Lecture 22Continue with evolution of Earth’s atmosphere(oxidation and reduction reactions on blackboard)Start on “Snowball Earth”Evidence for an oxic transitionBefore the oxic transition: Reduced minerals (before 2.4 Ga)1. Banded Iron Formations.2. Detrital Reduced minerals in riverbeds.3. Paleosols (ancient soils) show Fe loss.After the transition: Oxidized minerals.1. Red beds.2. Decline of reduced indicators (BIFs, detrital)paleosols, Fe retained.(Isotopic indicators are ‘smoking gun’ evidence (e.g. sulfur isotopesdescribed in the textbook, p.217-218). But the chemistry is complexand we will skip this).Banded Iron Formations (BIFs)Laminated chemicalsediments ≥15 wt% Fe.Western AustraliaFe2+HydrothermalinputOxide BIFPhotosynthesis, mixed layer O2Japanese carsUpwelling Fe 2+Photo: A. Knoll, HarvardFe3+Litte change in the O2 sourceRemember that net O2 input into the “atmosphere-ocean-surface”system comes from the burial of organic carbon.When we look at average sedimentary rock from 3.8 Ga to present,apart from a few relatively brief periods, remarkably we find:1) roughly constant 0.5% by weight organic carbon (no change)2) Isotopic evidence for litte change in organic carbon burial ratesIf the source of O2 has not changed, then what else could havecaused a rise of O2?Answer: the sink (i.e. mechanism for removal of O2).Leaky bucket analogy for growth in O2 atmospheric levelsIn each case: flux in = total flux out, but the water level is different.Water level - analogous to the O2 amount in the atmosphereHole size in base - analogous to amount of volcanic & metamorphicreducing gases (H2, CO) that react rapidly with O2 Holes in sides - analogous to oxidation of continents (e.g. loss tored beds) that kicks in only at higher O2 amounts.Theory for the rise of O2It is thought that the Earth’s rocky crust (and mantle)became more oxidized because of the loss of hydrogen to outer space.A more oxidized crust and mantle produces more oxidized outgassed gases, and the amount of outgassedoxygen-consuming gases ( H2, CO, etc.) diminish. Implies more O2.The set of reactions in the atmosphere that removeO2 are highly nonlinear. We do not get a steady increase in O2with a steady decrease of reducing gases.Instead, O2 reaches a critical point after which it leaps up in abundance.Aside: H2 emissions in today’s environmentScience, v. 300, p.1740 (2003)However, this paper took an extreme “worst case” view that 10% ofhydrogen would leak from the future production/fueling system.This is probably a huge overestimate; more like 1%.2000 Presidential election:George W. Bush mocksAl Gore as an environmental extremistfor wanting to eradicatethe combustionengine in the futureJanuary 2002:Pres. George W. Bushannounces a $1.2b programto research hydrogen fuel….to eradicatethe combustionengine in the futureBack to the Archean Earth:Enhanced H escape via methane, CH4Today vast amounts of methane are consumed in reaction withoxygen in the atmosphere, leaving CH4 at a trace abundanceof only 1.7 ppmv.In the low O2 Archean atmosphere, biogenic methane wouldbe abundant, at about ~1000 ppmv.But what ultimately happens to such methane in the upperatmosphere? Answer: It gets photolyzed and hydrogen escapesaway.Earth“Northern lights” ovalH atom geocorona in red:UV (121 nm) image of Earth.•Today, half of these H atomsoriginate from microbial CH4 ; theother half from H2O that makes itinto the stratosphere.•With greater CH4, H escape wouldbe significant and oxidize the Earth•In the low-O2 Archean, CH4 wouldbe ~1000 ppmv (compare 1.7 ppmvtoday).H escape rates were few hunderdtimes greater.H atomsToday: 93,000 metric tonnesof hydrogen escape to spaceeach year.Methane’s H escape causes O2 gain2CO2+2H2O = 2O2 + 2CH2O+ methanogenesisCO2+2H2O Æ CH4+2O2 Æ CO2 + O2 +4H(↑space)Photosynthesis + methanogenesis:2CH2O = CH4 + CO2photosynthesisnetCO2 +2H2O = CH4 +
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