Chapter 11: Evolution of the Earth

1. Chapter 11: Evolution of the Earth

2. Early history: • Earliest evidence for oceans? - oldest whole rock samples, 4 billion yrs (northern Canada) - composites of basalts typical of ocean crust, + rock somewhat like continents. • Oxygen record: A. Zircons: (zirconium sulfate): very hard gem like quality, dated by U-Pb system. - resistant to melting - dates 3.9 – 4.3 billion - ratio of O-18 / O – 16: formation environment, particularly presence of water. - 1% of zircons, dated 4.3 billion yrs, show evidence that water was circulating in crust.

3. B. Cherts: Sedimentary form for silica (SiO2) (small cyrstals or glassy forms) - isotopic ratio of oxygen is sensitive to type of environment in which chert formed - for ocean sediments, oxygen isotope ratio increases with increasing temperature (deduced from experiments). - uncertainties: local measure of T, not global; oceanic values of ratio affected by large scale glaciations; - find: has been a general decrease in ocean temperature. Earliest oceans much hotter than today, so only more heat resistant microbes would be selected.

4. Early Earth Atmosphere(s)…. • First Atmosphere - Composition - Probably H2, He - These gases were probably lost to space early in Earth's history because • Earth's gravity is not strong enough to hold lighter gases • Earth still did not have a differentiated core (solid inner/liquid outer core) which creates Earth's magnetic field (magnetosphere) which deflects solar winds. - Once the core differentiated the heavier gases could be retained • Second Atmosphere Produced by volcanic out gassing. - Gases produced were probably similar to those created by modern volcanoes (H2O, CO2, SO2, CO, S2, Cl2, N2, H2) and NH3 (ammonia) and CH4 (methane) - No free O2 at this time (not found in volcanic gases)

5. Earliest evidence for life… • Need a good clock for biochemical processes. - these rely on carbon uptake… so look at long-lived isotopic ratio. - 13C/12C is preferred since living things take up the lighter isotope 12C • Banded iron formation (BIF): layers of iron rich sediment interspersed with chert, show evidence of high isotopic ratio: • Embedded zircons provide age: 3.85 billion years. - iron sedimentation varies strongly with oxygen content…. perhaps as consequence of oxygen production such as early photosynthesis?

6. Banded iron formation: Red layers are iron cherts… (from http://geology.about.cotmm/library/bl/images/blbif.htm )

7. Banded Iron Formation (BIF)- Deep water deposits in which layers of iron-rich minerals alternate with iron-poor layers, primarily chert. Iron minerals include iron oxide, iron carbonate, iron silicate, iron sulfide. • BIF's - major source of iron ore, b/c they contain magnetite (Fe3O4) • Common in rocks 2.0 - 2.8 B.y. old, but do not form today.

8. Fossils of early micro-organisms? • Stromatolites – layered remnants of biological activity in bacterial colonies; enriched light carbon found in these - rocks < 3 billion yrs old – show widespread stromatolites and other biomarkers. - some evidence, although disputed, is that this is seen also at 3.5 Byr. • Best evidence: life began at least 3.5 Byr ago.

9. Oxygen history of early Earth….

10. CO2 and a massive early Green house effect • Equate the energy absorbed by the Earth from incoming radiation, with energy by the Earth (this is the black body radiation condition again): where A is reflectivity of the atmosphere, or “albedo”. - For A = 0.33; find T=256 K… below freezing point of water! - Atmosphere must have some way of retaining heat… allowing warmer temperatures. ***Greenhouse effect: optical light penetrates atmosphere – and warms surface of Earth - Reradiation is at infrared wavelengths… which is absorbed by atmosphere… - Lack of transparency due to carbon monoxide, methane, and water …. Gives T=288K for Earth

11. Evolution of the Sun… and effect upon Earth • When sun started burning on “main sequence, luminosity was lower (70% of present value) • So flux from Sun was lower – so that temperature of Earth would be lower as well: T=255K then (33K lower than today’s average temperature) • How much CO2 was needed – and how much was around? - CO2 is mostly locked up in sedimentary rock; in carbonates. Amount present comparable to total amount that is in Venus atmosphere - the above are 10^5 more massive than amount of CO2 in atmosphere..

12. This amount of CO2 placed in atmosphere, can keep ocean warm. Remember, early sun less luminous, and Earth is 30% farther from Sun than Venus. • Sustain over long time: need to decrease levels with time – ie dissolve CO2 in ocean depositing carbonates.

13. Carbon-silicate cycle; plate techtonics and weathering… • Removal time for CO2 out of atmosphere is only a million yrs! [conversion of calcium and bicarbonate accomplished by shell-forming organisms] • What replaces such rapid loss? - plate techtonics: Subduction of plates beneath others heats up rock, which melts at T around 1000K. • Calcium carbonate reacts with silicates and water in this high pressure environment, … releasing CO2. which gets back into atmophere by escape through volcanoes near subduction region. Cycle time: 60 million yrs.

14. Plate tectonics: (map of earthquake activity) • Plates are buoyant and float on underlying plastic mantle • Two types of plates: oceanic (basalt) and less dense continental (granite) Subduction zones located generally near edges of continents (eg. Japan, Alaska,..)

15. Stabilizing the Earth’s weather systems… • When climate unusually warm: - higher rainfall -> increased erosion -> increased binding of CO2 from atmosphere into carbonates -> temperature drops • When climate unusually cold: - lower rainfall and lower rate of loss of CO2 due to erosion, - CO2 input into atmosphere from volcanic activity -> raises temperature • Together, this constitutes “negative feedback loop” • Life affects this: plants accelerate trapping of CO2 ; decreasing it in atmosphere, increasing cycling rate of CO2 into mantle

16. Origins of continents… • Large continents - arose less than 3Byr ago – rock record shows ancient material very small fraction of total stable crust… even taking into account cycling time of continents. • Less than 0.2 % of Earth’s volume has been transformed into granite crust associated with continents • Formation of iron core leads ultimately to mantle depleted in iron – which when melted – produced basalt. • Making granite is different – not well understood. As it builds up -> larger continents, and fewer of them. • Collisions of these create supercontinents every 500 million yrs. -> reduces area of ocean floor that is subducted; reduces mountain building -> less erosion from rainfall engendered by high mountains… • Breakup of continents: -> much more rapid scrubbing of CO2 from atmosphere -> cooling -> Snowball earth episodes?

17. The rise of oxygen… • Today: largest source for oxygen: photosynthesis; largest sink is respiration and decay (Table 11.2) • When was atmosphere first oxygen rich: 2.2 - 2.4 Byr ago • Earliest appearance of oxygen… 3.5 Byr ago.. • Why the difference? Oxygen produced was first absorbed into minerals which deposited as ocean sediments… before it could build up and accumulate in atmosphere • Oxygen added to ferrous iron (FeO) in water produces Fe2O3 - which is much less soluble and precipitates out • Probably origin of BIFs…. These have ages 3.5 – 1.8 Byr ago.. Bottom Line… appearance of oxygen producing life had profound geochemical effects, as well as “chemical warfare” on earliest organisms which were “poisoned” in these new, oxygen rich environments…