoic.

• Early Proterozoic Glaciation

  • The first record of glaciation in Earth history appears in the early Proterozoic, around 2.4 Ga.

  • It was possibly the result of the destruction of methane by the increasing oxygen content of the atmosphere.

  • Its extent is not certain but some geologists think the Earth may have been entirely covered with ice at some times.

• Late Proterozoic Glaciation

  • There are four major geological paradoxes in the late Proterozoic:

    • glacial sediments at the equator

    • reappearance of the BIFs, temporarily replacing the oxidized redbeds

    • thick carbonate sequences on top of the glacial deposits

    • the greatest change in carbon isotope ratio in geological history

  • Snowball Earth Theory (SET) explains these paradoxes as follows.

    • Glacial sediments at the equator require the Earth to be entirely covered with ice.

    • A weakening greenhouse effect (possibly due indirectly to the arrangement of the continents) and the faint sun led to runaway glaciation.

    • Ice covered oceans became depleted in oxygen, allowing the BIFs to be formed once again.

    • Ice covered oceans could not absorb carbon dioxide emitted by volcanoes, allowing its buildup in the atmosphere.

    • When the carbon dioxide concentration in the atmosphere reached a critical value, the greenhouse effect overcame the cooling effect of the ice, the ice melted, and the Earth became a "hothouse".

    • These extremes drove life to the brink of extinction as reflected in the carbon isotope ratios.

    • Gradually, the now ice-free oceans absorbed the excess carbon dioxide, leading to another glaciation.

    • This snowball-hothouse cycle continued until, eventually, the atmosphere and the strengthening sun came into balance and the glaciation ended.

Precambrian Life

• Origin of Life: Materials and Processes Needed

  • Raw materials: carbon, hydrogen, oxygen, etc., are everywhere and plentiful.

  • Monomers: molecules that can be joined together to form larger molecules (amino acids, nucleotides, etc.) are easily formed by inorganic chemical processes in the absence of oxygen.

  • Polymers: consist of strings of monomers joined together. No theory of their formation is universally accepted.

  • Isolated cell: round organic structures can spontaneously form, but these are not close to being cells; how the first cells formed is unknown at present.

  • Reproduction: molecules that reproduce themselves have been made in the lab; however, the origin of biological reproduction is still unknown

  • Sexual reproduction was introduced with the eukaryotes and is important because asexual reproduction is no fun whatsoever. Another importance of sexual reproduction is that it allowed for more rapid evolution through new combinations of DNA. However, why the first eukayrotes got horny is unknown.

• Oldest Evidence for Life

  • The oldest organisms were probably anaerobic prokaryotes.

  • Biomarkers

    • Isotope ratios in sedimentary rocks that indicate the presence of life (such as carbon-13 to carbon-12 and iron-54 to iron-56)

    • Biochemicals that can only be produced by living organisms

  • Oldest biomarkers: carbon isotope ratios consistent with life in BIFs dated to 3.9 Ga in Greenland (very controversial)

  • Oldest prokaryotes (cells without a nucleus): 3.5 Ga "cyanobacteria fossils" in Australia (disputed; may have been produced inorganically)

  • Oldest eukaryote (cell with a nucleus) biomarker: 2.7 Ga sterane molecules, only known to be made by eukaryotes, found in Australian shales.

  • Oldest eukaryote fossil: 2.1 Ga algae preserved in Michigan.

  • Oldest fossil visible to the naked eye: spiral-chained algae, 1.9 Ga.

• Precambrian Fossils

  • are (except for the latest Precambrian) single-celled fossils found in typically in chert (a form of silica, like quartz) due to its resistance to metamorphic changes.

  • Stromatolites are round, mound-like structures formed by cyanobacteria (which are photosynthetic) that grow in shallow seawater and are common in the Precambrian.

  • Another common fossil form is the acritarch, a microscopic, hollow fossil resembling algae cysts but whose identification is unknown.

  • Origin of eukaryotes: Possibly derives from symbiosis between prokaryotes. Eukaryotes have organelles that have their own genetic material, which may originally have been individual organisms.

• Multicellular (Metazoan) Life

  • Metazoan life needs an enhanced source of energy, which was provided by a metabolism based on oxidation in the late Precambrian.

  • Hence free oxygen in the sea was necessary for metazoan life to develop. Isotopic evidence from Oman and geochemical evidence from Newfoundland indicate there was a jump in oxygen levels at the end of the late Proterozoic glaciation, 580 Ma. The cause of this sudden increase is unknown.

  • The first metazoans appeared in the Ediacaran Period (620 - 543 Ma) when invertebrates without shells are found in the fossil record in widely scattered parts of the Earth. This occurred at about the same time as the jump in oxygen levels.