Lecture 1 Slide Show_ Early Earth Lecture Notes PDF

Early Earth ATMO-310/OCN 310 August 28, 2024 Outline Overview of the Earth system, beginning with history of the planet, ”forcing” and variations of this forcing Next, we will look at what this forcing means, or influences, mean circulation patterns in the atmosphere and ocean We t...

Early Earth ATMO-310/OCN 310 August 28, 2024 Outline Overview of the Earth system, beginning with history of the planet, ”forcing” and variations of this forcing Next, we will look at what this forcing means, or influences, mean circulation patterns in the atmosphere and ocean We then will have a look at so-called “natural variability”, and then put this in context by looking at past climates Finally, we will look at impact of variability on different parameters A Brief History of Time —Formation of universe, —then the solar system, —then the sun, Earth and moon —Development of atmosphere, ocean and land Evolution of Earth as we know it today Georges Lemaître (1894-1966) Monthly Notices of the Royal Astronomical Society, Volume 91, Issue 5, March 1931, Pages 490–501, https://doi.org/10.1093/mnras/91.5.490 Origin of the Universe The universe began about 14.4 billion years ago The Big Bang Theory states that, in the beginning, all the energy and mass of the universe existed in an infinitely small point Then it exploded The material blown out by the explosion eventually formed the stars and galaxies After about 10 billion years, our solar system began to form  Origin of Our Solar Sysem The currently held hypothesis about how a solar system is formed is known as the “Nebular Hypothesis” Immanuel Kant (1724- 1804) 1. A large gas cloud (nebula) begins to condense; most of the mass is in the center, there is turbulence in the outer parts 2. Turbulent eddies collect matter measuring several meters across 3. Small chunks grow and collide, eventually becoming large aggregates of gas and solid chunks 4. Gravitational attraction causes the mass of gas and dust to slowly contract and it begins to rotate 5. The dust and matter slowly falls towards the center  Formation of Our Sun After sufficient mass and density was achieved in the Sun, the temperature rose to one million ºC, resulting in thermonuclear fusion. H atom + H atom = He atom + energy  Formation of Planets Gravitational forces allow the inner planets to accrue and compact solid matter (including light and heavy atoms) Solar radiation blew gases (primarily hydrogen, helium) away from inner planets These gases were collected and condensed into the gas giants (Jupiter, Saturn, Uranus, Neptune) Beyond Neptune, ice and frozen gases form Pluto, Sedna and the Kuiper Belt Objects Left-over debris form comets and asteroids And Finally… Formation of Earth Age of the Earth Meteorites give us access to debris left over from the formation of the solar system We can date meteorites using radioactive isotopes and their decay products Earth is ~ 4,570,000,000 years old For the first half billion years of its existence, the surface of the Earth was repeatedly pulverized by asteroids and comets of all sizes  Formation of the moon The Giant Impact Hypothesis asserts that around 50 million years after the initial creation of Earth, a planet about the size of Mars collided with Earth This idea was first proposed about 80 years ago, but it took calculations by modern high-speed computers to prove the feasibility Early Earth Heats Up Three major factors that caused heating and melting in the early Earth’s interior: 1. Collisions (Transfer of kinetic energy into heat) 2. Compression 3. Radioactivity of elements (e.g., uranium, potassium, or thorium)  Chemical Differentiation About 100 million years after initial accretion, temperatures at depths of 400 to 800 km below the Earth’s surface reach the melting point of iron In a process called global chemical differential, the heavier elements, including the melted iron, began to sink down into the core of the Earth, while the lighter elements such as oxygen and silica floated up towards the surface From (A) a homogeneous, low-density proto-planet to (B) a dense, differentiated planet  Whole Earth: Crust: Fe+O+Si+Mg = 93% O+ Si+Al = 82% Each of the major layers has a distinctive chemical composition, with the crust being quite different from the Earth as a whole Cross section of Earth Divisions of the Earth's interior showing in a rudimentary way the relation of the upper mantle to subduction zones and midocean ridges. Note also the region where basaltic magma is thought to form.  Differentiation of Chemical Elements in Earth Present distribution of major elements in the Earthʼs atmosphere, crust and in seawater (elements listed in order of abundance). Refinement Evolution of the continents, ocean and atmosphere Continents: Formed from solidified magma that floated up from the Mantle Oceans and Atmosphere: Fluid and gaseous outer layers believed to have been created by out-gassing of gases and fluids from volcanic eruptions (in a process called volatile transfer) Atmosphere Right after its creation, the Earth is thought to have had a thin atmosphere composed primarily of helium (He) and hydrogen (H) gases The Earths gravity could not hold these light gases and they easily escaped into outer space Today, H and He are very rare in our atmosphere For the next several hundred million years, volcanic out- gassing began to create a thicker atmosphere composed of a wide variety of