Lecture 9a: Archean, The Proterozoic – Dawn of the Modern World (Vocabulary Flashcards)

Archean–Proterozoic Overview

• Timeframe and terminology
  • Archean: crust formed and stabilized before 2.5~ ext{Ga}. Proterozoic: from around 2.5~ ext{Ga} to the start of the Phanerozoic (Devonian boundary around 419.2~ ext{Ma} in the ICS chart context).
  • Dawn of the modern world: rapid growth of continental crust, global tectonics, atmospheric and oceanic evolution setting the stage for complex life.
• Continental crust formation and growth (key ideas)
  • Crust grows by multiple processes, including accretion of crustal blocks, arc magmatism, and mantle-derived additions.
  • Major growth mechanisms cited:
  • Adding crustal pieces by collision (scraping off/thrusting up of pieces) → contributes to craton growth. ext{e.g., collision and accretion events across Archean}
    ightarrow ext{crustal thickening}.
  • Adding igneous (plutons) material in the crust → intrusions add silicic crustal volume.
  • Melting/magmatism from hydrated upper mantle/lower crust/lithosphere above subduction zones → generation of granitic-type crust (important for continental crust formation).
  • Melting oceanic crust on mid-ocean ridges is not listed as a primary mechanism for continental crust growth in this material (in many models, new crust forms mainly by upwelling mantle and extraction of silicic melts at ridges and in subduction-related settings).
  • Common crustal constituents in Archean Proterozoic settings:
  • Felsic granitoids and gneisses (early felsic crust remnants).
  • Greenstone belts (green metamorphosed volcanic-sedimentary sequences) consisting of meta-basalt, ultramafic rocks, and sediments.
  • Ultramafic rocks and greenstones often backarc basin crust; metamorphic rocks (serpentinite, greenschist) reflect high-temperature, low- to medium-pressure metamorphism.
  • Structure and deformation: Archean crust shows folded, faulted, and metamorphosed sequences forming elongated belts and granulite–gneiss terrains.
• Archean cratons and early tectonics
  • Archean cratons preserve evidence for plate tectonics in granulite and greenstone belt associations.
  • Granulites: highly metamorphosed igneous rocks and early sediments; remnants of the first felsic continental crust (metamorphosed).
  • Greenstone belts: sequences of metamorphosed mafic volcanic rocks and accompanying sediments; green color due to metamorphic minerals (e.g., chlorite, hornblende).
  • Greenstone belts documented in Ontario as classic Archean examples (Abitibi belt in the Superior Province, ~2.8–2.6~ ext{Ga}). Often host ore deposits (e.g., Au).
• Archean Greenstone Belts in Ontario and significance
  • Abitibi Greenstone Belt (Superior Province): 2.8–2.6~ ext{Ga}; important ore deposits, including gold (Au).
  • Greenstone belts are key observational evidence for early plate tectonics and crustal growth, linking volcanic activity, metamorphism, and sedimentation into early continental crustal blocks.
• Question 2A and 2B (from Lecture 13): quick recap
  • 2A: How does continental crust form and grow? Correct selections include:
  • c. Melting hydrated upper mantle / lower crust / lithosphere above subduction zones
  • d. Adding crustal pieces/material by collision (scraping off / thrusting up of pieces)
  • f. Adding igneous (plutons) material in the crust
  • 2B: Which statement is NOT correct about (Granulite) Greenstone Belts? The incorrect statement is:
  • a. They are mostly Phaneritic of age (e.g., Paleozoic: 365–285 Ma old)
  • Notes on general context: these belts are Archean–early Proterozoic, not Paleozoic, and are not primarily preserved as parts of cratons in the sense that their age makes the option implausible.

