Effect of Life on the Atmosphere: The Rise of Oxygen and Ozone

Transcription

1 Some preliminary chemistry Chapter 11 Effect of Life on the Atmosphere: The Rise of Oxygen and Ozone Chemical reactions involve the giving and taking of electrons between atoms. the nucleus is not affected in chemical reactions. e.g. when C is combined with O2, oxygen gives electrons. isotopes same number of protons and electrons, different number of neutrons. Affects the atomic weight, but not the electrons, and therefore not the types of chemical reactions it undergoes. Oxidation states associated with the number of electrons either given or taken in a chemical lbond. Oxygen is considered dto have an oxidation state of -2, so when a mineral is oxidized it gives 2 electrons to the oxygen molecule. Each element can be found on earth in one or more oxidation states, but usually only certain states are stable and are more frequent. How did early organisms affect the atmosphere? Speculative these paleo-studies of the origin of the earth and life (chapter 10) and the rise of oxygen and ozone (chapter 11) are highly speculative. How did organisms affect oxygen, methane, nitrogen The answer depends on the metabolic functions of the organisms: oxygenic photosynthesis anoxygenic photosynthesis th methanogenic chemosynthesis How did early organisms survive? Stromatolites 3.5 b.y. old (?), fossil remains of bacteria today found in shallow-salty environment in Western Australia. Layers sunlight and that they can photosynthesize! But produce O 2? Not necessarily. Recall: oxygenic photosynthesis sunlight + CO 2 + H 2 O organic C- compounds + O 2 anoxygenic photosynthesis sunlight + CO 2 + H 2 N or H 2 (instead of H 2 O) organic C-compounds + no O 2! And dthere is cyanobacteria that t can perform both!! and can switch from one to the other depending on whether H 2 N is there or not!! But there is more to the rise of O 2 in the atmosphere than O 2 ; we need to look into the production of CH 4 and the cycling of atmospheric N 2 1

2 We need some other evidence to indicate which form of photosynthesis these bacteria performed, and therefore how they affected oxygen levels, or how they affected any other atmospheric constituent. How did early organisms survive? Methanogenic bacteria (methanogens) hyperthermophiles, which live at high temperatures (deep sea vents), get energy from chemical reactions, not sunlight CO 2 + 4H 2 CH 4 + 2H 2 O Both CO2 and hydrogen were probably present in the deep sea vents as well as the atmosphere, in high enough concentrations. So, the early atmosphere may have had a lot of methane (CH 4 ), like Titan (Saturn s moon) which looks orange. Sequencing of ribosomal RNA results in these categories of organisms Bacteria and Archaea are single-celled organisms (Bacteria is used for both) and Eucarya include some single-cell but all the rest are there too (this includes humans) Ratio of 13 C to 12 C definition: 13 δ C sample = C/ C 1 x sample 13 C/ x 12 CPDB standard from K marine fossil CH 4 produced by methanogens is depleted in 13 C in relation to 12 C so the their ration can be used as evidence of methanogenic activity. But this requires a little bit extra: IF the CH 4 thus produced is used by other organisms, which died and eventually made it to the sediments (the preserved geological record which is analyzed), then the record will show low 13 C content (again, in relation to 12 C) see figure next. Evidence in the geologic record: particularly low 13 C levels in ancient sediments at around 2.7 b. y. ago may be associated with methanogens, because they preferentially incorporate light C ( 12 C). 12 C is taken up faster during photosynthesis than 13 C so organic C formed by photosynthesis is depleted in 13 C but methane is even more depleted in 13 C photosynthesis alone cannot explain the extreme low concentrations of 13 C in the record around 2.7 b. y. ago. 2

