PTYS 214 Spring Announcements Midterm #4: two weeks from today!

Transcription

1 PTYS 214 Spring 2018 Announcements Midterm #4: two weeks from today! 1

2 Previously Radiometric Dating Compare parent / daughter to determine # of half lives 14C, 40K, 238U, 232Th, 87Ru Evidence for Early Life Fossils Biomarkers Isotopes O2 2

3 Early Life Summary Evidence of the earliest life on Earth is difficult to validate: Isotopic evidence seems to date it back to about 3.5 Gyr (Pilbara craton, Australia) Oldest stromatolites are about 3.46 Gyr old Earliest microfossils date back to about 2.55 Gyr (Transvaal Supergroup, South Africa) Earliest molecular biomarkers date back to about Gyr old rocks (Pilbara, Australia) 3

4 Atmospheric Oxygen All terrestrial life requires energy, nutrients, and liquid water Why is atmospheric oxygen important for life? 4

5 Atmospheric Oxygen All terrestrial life requires energy, nutrients, and liquid water Why is atmospheric oxygen important for life? All terrestrial multicellular life requires high O2 CH2O + O2 H2O + CO2 + energy Almost all terrestrial life requires some protection from UV 5

6 Ozone (O3): Good and Bad 90% 10% 6

7 Stratospheric Ozone Most of the ozone is in the stratosphere (above 15 km), where it is produced Production: O2 + (UV radiation < 240 nm) 2 O O + O 2 O3 Destruction: O3 + (UV radiation nm) O2 + O UV-C UV-B UV-C O3 + O 2O2 The Good Stratospheric ozone absorbs part of the UV spectrum (<310 nm) where other gases do not absorb That s why ozone in stratosphere is good for us 7

8 Tropospheric Ozone In the troposphere ozone is the result of pollution: OH + COpollution H + CO2 H + O2 HO2 HO2 + NOpollution OH + NO2 NO2 + hν NO + O O + O 2 O3 Net reaction: CO + 2O2 CO2 + O3 The Bad Ozone is a very chemically active gas and can cause eye and respiratory problems 8

9 Atmospheric Oxygen! Both O2 and O3 are important to the biosphere but O3 cannot form without O2 What are natural sources of O2? 9

10 Atmospheric Oxygen! Both O2 and O3 are important to the biosphere but O3 cannot form without O2 What are natural sources of O2? Volcanoes: NO Major volcanic gases are H2O, CO2, SO2 etc., but no O2 Today the major source of O2 is LIFE H2O + CO2 CH2O + O2 Mt. Pinatubo eruption,

11 Oxygen Sources Hydrogen escape Water dissociation (minor): 2H2O+hν O2 + 4H Photosynthesis: CO2+H2O O2 + CH2O 11

12 Oxygen Sinks Atmospheric O2 Aerobic Respiration CH2O+O2 CO2 + H2O Oxidation of volcanic reduced gases O + H2O H2O2 Outgassing SO2 + H2O2 H2SO4 (volcanoes) Oxidative weathering of rocks Fe2+ Fe3+ Methane Oxidation CH4 + O2 CO2 + 2H2 SO2, H2S, H2 (FeO Fe2O3) Land Ocean Land BIFs Main source is biological, but many abiological sinks 12

13 When did life start to produce O2? Molecular biomarkers Earliest biomarkers for cyanobacteria : ~ Gyr ago Some photosynthetic O2 flux about 2.7 Gyr ago Geologic Evidence Atmosphere with low oxygen until about 2.3 Gyr ago: BIFs (Banded Iron Formations) Sulfur Isotope Ratios Charcoal 13

14 Banded Iron Formations (BIFs) Varying O2 amount Alternating iron-rich layers and ironpoor shale or chert layers Iron-rich: include iron oxides (Fe3O4 or Fe2O3) formed in the oceans by combining oxygen with dissolved iron 2.47 Gyr old Brockman Iron Formation, Western Australia Iron-poor: deep ocean should have been anoxic, causing deposition of shales and cherts 14