Early Atmosphere and Ocean Chemistry

• Hadean–Archean atmosphere composition
  • Very low to zero free oxygen: ~0 extrm{-} ext{O}_2; high temperatures (~300^ ext{°C}) and high pressures (~ ext{100 atm}).
  • Dominant gases: water vapor H$2$O, carbon dioxide CO$2$, nitrogen N$_2$, plus other volcanic/outgassing species.
  • Primary sources: accretion, differentiation, outgassing (volcanism), delivery of volatiles by impacts.
  • Modern atmosphere: roughly 78 ext{% N}2, ext 21 ext{% O}2, ext 1 ext{% ext{other gases}}.
  • Change driver: photosynthesis increasing O$_2$ levels; weathering and water–rock interactions began cycling oxygen more effectively.
• Evidence for lack of free oxygen in the early atmosphere
  • Absence of oxidized iron minerals in oldest sedimentary rocks (no FeO oxidation signatures).
  • Sulfide minerals (e.g., pyrite FeS$_2$) formed instead of oxides, indicating anoxic conditions.
  • No carbonate rocks (CaCO$_3$) formed due to acidic oceans; carbonate precipitation requires more oxygenated conditions.
  • Archean microbes were largely anaerobic, living in oxygen-poor oceans.
  • Emergence of oxygenic photosynthesis in oceans gradually increased dissolved O$_2$.
  • Banded Iron Formations (BIFs) begin in the Archean and become widespread in the late Archean to early Proterozoic, signaling oxidative events in ocean water (Fe$^{2+}$ oxidation to Fe$^{3+}$ minerals under some oxygen presence).
• Banded Iron Formations (BIFs)
  • Initiation around 3.8~ ext{Ga}; most significant activity between 2.5–1.8~ ext{Ga}.
  • Pre-oxygen atmosphere: chemical sedimentary rocks with iron sulfides/metals precipitated on seafloor; later bands show ferric oxides (hematite Fe$2$O$3$, magnetite Fe$3$O$4$) interbedded with chert (SiO$_2$).
  • Transition: as atmospheric and oceanic oxygen increased, dissolved iron in oceans precipitated as oxide minerals, forming red-rich BIF layers. After ~$1.8~ ext{Ga}$, much iron is bound in BIFs, allowing oxygen to escape from oceans to the atmosphere.

Early Hydrosphere and Ocean Chemistry

• Formation of oceans and early seawater characteristics
  • Oceans formed as early as 4.4~ ext{Ga} via outgassing, delivery of volatiles, and cooling of the planet.
  • Early seawater was very acidic (pH around 0.6–7 range in the cited text), with low salinity; subsequent geochemical evolution increased alkalinity.
  • Transition to alkaline/neutral oceans: rapid geological and hydrothermal inputs resulted in higher pH (~8) and higher salinity; cations such as Ca, Na, Fe contributed to neutralize acidity.

Origin of Life and Early Biochemistry

• The prebiotic world and origin of life (abiogenesis)
  • Proposed scenario involves delivery of organic matter from space, conjunction of essential chemical elements (C, O, H, N, S, P) in ancient oceans, UV radiation and lightning driving synthesis of larger organic molecules.
  • Abiotic synthesis of proteins (amino acids), nucleic acids (RNA or other genetic polymers), and phosphorous-containing compounds; formation of a protocell membrane and metabolism.
  • Pan-Spermia hypothesis: life’s building blocks or life itself may have originated elsewhere and was delivered to Earth.
• Early life forms (proto-life) and prokaryotes
  • Prokaryotes: lack membrane-bound organelles and nucleus; do not have a true cell wall in the same sense as later organisms (some have cell walls, many archaea and bacteria do).
• Evidence for early life and fossilization
  • Isua, Greenland (kerogen evidence) around 3.8~ ext{Ga}; Warrawoona Group, Australia around 3.46~ ext{Ga}; other potential early life indicators in greenstone belts (filamentous fossils).

Extreme Environments and the Beginning of Life

• Modern analogues and the origin of life in extreme environments
  • Midocean ridges and hydrothermal vents (black smokers) host hyperthermophilic Archaea-type organisms.
  • Chemoautotrophic life thrives on chemical energy from vent fluids, suggesting a plausible pathway for early life in high-temperature, reducing environments.
  • 3D modelling resources and references are available (e.g., giant tube worm on hydrothermal vent).

