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(6) Earth in space and time. The student knows the evidence for how Earth's atmospheres, hydrosphere, and geosphere formed and changed through time. The student is expected to: (C) investigate how the formation of atmospheric oxygen and the ozone layer impacted the formation of the geosphere and biosphere; and (D) evaluate the evidence that Earth's cooling led to tectonic activity, resulting in continents and ocean basins. The geosphere refers to everything, from the core of the Earth to its surface. As you can see, there are many features within the Earth, many of which we already have learned about. The biosphere is the layer of life on Earth. It exists beneath, upon, and above the surface in the atmosphere as well. Axolotl Soil Nematodes Airborne Bacteria Harvestman One thing most geologists agree on is that the Earth’s first atmosphere contained no free oxygen. There were trace amounts of Oxygen bound in water molecules, and Carbon dioxide…but none of it was “free”, or molecular oxygen. (O2) Photochemical Dissociation Hypothesis states that the Sun’s energy helped the atmosphere evolve through the following processes: • The UV light combined with water vapor to set the hydrogen off into space and freeOxygen the oxygen. 2H2O + UV light energy 2H2 (freed into space) + O2 Oxygen • The newly freed oxygen reacted with methane, forming carbon dioxide and additional water vapor. + + CH4 + 2O2 CO2 + H2O Oxygen • The oxygen also reacted with ammonia, producing nitrogen and water. 4NH3 + 3O2 2N2 + 6H2O • After converting the ammonia and methane to carbon dioxide and nitrogen, free oxygen began to accumulate as further dissociation of water vapor continued. This theory states that our atmosphere was delivered to us from the Earth’s interior through volcanic eruptions. In contrast to the Photochemical Dissociation Hypothesis, the Outgassing Hypothesis argues that the free oxygen came from the photosynthesis of primitive organisms which existed 1.5 - 3.5 billion years ago. The oxygen took approximately 2 billion years to become free, but when it did, it formed the ozone layer, within Earth’s stratosphere, eliminating the dangerous radiation and setting up the foundation for a habitable planet. Without ozone, life would not have been possible. It is obvious that Earth contains O2 now, and without it, aerobic life would not be possible. What life could have evolved all those billions of years ago, before there was significant O2 in our atmosphere? Anaerobic life forms http://www.teachersdomain. org/resource/tdc02.sci.life.ce ll.stetteroxygen/ 1. How is the geosphere different from the biosphere? 2. Describe how photochemical dissociation could have freed oxygen. 3. What other molecules did free O2 interact with to create water? 4. Oxygen reacts with _______ , a part of volcanic outgassing, to produce nitrogen gas in our atmosphere. 5. Outgassing hypothesis states our oxygen came from __________, due to all the CO2 generated by volcanoes. 6. Where does ozone form a protective barrier in our atmosphere? 7. What type of life evolved first, and is killed in the presence of O2? Although the early Earth was mostly devoid of molecular oxygen, high volcanic activity released significant amounts of molecular hydrogen. With little oxygen available to convert that hydrogen into water, hydrogen gas probably accumulated in the atmosphere and oceans in concentrations as high as hundreds to thousands of parts per million. The evolution of O2 in our atmosphere spelled doom Thus, the early Earth was likely a for the proliferate paradise for methanogens methanogens, and other (methane-producing) that fed types of extremophiles that directly on hydrogen and carbon had evolved during this early dioxide, at least until the period in Earth’s past. Once atmospheric hydrogen was depleted. again…this was the GOE. Despite their small stature, one of the first aerobic organisms (require the presence of O2) set in motion a process that would change everything. These cyanobacteria (also known as blue-green algae), were remarkably selfsufficient creatures that could use the sun’s energy to make their own food, and fix While this may not seem significant, the cycling of nitrogen on nitrogen, a process where Earth is essential for life. It is found in amino acids, proteins, nitrogen gas is converted into and genetic material. Nitrogen is the most abundant element in ammonia or nitrate. the atmosphere (~78%). However, gaseous nitrogen must be 'fixed' into another form so that it can be used by living organisms. Because cyanobacteria were the first photosynthetic organisms, they are also responsible for getting the cycling of carbon and oxygen going too! And then...nothing else happened. At least, not for another two billion years! It wouldn't be until about 600 million years ago, that the first multicellular organisms finally emerged. So what happened during that immense, multi-billion year gap? Why did it take so long for more complex life to arrive on the scene? For that matter, why did oxygen suddenly spike 2.5 billion years ago? The spike was very likely due to photosynthesis, but as to why complex life took so long… The simple, uncomfortable answer is that we don't really know. ??? Formation of Silicates: BIFs and Red Beds: Oxygen combines Without oxygen inwith the silicon atmosphere, in various neither of these would have evertoexisted configurations create on 80% ourofplanet. the Earth’s lithosphere…a silicate material. 8. What anaerobic organisms used hydrogen in chemosynthesis to produce food? 9. What are cyanobacteria? 10. Why were cyanobacteria so important to the nitrogen cycle? Carbon and oxygen cycles? 11. Why did O2 spike in the atmosphere 2.5 billion years ago? 12. How did oxygen influence the geosphere? We already know that over time, the Earth’s crust cooled. The crust is thin, varying from a few tens of kilometers thick beneath the continents to less than 10 km thick beneath the many of the oceans. The crust and upper mantle together constitute the lithosphere, which is typically 50-100 km thick and is broken into large plates. These plates sit on the asthenosphere. The asthenosphere is kept a plastic fluid consistency largely through heat generated by radioactive decay. This heat source is small, but nevertheless, because of the insulating properties of the Earth's rocks at the surface, this is sufficient to keep the asthenosphere flowing. Thermal energy can be transferred in three ways… Radiation Energy transfer across the vacuum of space Conduction Energy transfer directly from molecule to molecule (solids) Convection Energy transfer through fluids (liquids and gases) Very slow convection currents flow in the asthenosphere, (upper portion of the mantle) and these currents provide horizontal forces on the plates of the lithosphere much as convection in a pan of boiling water causes a piece of cork on the surface of the water to be pushed sideways Of course, the timescale for convection in the pan is seconds and for plate tectonics is 10-100 million years, but the principles are similar. Differentiation within the Earth is crucial to plate tectonics, because it is responsible for producing an interior that can support tectonic motion. It is this tectonic motion that is responsible for the widening Atlantic Ocean, along the divergent mid-Atlantic ridge, and the The heat generated byconstricting the lower mantle, drives the convection currents Pacific Ocean, which is upward against the lithospheric plates. As the currents cool, they move laterally, pushing subducting beneath themove North and again, South and pulling the lithosphere apart. Then, the currents downward where they begin to heat up once more duetectonic to proximity to lower mantle heat. American plates. Pangea Ultima How did we get here? https://youtu.be/C m5giPd5Uro Being dynamic, the Earth is still changing. 150 million years in the future, the continents should look something like this. In 250 million years, we will have another pangea supercontinent. 13. Where is Earth’s crust thickest? Thinnest? 14. By what three ways can thermal energy be transferred, and how 15. What type of energy transfer is at work in Earth’s mantle, which drives tectonic plate movement? 16. What is tectonic motion doing to the Atlantic Ocean? 17. When will the next supercontinent form?