Origin of the Atmosphere - Physical Geography

Readings

• Geosystems: An Introduction to Physical Geography, 8th Edition by Robert W. Christopherson, American River College - ©2012 Pearson
• An Introduction to Physical Geography and the Environment – 3rd Edition by J. Holden - ©2012 Pearson

Origin of the Atmosphere - Summary

• Formation of Earth by Accretion (4.5 Ga): This process lasted 50-100 million years.
• Differentiation: Elements sorted by density, leading to the formation of layers (crust, mantle, core, atmosphere).
• Degassing of the Mantle (changes in redox status): Release of CO2 and H2O (approximately ±70 million years).
• Primitive Atmosphere composition: H2O, CO2, CH4, N2, and O2.

What caused the Early Life to kick off?

• After the Intense Bombardment episode (4.4 Ga): Earth's surface cooled, water vapor condensed, forming the first ocean.
• Hydrogen isotope features of ocean water suggest the source of water is icy asteroids, not condensation.
• The Cool Early Earth period lasted from approximately 4.4 Ga to 4.0 Ga. (Carbonaceous Chondrites are rocky meteorites with less than 35% metal).

Evolution of the Atmosphere

• Second Atmosphere: Compatible with early life conditions (CO2, CH4, H2O, O2, and N2), warm climate (4.5 - 3.5 Ga).
• Weathering of Rocks: Plays a crucial role in long-term carbon cycle.

Weathering of Rocks and the Long-term C Cycle

• Early oceans and atmosphere were in equilibrium with atmospheric CO2 levels.
• Weathering: Chemical reactions between atmospheric and surface compounds. A significant carbon sink over millions of years. (approximately ± 200 million years)
• Atmospheric CO2 dissolves in water droplets in clouds, forming carbonic acid.
• Carbonic acid reacts with minerals at varying rates, locking away atmospheric CO2 in carbonate sediments.

Chemical Stability of Minerals

• Most Minerals are stable and react slowly with carbonic acid.
• Gold and quartz (silicon dioxide) are difficult to erode.
• Less stable minerals (e.g., sulfides and sulfur compounds with various metals) weather readily and leave behind accompanying metal salts.

Carbon Regulation

• CO2 and H2O form carbonic acid during the silicate weathering process.
• Diagram of the carbon cycle showing processes like silicate weathering, outgassing, continental plate, and oceanic plate.
• Carbonate compensation depth is where dissolution of calcium carbonate exceeds precipitation, causing CO2 to outgas from the ocean.

Equations of Weathering

• H2CO3 = HCO3- + H+ (Acid rains) Carbonic acid dissociates.
• H2CO3 + CaCO3 = Ca(HCO3)2 (Weathering of carbonate rocks)
• 2CO2 + 3H2O + CaSiO3 = Ca2++ 2HCO3- + H4SiO4 (Weathering of silicate rocks)
• Release of CO2 + H2O.

First Organic Molecules

• First organic molecules appeared early in Earth's formation.
• Some authors believe that life was prevented from developing due to the Intense Bombardment episode.
• Miller-Urey experiment (1953) demonstrates the feasibility of organic molecule synthesis.

The Swan-Neck Experiment (Pasteur, 1850)

• Demonstrated that life doesn't arise spontaneously from non-living matter under specific conditions.
• Provided evidence for biogenesis.
• Different scenarios and outcomes depending whether the flask's neck was removed of not

Coacervates (Bungenberg de Jong, 1932)

• Spherical aggregates of colloidal droplets. Held together by hydrophobic forces.
• Proposed by Alexandr Oparin and John Haldane as crucial in the early theory of abiogenesis (origin of life).
• Metabolism may have predated information replication in the origin of life.

The Miller-Urey Experiment

• Experiment (1953) simulated early Earth conditions and showed organic molecule synthesis is possible in a reducing atmosphere.
• Within a week, methane converted into compounds – formaldehyde, hydrogen cyanide, formic acid, urea, and amino acids.
• Further experiments confirmed varied gas mixtures and energy sources like lightning, UV, radiations.

The Origin of Life

• Pre-Biogenesis theories: Panspermia (microscopic life forms in space, extremophiles can survive in space), Spontaneous Generation (life arises from non-living material)
• Biogenesis: Life arises from pre-existing life, opposed to spontaneous generation. Francesco Redi (1668) and Louis Pasteur (1850) provided evidence for this hypothesis.
• Chemical evolution (1920s): Different types of coacervates formed in the primordial ocean, subject to a selection process that eventually leads to life.

Natural Synthesis vs. Extraterrestrial Origin

• "Nuclear geyser model" suggests a terrestrial origin of first organic molecules, contrasting models with an extraterrestrial origin.
• Organic molecules delivered to Earth by early Solar System bodies (e.g., comets, meteorites)
• Murchison meteorite (1969) contained organic compounds similar to the Miller-Urey experiment. Analysis (2010) confirmed 14,000 molecular compounds, including 70 amino acids and di-amino acids.

Where did the organic building blocks of life come from?

• Chyba/Sagan (1990s): Comets rich in organic molecules, an extraterrestrial source, potentially delivered to early Earth.