Oldest Fossils and Early Biota

• Oldest fossil evidence and preserved materials
  • Apex Chert, Western Australia: approx. 3.46~ ext{Ga}; Warrawoona Group remains; other paleosols and filamentous structures in greenstone belts (e.g., 3.77~ ext{Ga} Nuvvavigutuq: old paleosol; Quebec Greenstone Belt with filamentous tubes).

Phanerozoic Chronostratigraphy (ICS Chart Overview)

• Chronostratigraphic framework (ICS): Eonothem — Eon; Erathem — Era; System — Period; Series — Epoch; Stage — Age
  • The International Chronostratigraphic Chart (ICS) provides the standard for naming and dating rocks and fossils, with boundary ages assigned to GSSP (Global Boundary Stratotype Section and Points).
  • Key elements include the recognition of Archean–Proterozoic boundaries and Phanerozoic subdivision (Paleogene, Neogene, Quaternary, etc.).
  • Example: Boundary ages for recent oceans and land surfaces are defined with numerical ages (Ma) and stratigraphic boundaries; some numerical ages are ~≈ and depend on ratified GSSPs.
  • The chart also indicates GSSA (Global Standard Stratigraphic Age) usages for lower boundaries prior to ratified GSSPs.
• Practical takeaway for exam: ICS chart provides framework to place events like the GOE, Huronian glaciation, Cryogenian glaciations, and Proterozoic–Phanerozoic transitions within a global timescale.

Global Oxygenation Events and Climate Shifts

• Global oxygenation of oceans and atmosphere
  • Oxygen began to accumulate in oceans around 2.5~ ext{Ga} (GOE related processes) with a major oxygenation step in the atmosphere around 2.4~ ext{Ga} (Great Oxidation Event, GOE) and later peak events.
  • Cooling events accompany oxygenation: Neoproterozoic Cryogenian glaciations (~0.8~ ext{Ga}) occur after initial oxygenation waves.
  • The Great Oxidation Event contributed to altered ocean chemistry, ozone formation, and later atmospheric oxygen buildup.
• Huronian glaciation (Ontario, Canada)
  • Occurred roughly 2.4–2.2~ ext{Ga} in the Huronian Supergroup of Ontario.
  • Indicators include glaciogenic sedimentary rocks, diamictites, drop stones, varves, and evidence for ancient ice sheets covering large regions (>50,000 km²).

Ontario and North American Proterozoic Glaciations

• Huronian Supergroup (Ontario) details
  • Age: 2.5 ext{–}2.0~ ext{Ga}
  • Preserved rocks: metamorphosed igneous and sedimentary rocks; includes evidence for Huronian glaciation and passive margin sediments.
  • Continental margin dynamics: Margin records include siltstones, sandstones, quartzites, and conglomerates, reflecting transition from continental to open-ocean margin settings during the Proterozoic.
• Evidence and manifestations of glaciations in North America
  • Varves, clays and silts in cyclic layers (seasonal deposition) mark glacial-interglacial cycles.
  • Diamictites (e.g., Gowganda Formation in Ontario) as glacial depositional products.
  • Other features: drop stones, tillites, scratched/facetted bedrock, cobbles, and boulders indicating glaciation episodes.
• Global perspective on Proterozoic glaciations
  • North America provides a well-documented record of multiple glaciations during the Proterozoic, consistent with global climate fluctuations during this era.