gases The gases that were released were probably similar to those created by modern volcanic eruptions  These would include: Water vapor (H2O) Sulfur dioxide (SO2) Hydrogen sulfide (H2S) Carbon dioxide (CO2) Carbon Monoxide (CO) Ammonia (NH3) Methane (CH4) Note that oxygen (O2) gas is not created by volcanic eruptions Oceans It is hypothesized that water vapor escaping from the interior of the Earth via countless volcanic eruptions created the oceans (this took hundreds of millions of years) The earliest evidence of surface water on Earth dates back about 3.8 billion years Astronomers also hypothesize that comets impacting the Earth were a major source of water that contributed to creation of the oceans Continents By 2.5 billion years ago, the continents had been formed The density of the continental crust (2800 kg/m3) is lighter than the crust found on ocean bottoms (3200 kg/m3), so the continents rise above the ocean floor Review By 3.5 billion years ago, when the Earth was a billion years old, it had a thick atmosphere composed of CO2, methane, water vapor and other volcanic gases. By human standards this early atmosphere was very poisonous; It contained almost no oxygen (today our atmosphere is 21% oxygen) By 3.5 billion years ago, the Earth also had extensive oceans and seas of salt water, which contained many dissolved elements, such as iron. The First Atmosphere The early atmosphere would have been similar to the Sun-- mainly hydrogen and helium, but this atmosphere was lost quickly for two reasons: 1. The gravity of the modest size earth was not strong enough to prevent such light gases from escaping to space. 2. The impact leading to the origin of the moon contributed to the loss of Earth’s early H, He atmosphere. Hell on Earth The surface of the earth during this period was extremely hot with numerous volcanoes The earth was under near constant bombardment by objects of varying sizes Slowly, the earth started to cool down and the second atmosphere began to form. Earth’s Second Atmosphere —A new atmosphere was established by the out- gassing of volcanoes…the mixture of gases was probably similar to those of today’s volcanoes: —H20 vapor (roughly 80%) —CO2 (roughly 10%) —N2 (few percent) —Small amounts of CO, HCL, HS (Hydrogen Sulfide), SO2, CH4 (Methane), Ammonia (NH3), and other trace gases. Earth’s Second Atmosphere —Virtually no oxygen in that second atmosphere. —Thus, no ozone layer, so ultraviolet radiation flooded the earth’s surface. —With a huge influx of water vapor and the cooling of the planet, clouds and earth’s oceans formed. —At that time the sun was about 30% weaker than today…why didn’t the earth freeze over? —The apparent reason: so much CO2 so there was a very strong greenhouse effect.  The Rise of Oxygen and the Third Atmosphere —In the first two billion years of the planet’s evolution, the atmosphere acquired a small amount of oxygen, probably by the splitting of water (H20) molecules by solar radiation. —The evidence of this oxygen is suggested by minor rust in some early rocks. —The oxygen also led to the establishment of an ozone layer that reduced UV radiation at the surface. —With the rise of photosynthetic bacteria (cyanobacteria) and early plants, oxygen levels began to rise rapidly as did indications of rust in rocks —Between 2.5 billion years ago to about 500 mya, 02 rose to near current levels. The Third Atmosphere While O2 was increasing, CO2 decreased due to several reasons: In photosynthesis CO2 is used to produce organic matter, some of which is lost to the system (e.g., drops to the bottom of the ocean or is buried), and chemical weathering, which removes CO2  Fig. 6.14 Stage I to Stage II: evolution of nitrogen, (N) the virtual disappearance of hydrogen (H) and methane (CH4). Stage II to Stage III: rise in oxygen (due to evolution of photosynthetic algae). Note the presence of the noble gases, Ar, Ne, He and Kr. Most likely from the degassing upper mantle which continues to today. Chemical Weathering —H20 + CO2 --> H2CO3 carbonic acid —CaSiO3 + H2CO3 --> CaCO3 + SiO2 + H20 Silicate Rock Carbonate —At first this happened without life, but the process was sped up tremendously by living organisms —Marine organisms would incorporate carbonate into their shells, which would fall to the ocean bottom when they died---thus, removing them from the system for a long time. —The bottom line…CO2 was being removed from the system. A Problem —With lower CO2 levels the earth became more susceptible to ice ages when solar radiation decreases due to orbital variations, —It appears that around 750-550 million years ago the earth cooled down and became nearly entirely glaciated. —Note: one can get into a feedback with snow reflecting solar radiation, producing cooler temperatures and more snow, leading to less radiation, etc. -- We’ll talk about this later… How Did We Get Unfrozen? Volcanoes were still putting CO2 into the atmosphere Weathering was greatly reduced…since little liquid water. So CO2 increased until the greenhouse effect was so large the earth warmed up. Once warming started it would have happened very rapidly. The Last 700 Million Years The climate has not been constant, with warm periods interrupted by ice ages. Much of the variability forced by changing solar radiation due to periodic changes in the earth’s orbital characteristics and tilt (Milankovitch cycles) and major volcanic eruptions (putting out massive CO2 that caused warming).