3 Effect of organisms (life) on the early atmosphere: 1. The production of CH 4 by methanogenic bacteria (simple autotrophic) CO 2 +4H 2 CH 4 +2HO 2 2. The cycling of N 2 through the atmosphere-ocean system - organisms need fixed nitrogen compounds (ammonia NH 3 and nitrate NO 3- ) Today, Nitrogen may be fixed abiotically (lightening) N 2 + O 2 2 NO (nitric oxide, a radical) NO is oxidized to HNO 3 highly soluble (dissociation) HNO 3 H + + NO 3- (can be used) However, in the early atmosphere there would have been little O2. How did early organisms affect the atmosphere In the ancient atmosphere, Nitrogen may be fixed abiotically, in the absence of O 2, the most likely reaction to get fixed nitrogen would have been (lightening) (dissociation) N 2 + 2CO 2 2 NO + 2 CO NO is oxidized to HNO 3 highly soluble HNO 3 H + + NO 3- (can be used) Nitrogen may also be fixed biologically i ll cyanobacteria in the oceans, prokaryotes (believed to have been first) such as methanogens, simpler, more primitive organisms that resist UV well How did early organisms affect the atmosphere Nitrogen if Nitrogen was being fixed, there had to have some reverse process to balance this, otherwise nitrogen would have been depleted d from the atmosphere in 20 million years Some evidence suggests that oxygenic cyanobacteria first produced O 2 about 2.7 b. y. ago (organic chemicals in sediments) chloroplasts prokaryotes, perform oxygenic photosynthesis. They were later incorporated into eukaryotes by endosymbiosis Denitrification some bacteria derive energy by reacting dissolved nitrate with CH 2 O. They release either N 2 or N 2 0. N 2 0 is then converted to N 2 by photolysis. Particularly in anoxic environments. 3

4 But, evidence also suggests that atmospheric oxygen did not start rising until 400 my later, 2.3 bya. Perhaps oxygen was reacting with something at the surface, and not remaining in the atmosphere. Evidence for the rise of oxygen comes from geological sources: 1. banded-iron formations 2. detrital uraninite and pyrite 3. paleosols and redbeds 4. mass-independent sulfur isotope ratios - most conclusive banded-iron formations (BIFs) banded because of alternating layers with/without iron: at certain times iron precipitated out of the water, and at other times it did not. Iron in different oxidation states is more or less soluble. The two common oxidation states of iron are: Fe 2+ is soluble in water Fe 3+ is not soluble During times when O 2 is present, iron will preferentially be in the Fe 3+ state, which h will not dissolve in water, not be transported by the oceans large distances, and therefore not end up in sediments. So, at times when the sediments have iron (the iron bands), oxygen was not present. BIFs occur prior to 1.9 bya, but almost no other time detrital uraninite Detrital refers to minerals that were transported from their origin to the site of deposition in its original solid form, not in solution. Geologists can usually identify such rocks. For uranium, the more oxidized (U 6+ ) is soluble, while the more reduced (U 4+ ) is not soluble. Thus, the presence of detrital uranium in the 4+ oxidation state indicates low atmospheric oxygen levels, while oxidized uranium in the 6+ oxidation state indicates the presence of atmospheric oxygen Sediments older than ~2.2 bya contain detrital uraninite in the 4+ oxidation state, indicating low atmospheric O 2 levels. detrital pyrite Pyrite (FeS 2 ), when weathered in the presence of oxygen, is oxidized to SO 2-4 and Fe 3+. When conditions are anoxic, the detrital form is carried all the way to the sea and deposited. Sediments older than ~2.2 bya contain detrital pyrite, indicating little or no oxygen in the atmosphere. 4

5 paleosols Paleosols (ancient soils) are found to be depleted in iron prior to around 2.2 bya, while since 1.9 bya they have more iron. If atmospheric oxygen is low, iron released during weathering remains as soluble Fe 2+, and gets washed out of the soil. If atmospheric oxygen is higher, iron released during weathering is oxidized to Fe 3+, which is insoluble and does not get washed out of the soil. redbeds Redbeds are sandy sediments with reddish color, such as in the southwest US (dry conditions). The color is associated with the presence of hematite (Fe 2 O 3 ), which is oxidized iron. The presence of hematite indicates the presence of oxygen in the atmosphere. Earliest known redbeds are ~2.2 bya. NOTE: oxidized iron? RUST! sulfur isotope ratios considered most conclusive evidence of low oxygen levels prior to 2.3 bya. Biological sulfate reduction (bacteria use sulfur to oxidize CH 2 0) preferentially uses light sulfur (S 33 instead of S 34 ). In sulfur bearing rocks today, there is a regular ratio of the different isotopes of sulfur (the mass fractionation line, or MFL). Pyrite (FeS 2 ). sulfur isotope ratios considered most conclusive evidence of low oxygen levels prior to 2.3 bya. In rocks older than ~2.3 bya, the relative ratios of the different Sulfur isotopes in Pyrite deviates from more recent values. SOMETHING CHANGED! Sulfur isotope concentrations in Archean rocks ( b. y. ago) Time series of deviations of S concentrations from the MFL 5