Proterozoic Supercontinents and Tectonics

• Kenorland, Nuna, and Rodinia: key assemblages and events
  • Kenorland (Archean to around 2.5~ ext{Ga}): precursor supercontinent; includes the Superior Province and Kenoran crustal blocks.
  • Opening of the Huronian Ocean (~Paleoproterozoic, 2.4–2.2~ ext{Ga}): initiation of ocean basin between early cratons.
  • Grenville Orogeny (Neoproterozoic, around 1.3–1.0~ ext{Ga}): major orogenic event building the later assembly of Rodinia.
  • Nuna (Neoproterozoic): a proposed supercontinent that formed after Kenorland and prior to Rodinia; involved major cratons such as Laurentia, Baltica, and others in assembly around 1.7–1.0~ ext{Ga} depending on reconstructions.
  • Rodinia (Neoproterozoic): late Proterozoic supercontinent formation; break-up initiated around 750–600~ ext{Ma} with subsequent dispersal and ocean opening events.
• Structural and magmatic indicators
  • Dykes and dyke swarms as evidence of breakup and mantle plume activity around 2.18–2.21~ ext{Ga} in multiple cratons (e.g., Slave Province, Kenorland fragments, Kola–Karelia regions).
  • Orogenies associated with accretion and collision (e.g., Penokean, Grenville) documenting long-built orogenic belts and continental margins.
• Concept of passive margins in the Proterozoic
  • Transition from active margin tectonics to passive margins accompanies craton formation and supercontinent assembly; Proterozoic margins show metamorphosed sediments (siltstones, sandstones, quartzites) and conglomerates indicating stabilization and long-term crustal evolution.

Structural and Stratigraphic Transitions on the North American Craton

• Proterozoic transition of continental crust to oceanic crust interactions
  • Transition phase features older stretched continental margins initially overlain by younger continental and marine sediments; reflects a move toward passive margin conditions.
  • Ontario and surrounding cratons illustrate a preserved pattern similar to modern crustal evolution: large-scale deformation, metamorphism, and sedimentary sequences record the overall tectonic history.
• Summary of North American crustal evolution patterns
  • Archean craton formation → early plate tectonics and crustal accretion through greenstone belts and granulites.
  • Paleoproterozoic–Mesoproterozoic: major transcurrent and collisional events, formation of the Huronian Ocean, and initiation of larger oceanic basins.
  • Neoproterozoic: supercontinent assembly (Nuna, Rodinia) and Grenville orogeny marking the later crustal growth and reorganization leading toward the Phanerozoic framework.

Key Takeaways for Exam Preparation

• Archean crust shows evidence for early plate tectonics through granulite and greenstone belt assemblages; formation of the first felsic continental crust remnants and elongated belts through tectonic processes.

• Greenstone belts are critical archives of early Earth tectonics, metamorphism, and ore deposits; their green color reflects chlorite/hornblende metamorphic phases; they are not Paleozoic in age.

• The early atmosphere and hydrosphere were reducing and acidic; oxygenation was gradual, proceeding from oceans to atmosphere with major milestones around 2.5–2.4~ ext{Ga} and later.

• Banded Iron Formations document the rise of oxygen in oceans and the eventual atmospheric oxygenation; the redox evolution shaped both ocean chemistry and sedimentary records.

• Life likely originated in prebiotic conditions with energy from UV, lightning, and hydrothermal systems; evidence spans Isua, Warrawoona, and other early assemblages; RNA-world hypotheses are often discussed in origin-of-life debates.

• The Proterozoic hosts glaciations (e.g., Huronian) and major climatic swings; varves, diamictites, and dyke swarms across basins reflect global change and continental scale processes.

• Chronostratigraphy (ICS) provides the framework to place events in time with GSSPs; Phanerozoic boundaries anchor modern time, while Archean–Proterozoic transitions require careful interpretation of GSSA and provisional ages.

• Ontario’s Huronian Supergroup serves as a canonical record of Proterozoic glaciations and passive-margin sedimentation, illustrating the broader Northern American tectonic narrative.

• References and further reading

  • Greenstone belt videos and Ontario examples: Granulite/Greenstone belts and Abitibi belt discussions.
  • Timescale resources: ICS Chronostratigraphic Chart (latest version) for boundaries and GSSP data.
  • General background on early Earth atmospheres and oxygenation events (Great Oxidation Event) and glaciations (e.g., Cryogenian).
  • External video references mentioned in the transcript for additional context on plate tectonics and early Earth processes.