Chapter 11: Evolution of the Earth

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.

B. Cherts: Sedimentary form for silica (SiO 2 ) (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.

Early Earth Atmosphere(s)…. First Atmosphere - Composition - Probably H 2, 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)

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?

Banded iron formation: Red layers are iron cherts… (from )

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 (Fe 3 O 4 ) Common in rocks B.y. old, but do not form today.

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.

Oxygen history of early Earth….

CO 2 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

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 CO 2 was needed – and how much was around? - CO 2 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 CO 2 in atmosphere..

This amount of CO 2 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 CO 2 in ocean depositing carbonates.

Carbon-silicate cycle; plate techtonics and weathering… Removal time for CO 2 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 CO 2. which gets back into atmophere by escape through volcanoes near subduction region. Cycle time: 60 million yrs.

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,..)

Stabilizing the Earth’s weather systems… When climate unusually warm: - higher rainfall -> increased erosion -> increased binding of CO 2 from atmosphere into carbonates -> temperature drops When climate unusually cold: - lower rainfall and lower rate of loss of CO 2 due to erosion, - CO 2 input into atmosphere from volcanic activity -> raises temperature Together, this constitutes “negative feedback loop” Life affects this: plants accelerate trapping of CO 2 ; decreasing it in atmosphere, increasing cycling rate of CO 2 into mantle

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 CO 2 from atmosphere -> cooling -> Snowball earth episodes?

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: 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 Fe 2 O 3 - 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…