22 20 Early Earth Atmosphere and Ocean (2)

Introduction to Earth and Marine Geology (MSCI *112)

  • Instructor: Prof Stewart

  • Contact:

    • Phone: [Not provided]

    • Email: jstewar6@coastal.edu

Early Earth during the Precambrian

  • The Earth is approximately 4.56 billion years old.

  • The oldest known rocks date back to about 4.04 billion years.

  • The largest unit of geochronology on Earth is the Precambrian, lasting more than 4 billion years.

  • Precambrian is further divided into three eons:

    • Hadean

    • Archean

    • Proterozoic

Hadean Eon

  • Prelude to the Earth’s formation from approximately 4.6 to 3.8 billion years ago.

  • Characterized by:

    • Initial growth of the Earth through accretion (the process of gradual accumulation).

    • Formation of the Earth's core and the Earth's magnetic field.

    • Presence of a magma ocean approximately 400 km thick.

    • Formation of the crust and protocontinents, possibly indicative of early plate tectonics.

Atmospheric Conditions during the Hadean

  • Extremely reducing primary atmosphere containing:

    • Methane (CH₄)

    • Ammonia (NH₃)

    • Hydrogen (H₂)

  • Transition to moderately reducing secondary atmosphere composed of:

    • Water vapor (H₂O)

    • Carbon dioxide (CO₂)

    • Nitrogen (N₂)

    • Sulfur dioxide (SO₂)

  • Major contributors to atmospheric conditions:

    • Volcanic outgassing.

    • Condensation of water vapor leading to the formation of primitive oceans.

Formation of Oceanic Crust

  • First oceanic crust (mafic in composition) was formed from the cooling of the magma ocean as early as 4.3 billion years ago.

  • Development of small felsic protocontinents, which were thinner compared to modern continents, eventually forming the core of late continents.

Potentially Biogenic Carbon

  • Evidence of potentially biogenic carbon has been found in a 4.1 billion-year-old zircon.

  • Studies involving zircon indicate the presence of both inorganic and organic carbon.

Archean Eon

  • The Archean Eon spans from 3.8 to 2.5 billion years ago, marked by significant geological and biological developments:

    • Continued development of oceanic crust and protocontinents initiated subduction processes leading to the formation of continents.

    • Formation of gneiss within subduction zones.

Greenstone Belt

  • Characteristics of the Archean crust include the presence of greenstone belts, which are:

    • Comprised of large chunks of weakly metamorphosed mafic crust and deep-sea sediments.

    • Metamorphosed felsic (continental) crust surrounds these belts.

    • The term derives its name from the green hue of the metamorphic mineral chlorite.

    • Often originated from oceanic environments or forearc/back-arc basins.

Cratons

  • Definition: Cratons are the ancient, stable portions of a continent that have remained relatively unchanged for vast geological timescales.

  • Cratons can be categorized into:

    • Shields: Areas where Precambrian rocks are exposed.

    • Platforms: Areas with buried Precambrian rocks.

Banded Iron Formation (BIF)

  • Definition: Banded Iron Formations are stratified sedimentary deposits, typically consisting of alternating thin layers of chert, hematite, and magnetite, formed primarily between 3.7 and 1.8 billion years ago.

  • Context: Indicates the presence of dissolved iron in early oceans and serves as evidence of the oxygen produced by early photosynthetic life.

    • The presence of cyanobacteria likely contributed to the release of oxygen during photosynthesis, influencing the iron precipitation patterns observed in BIF deposits.

Archean Eon - Beginning of Life

  • Archean life forms are predominantly characterized as:

    • Single-celled prokaryotes, lacking membrane-bound nuclei and cellular organelles, representative of early bacterial forms.

  • A significant evolutionary event during this period was the development of autotrophic processes, particularly photosynthesis, evidenced by 3.5 billion-year-old prokaryotic fossils from Australia.

Oxygenation Events

  • Increase in atmospheric and oceanic oxygen levels can be associated with various geological formations and dynamics, such as:

    • Banded Iron Formations, pointing to a significant role of cyanobacteria in atmospheric changes.

    • Stromatolites: sedimentary structures formed by cyanobacterial activity which trap sediments within mat-like formations.

Proterozoic Eon

  • The Proterozoic Eon spans from 2.5 to 0.542 billion years ago, characterized by major geological and biological developments:

    • Development of a continental crust that accounts for 80-90% of Earth's surface area.

    • Continents become larger and stabilize considerably.

    • Emergence of extensive sedimentary rock formations and sedimentary basins across the planet.

    • A rise in atmospheric oxygen concentration roughly 10%.

Tectonic Activity

  • Active plate tectonics during the Proterozoic Eon led to:

    • Development of the highest grade metamorphic rocks.

    • Occurrences of supercontinent cycles, notably the formation of Rodinia around 1 billion years ago, followed by its fragmentation around 750 million years ago.

    • Periodic significant climate oscillations alternating between greenhouse and icehouse conditions known as Snowball Earth episodes.

Geological Features of the Grand Canyon

  • Investigation of geological layers in the Grand Canyon reveals crucial information about Earth's history, illustrating formations from various periods.

  • Key formations observed include:

    • Kaibab Plateau

    • Kaibab Formation

    • Torowap Formation

    • Coconino Sandstone

    • Hermit Shale

    • Redwall Limestone

    • and several unconformities, both within and across geological time frames.

Late Proterozoic Eon Developments

  • Oxygen accumulation through the Archean and Proterozoic Eons played an essential role in the evolution and complexity of living organisms.

  • Although numerous Banded Iron Formations formed during this time, fossil evidence remains scarce, primarily due to the preeminence of microscopic, soft-bodied organisms that became subject to metamorphosis over time.

Early Eukaryotes

  • Emergence of single-celled eukaryotes and multicellular organisms, largely depended on free oxygen in the atmosphere to sustain their metabolic processes.

  • Notable features distinguishing eukaryotes include:

    • Membrane-bound nucleus

    • Presence of organelles

    • Larger quantity of DNA when compared to prokaryotes

    • Inflexible cell walls versus flexible cell walls in eukaryotes, allowing for more complex cellular interactions and possible endocytosis.

    • Asexual and sexual reproductive capabilities in eukaryotes versus asexual reproduction in prokaryotes.

Contrast Between Prokaryotes and Eukaryotes

  • Prokaryotes:

    • Lack of membrane-bound nuclei

    • Less complex DNA structure

    • Absence of internal cellular subdivisions

    • Inflexible cell walls leading to limited cellular engulfment abilities.

  • Eukaryotes:

    • Contain membrane-bound nuclei and organelles

    • More extensive DNA content (approximately 1000x) than prokaryotes

    • Flexible cell walls permitting more complex interactions.

Summary of Eons

  • The earth underwent significant developmental changes throughout its history marked by the following eons:

    • Hadean:

    • Known as “Hell” (4.6 to 3.8 Ga), characterized by internal differentiation and the formation of oceans and protocontinents.

    • Archean:

    • Termed “Ancient” (3.8 to 2.5 Ga), marked by the birth of continents and the emergence of the earliest life forms.

    • Proterozoic:

    • Labeled “Before life” (2.5 to 0.542 Ga), noted for the development of tectonic plates, significant oxygen buildup in the atmosphere, and the appearance of multicellular life.