6 sulfur isotope ratios considered most conclusive evidence of low oxygen levels prior to 2.3 bya. In rocks older than ~2.3 bya, the relative ratios of the different sulfur isotopes in Pyrite deviates from more recent values. SOMETHING CHANGED! This indicates that no preferential sulfur isotope was being incorporated into rocks. The process was probably photolosys of SO 2, which does not occur today because of ozone. This indicates much lower ozone, and therefore oxygen, values prior to 2.3 bya. Also, in a low-oxygen atmosphere, sulfur undergoes different chemical reactions in the atmosphere, and is deposited to the surface in a variety of molecules (and oxidation states). Today, atmospheric sulfur is oxidized to sulfuric acid (H 2 SO 4 ), which is soluble and gets rained out, which would wash out the effects of photolytic reactions. Problem: why does so much geological evidence suggest that cyanobacteria, which perform photosynthesis and release oxygen to the atmosphere, existed ~2.7 bya, but oxygen levels did not begin to rise until at least 2.3 bya? SOME THEORIES 1. it took that long for the oxygen to oxidize all the iron in the oceans (researches believes it would have been much quicker) 2. Perhaps the cyanobacteria were not really producing any, or much, oxygen, but rather they were performing anoxygenic photosynthesis 3. Perhaps volcanic emissions were much more reduced back then, meaning that the oxygen emitted to the atmosphere would have reacted with (i.e. oxidized) those gases, and not remained as O 2. The Rise of Ozone The Rise of Ozone 6

7 Variations in Atmospheric O 2 Over the Last 2 Billon Years initial rise of O b. y. ago - conclusive evidence for rise of O 2 in atmosphere 2.3 b. y. ago multicellular li ll l organisms seem to date from b. y. ago only (h (these need a good amount of O 2!) The Cambrian begins about 0.5 b. y. ago - Phanerozoic - this is when organisms start to develop hard shells, more detailed fossil records since then. Direct evidence for O 2 poor for this period (as it was before) but the estimates come from studies of Carbon isotopes. Variations in Atmospheric O 2 Over the Last 2 Billon Years Phanerozoic Carbon Isotope Record 1. Burial of organic carbon produces O 2 for atmosphere 2. Organic carbon is isotopically light : lower C 13 /C During periods of more organic carbon burial, the buried organic sediments are light; the carbon remaining in the atm/ocean system is heavy; and the inorganic sediments (which are the same del 13 C as the ocean) are also heavy. 4. So, del 13 C is indicator of rate of O 2 production Phanerozoic del 13 C from carbonate (inorganic) rocks Indicator of O2 production rate (not atm concentrations) carboniferous coal deposits Calculated variation in atmospheric O 2 during the Phanerozoic (0-0.5 b. y. ago) 7

8 Modern controls of atmospheric O 2 Modern controls of atmospheric O 2 Marine sediments - Most organic-c burial is Oceanic (controls atm O 2 ) Vertical profile of DO in the ocean (low-lat.) The solubility pump Atmosphere Atmospheric O 2 fire window Negative feedback for control of O 2 (?) involving DO and the C:P ratio 1. High O 2 -> lower C:P (more P required for every C) 2. If P is limiting nutrient, then lower organic C burial 3. Lower Org-C burial, slower O 2 production rate Modern controls of atmospheric O 2 Forests - possible negative feedback for O2 control involving forest fires and the C:P ratio 1. High O2 -> more forest fires 2. More forest fires -> Less terrestrial burial (C:P = 1000:1) 3. More forest fires -> more sediment transport, more marine burial (C:P = 105:1) 4. If P is limiting, each mol of P buries less C, and produces less O 2 8