15 Sulfur Mass-Independent Fractionation (MIF) Normally, isotopic ratios of an element follow a standard mass fractionation line (MFL): 33S S Prior to 2.5 Gyr ago the isotope ratios fall off the MFL line! 33S = 33S S 0 S-isotopes: 32 S: 95% 33 S: <1% 34 S: 4 % 36 S: trace Gyr old samples.5 0 > S S 15 Mass-dependent 3 3 S 34 S <0 Farquhar et al

16 Sulfur Mass-Independent Fractionation Sulfur MIF can only occur in an oxygen-free atmosphere 33S = 33S S Large Sulfur MIF effects are associated with photochemical reactions (involving UV radiation) mass-independent mass-dependent Kump (2008) Nature 451, p

17 Forest Fires and Atmospheric Oxygen CH2O + O2 CO2 + H2O Fires produce charcoal that is preserved in the geologic record 17

18 30% 20% Fire 21% 15% Present Atmospheric Level (PAL) 10% 0% Atmospheric oxygen There has been a continuous record of charcoal in sediments younger than 360 million years old O2 levels have not been lower than 15% during the past 360 million years 18

19 Atmospheric Oxygen Summary ~1ppm A few% 15-35% No O2/O3 19

20 What About Ozone (O3)? O2 rise causes O3 rise An O2 level of 1% PAL is sufficient to create an ozone screen The O3 layer should have been absorbing most UV radiation by 2.3 Ga, as soon as O2 levels began to rise 20

21 Oxygen and the evolution of life Phanerozoic Eon (542 Myr ago - present) Visible life (macroscopic animals and plants) Proterozoic Eon ( Gyr ago) Mostly single-celled and some primitive multicellular organisms Archean Eon (3.5? Gyr ago) Single-celled organisms, prokaryotes (cyanobacteria) and some eukaryotes 21

22 Slow Early Evolution Oxygen was in the atmosphere by 2 Gyr ago However, life was limited to unicellular organisms or very simple multicellular organisms until ~540 Myr ago The oldest known possible multicellular eukaryote is Grypania (~1.9 Gyr old) 22

23 Cambrian Explosion About 540 Myr ago there was a seemingly rapid appearance of complex multicellular organisms Coincides with O2 increase above 15%. All known complex multicellular organisms need at least 10-20% of the present oxygen 23

24 Phanerozoic Eon Paleozoic Era ( Myr ago) - Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian periods - Age of sea life (trilobites) Mesozoic Era ( Myr ago) - Triassic, Jurassic, Cretaceous periods - Age of dinosaurs Cenozoic Era (0-65 Myr ago) - Paleogene, Neogene periods - Age of mammals 24

25 What does the fossil record say about biodiversity? Need to count Species: ability to interbreed, producing fertile offspring similar morphology (body shape) or DNA Species continually come and go due to genetic mutations and natural selection 25

26 Logistic Growth Curve Change in number of species = origination rate - extinction rate 26

27 Logistic Growth Curve Initial exponential growth # of species or individuals Change in number of species = origination rate - extinction rate Eventual equilibrium So what does the fossil record say? 27

28 No logistic growth curve in the fossil record! Why? 28

29 Species Sampling bias! Crust There are many more recent rocks than ancient rocks available to study Sediments 29

30 Evolution of Complex Life Forms Earth-like complex life requires not only energy, water, nutrients but also oxygen and ozone (UV protection) Suppose the environment has everything indicated above (Phanerozoic eon) Does it mean that evolution of complex species will proceed smoothly? 30

31 Evolution of Complex Life Forms Earth-like complex life requires not only energy, water, nutrients but also oxygen and ozone (UV protection) Suppose the environment has everything indicated above (Phanerozoic eon) Does it mean that evolution of complex species will proceed smoothly? No! 31

32 Mass Extinction Sharp decrease in the number of species in a relatively short period of time Rapid event - 10,000 to 100,000 years A significant part of all life on Earth became extinct (use of families is more reliable than species; for example extinction of 18% of all families corresponds to about 40% of all genera and 70% of all species) Extinct life forms from various phyla, lived in different habitats, spread out over the whole world 32

33 Homework Homework #15 available shortly on the web site. 33