1. Astronomy190 - Topics in Astronomy Astronomy and Astrobiology Lecture 10 : Earth History Ty Robinson

2. Questions of the Day How do isotopes help us study the climate history of Earth? What is the ‘Faint Young Sun Paradox’? How and why have levels of CO 2, CH 4 (methane) and O 2 changed through Earth’s history?

3. The Geological Time Scale Phanerozoic, 540Mya to present day –“visible life” fossils of plants and animals Proterozoic, 2.5Gya to 540Mya –“earlier life” fossils of single-celled organisms Archean, 3.9 to 2.5Gya Hadean, 4.6 to 3.9 Gya

4. A Word About Isotopes… isotopes can indicate climate variations on Earth over its history need stable isotopes of the same element the isotopes’ mass difference can lead to “fractionation” of the isotopes in natural systems at different temperatures to use as climate indicators we need –stable isotopes of the same element whose fractionation is temperature dependent –storage of the fractionated mixture over time –ability to measure the fractionation in samples –a means to date the stored isotope data typically use isotopes of carbon, oxygen, hydrogen, and shells or minerals as the storage

5. Oxygen as a Climate Probe water has three stable isotopes, H 2 16 O, H 2 17 O and H 2 18 O H 2 16 O is preferentially evaporated from the oceans to form clouds during ice ages, the clouds precipitate snow to form ice sheets there is a net loss of H 2 16 O from the oceans and an enhancement of H 2 18 O vs H 2 16 O in the oceans enrichment in 18 O In tiny shelled sea creatures indicates cooler climate conditions

6. Questions? Why must we use stable isotopes as climate indicators? radioactive isotopes are dangerous to work with radioactive isotopes decay, skewing results stable isotopes are more abundant stable isotopes are environmentally friendly

7. Questions? Why is H 2 16 O preferentially evaporated from the oceans? it is the lightest of all water isotopes it is the most abundant of all water isotopes it is the heaviest of all water isotopes it is the only water isotope present in ocean waters

8. The Hadean Eon: “Cryptic” Era The Earth forms 4.6 Gya 50-70 My later, a Mars-sized object (0.1Me) collides with the Earth and the Moon is formed.

9. The “Hadean” first 0.6Gy of Earth’s history Sun was 30% fainter than at present formed with no “primary” atmosphere, but outgassed first atmosphere –atmosphere was likely CO 2 and water vapor dominated, no O 2 at that time zircons show oceans and continental material had formed by 4.4Gya (0.1Gy after formation)

10. A Planet Under Siege impacts occurred from formation through until the Late Heavy Bombardment (about 3.9Gya) sterilizing impacts probably occurred 6-12 times during the Hadean –250-400 km diameter asteroid will vaporize oceans

11. The Archean Eon 4.0-2.5 Gya (starts with the oldest whole rock samples and ends with the rise of oxygen) sedimentary rocks are seen after 3.85 Gya life first detected by 3.7-3.5 Gya Earth’s heat flow would have been much higher and may have resulted in small plates with hotspots in between –produced basaltic proto-continents –Archean continents 10-60% of present

12. Questions? Why was the Earth’s heat flow higher in the Archean? leftover heat from the Moon-forming impact the Sun was brighter in the Archean plate tectonics were not functioning in the Archean more the rate of energy production from radioactive decay was higher in the Archean

13. 7% of the modern continents contain surviving Archean rocks

14. The Faint Young Sun Paradox 3.8 Gya the Sun’s luminosity was 75% of the present value an atmosphere with the modern composition, gives a surface T of 255K (-18C) yet, water was liquid on the surface to warm the early Archean above freezing requires 1000 times the amount of CO 2 in the present atmosphere (it would have been 25% of atmosphere) evidence from the late Archean indicate that the atmosphere could have been no more than 3% carbon dioxide

15. …and to make matters worse 18 O isotope data implies that the Archean ocean may have been 50C, almost twice as warm as today’s hottest oceans!

16. Archean Atmospheric Composition ratios of different sulfur isotopes in rocks (combined with other data from the rock record) suggest that O 2 was very low in the Archean atmosphere O 2 rose dramatically in the late Archean/early Proterozoic

17. The Proterozoic 2.5 Gya to 540 Mya evolution from “small plates” to more modern large plate tectonics “supercontinents” may have formed near the beginning of the Proterozoic (~2.3Gya) atmospheric O 2 rose dramatically from almost nothing in the Archean, to about 10% of the present concentration

18. The Phanerozoic 540 Mya to present characterized by complex multicellular organisms in the fossil record total continental mass comparable to today modern-style plate tectonics operating O 2 is at near modern levels, and is maintained at levels between 15- 35% of the atmosphere

19. Carbon as a Climate Probe Carbon has two stable isotopes, 12 C and 13 C plants preferentially take up 12 C consequently they preferentially enrich the atmosphere in 13 C atmospheric carbon enters the oceans through the chemical erosion of rocks this fractionation in carbon in the atmosphere is recorded in the shells of tiny sea creatures the higher the 13 C/ 12 C ratio, the warmer the climate was at the time the sea creature lived

20. CO 2 in the Phanerozoic

21. The Earth’s Ice Ages “Ice Age” is a loose term for times when glaciers cover down to mid-latitudes. Ice ages can be triggered by moving continents poleward, and may have been mediated by life- induced changes in atmospheric composition. –Loss of methane at the Archean/Proterozoic boundary –Loss of CO 2 after widespread colonization of continents by plants Snowball Earth?

22. The Rise of Oxygen Modern N 2 O 2 CO 2 CH 4 N 2 CO 2 CH 4

23. Questions? Why might you expect that rises in oxygen levels correspond to decreases in carbon dioxide levels? I wouldn’t expect this! CO 2 can come from burning organic material, which requires O 2 oxygenic photosynthesis converts CO 2 into O 2 oxygen comes from plants while CO 2 comes from volcanoes

24. The Earth’s Ice Ages

25. Questions of the Day How do isotopes help us study the climate history of Earth? What is the ‘Faint Young Sun Paradox’? How and why have levels of CO 2, CH 4 (methane) and O 2 changed through Earth’s history?

26. The Phanerozoic (cont). The Phanerozoic eon is divided into eras, and eras into periods. Boundaries between most of the periods are extinction episodes. –5 mass extinctions –Possibly due to drastic climate change, ice ages, large-scale volcanism and impacts

27. Summary Earth has evolved from: –small amounts of continental mass to large continents –small plates to large plates –low O 2 in the Archean to ~20% O2 in the Phanerozoic rapid rise at the Archean/Proterozoic boundary (2.5Gya) –higher CO 2 to lower CO 2 –possible higher T in the early Archean to lower temperatures During its history, Earth has suffered several ice ages, impacts, and volcanic episodes.