Natural Synthesis vs. Extraterrestrial Origin

• Molecules exist in both left and right-handed structures (racemic mixture). Living organisms use only one form. Chiral synthesis may be possible.

Chirality

• Chirality is a geometric property of some molecules. A molecule and its mirror image are not superposable.

In the most recent developments:

• Scientists found an explanation for the chirality of molecules and specifically the reason why they are generally levorotatory.

Purine and pyrimidine compounds in Murchison Meteorite

• Carbon isotope ratios for uracil and xanthine in the Murchison meteorite indicate a non-terrestrial origin for these compounds.

Origin of life - Theory and Definition

• A generally accepted definition involves metabolism, membrane, and self-replication.
• Coacervation may explain the appearance of membranes in the origin of life, but how to account for self-replication and metabolism.

The First Cells

• Microfossils of cells: 3.5 billion years old, resembling present-day prokaryotes.

The Prokaryotes: Bacteria and Archaea

• Diagram of changing classifications over time, showing evolving perspectives on organisms.
• Timeline showing origin of life, prokaryotes, and eukaryotes. Timelines of major and minor extinction events.
• Comparisons of the Archaea and Bacteria showing different structures and compositions.

The Archean Eon (3.8 Ga – 2.5 Ga)

• The Earth was much hotter and had more active tectonic processes.
• Water existed in liquid form.
• Early prokaryotic life evolved, including oxygen-producing bacteria.
• Continents formed during this Eon but not in the form they are known today.
• Atmospheric oxygen levels were low during this time.

The Development of an Oxidizing Atmosphere

• Cyanobacteria led to oxygenic photosynthesis.
• Molecular oxygen (O2) is a product of photosynthesis and was toxic to certain early life.
• Banded Iron Formations (BIFs) are evidence of oxygen reacting with iron in oceans.
• Possible link between snowball Earth events and the timing of oxygen build-up.

The Snowball Earth Hypothesis

• Possible episodes of global glaciation.
• Ice sheets covering most of the Earth's surface, with limited exposed, open water.
• Lowered reflectivity (albedo), positive feedback loop exacerbating cooling.
• Release of CO2 by volcanic activity and thawing of ice lead to the onset of hothouse conditions.

Side effects of Oxygen production

• Ozone formation, protecting organisms from harmful UV radiation.
• Inhibition of methane-producing organisms.
• Toxicity of O2 for early anaerobic life.

The alternative Slushball Earth Hypothesis

• A hypothesis of an ice-covered Earth, with open water near the equator.

Deep Freeze and Complex Life

• Some microscopic organisms flourished in challenging conditions during glacial episodes.
• These refuges fostered adaptability and persistence of life.

Ice Ages and Glaciations

• Episodes of global glaciation occurred during the Proterozoic Era.
• The Cryogenian Period saw significant glacial events and subsequent mass extinctions, potentially leading to diversification of life.

The Great Oxygenation Event

• Key event in the evolution of Earth's atmosphere.
• A phase-wise depiction of the development of oxygen in the atmosphere over time.
• The increase of O2 in the atmosphere created an ecological crisis, because of its toxicity to early anaerobic life.

The Proterozoic Eon (2.5 Ga – 0.54 Ga)

• Earth cooled, re-heated, offering new opportunities for life.
• The supercontinent Rodinia formed and later broke up.
• Multi-cellular organisms developed.

The Textbook Paradigm

• Modern classification of living organisms into three domains (Bacteria, Archaea, Eukarya).
• rRNA sequencing and biochemical developments changed the classification of life (E. Haeckel's concept of monera).
• Development of the three-domain model showing the evolutionary relationships between domains

Aerobic Pathways and the Cyanobacteria

• Cyanobacteria perform oxygenic photosynthesis, producing O2 as a byproduct.
• Cyanobacteria's role in shaping Earth's atmosphere.
• Early photosynthetic organisms.
• Stromatolites are fossil evidence of their activity.

Domain Archaea

• Kingdom Archaebacteria: Organisms that use inorganic chemical reactions to produce organic matter. (e.g. use sulfur or other energy sources).
• Methanogens: Produces methane as a byproduct (uses carbon dioxide).
• Habitats in extreme environments such as hot springs and salt lakes.
• Diverse metabolic adaptations.

Domain Eukarya

• Eukaryoti cells are more complex than prokaryotes, having nucleus. The complexity and diversity of eukaryotic cells are likely results of endosymbiotic events.

The Endosymbiotic Theory

• Explains the evolution of eukaryotic organelles (mitochondria, plastids) from symbiotic prokaryotic cells (engulfed by ancestral prokaryotic cells).
• Genetic material of organelles differs from the cell itself. The existence of organelles' independent reproduction suggests their symbiotic relationship.
• The theory explains the evolution of the cellular complexity of eukaryotic cells.

The Kingdom Fungi, Plantae, Animalia

• General characteristics of fungi, plants, and animalia, including methods of nutrition, structures, reproduction, and overall biological roles in ecosystems.

Explore the fundamental concepts of the origin of the atmosphere as outlined in your Physical Geography readings. This quiz covers the formation of Earth, atmospheric composition, and the factors contributing to early life. Test your understanding of these essential geological processes and their implications for the environment.