Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
The Oxygen Revolution as a premise to the rise of animal life PAL = present-day O2 level in atmosphere = 0.2 atm Oxygen rise between 2.5 and 2.0 By Oxygen proxies: 1- MDF vs MIF of the S cycle 2- BIF vs redbeds 3- Uraninite - pyrite - siderite 4- The C cycle 5- Paleosols The Sulphur cycle Forms of Sulfur Valences -2 0 +2 +4 +6 O X I D A T I O N R E D U C T I O N Forms Sulfides (Solfuri) H2S, FeS2 Elemental Sulfur S Hyposulfites Sulfites Sulfates CaSO4, BaSO4 The Sulphur cycle SO4 solfato H2SO4 H2S SO4 Oxidation of FeS2 => Hem+SO4; then H2S SO2 SO4 solfato batteri H2S+Hem FeS2 Solfuri On modern Earth, the isotopes (e.g., 32S, 33S, 34S, 36S, or 16O, 17O, 18O, or 12C, 13C) are fractionated during eg biologic processes depeding on their mass in accord with precise rules. Two types of mass-dependent fractionation:equilibrium and kinetic Normally, the product will be lighter than the reactant. intuitive example Rain is enriched in 18O due to the equilibrium isotope effect as 18O has stronger bonds in H2Owater than H2Ovapour Ad esempio nel passaggio di stato da H2Oaq ad H2Ovap: H2Ovap sarà più o meno ricco di 16O rispetto all’acqua di partenza? Nella trasformazione da liquido a gas l’acqua si arricchisce di 16O (diventa più negativa, più leggera) per frazionamento cinetico. Nel ritrasformarsi in liquido (condensazione), la pioggia prende 18O (più pesante) per frazionamento di equilibrio, poiché 18O ha un legame più forte con H in H2Oaq. Il vapore rimanente è ancora più negativo. 1- MDF vs MIF of the S cycle On modern Earth, the S isotopes (32S, 33S, 34S, 36S) are fractionated during eg biologic processes depeding on their mass in accord with precise rules Notations: D33S = d33S - 0.515 d34S D36S = d36S - 1.90 d34S D33S, D36S = 0 in MDF D33S, D36S ≠ 0 in MIF Example of biologically mediated MDF of S: Anaerobic bacterial sulfate reduction & sedimentary pyrite formation Sulfate-reducing bacteria (e.g. Desulfovibrio or Desulfotomaculum) are those bacteria that obtain their energy by oxidizing organic compounds or molecular hydrogen H2 while reducing sulfates to sulfides, especially to hydrogen sulfide. In a sense, they "breathe" sulfate rather than oxygen. 1. 2H+ + SO42- + 2(CH2O)->2CO2+H2S+2H2O+energy 2. Reaction of H2S with Fe+3 or Fe+2 mineral 4Fe2O3+9H2S ->8FeS+SO42-+8H2O+2H+ 3. Formation of pyrite FeS + S0 -> FeS2 Example of biologically mediated MDF of S: Anaerobic bacterial sulfate reduction & sedimentary pyrite formation Biological sulfate reduction shows a chemical preference for 32S (16 protons+16 neutrons) over the heavier isotopes 33S and 34S, resulting in sedimentary pyrite that is enriched in 32S relative to gypsum formed from the same water body. d33S biogenic pyrite less than d33S chemical gypsum d34S biogenic pyrite much less than d34S chemical gypsum In any case, the isotopic signature of gypsum and pyrite formed in this way respond to MDF rules: d33S = 0.515 d34S; D33S = 0 (i.e., the fractionation of 33S relative to 32S is half the fractionation of 34S relative to 32S) And also d36S = 1.9 d34S; D36S = 0 (i.e., the fractionation of 36S relative to 32S is twice the fractionation of 34S relative to 32S) Example of physically mediated MIF of S: Photodissociation of SO2 in atmosphere No O2 in atmosphere = no ozone (O3) layer. UV-induced photodissociation of volcanic SO2 generates isotopically distinct elemental sulfur (S0) and sulfur trioxide (SO3) characterized by large anomalous (i.e., MIF) fractionations. SO3 is soluble and reacts with water producing H2SO4, whereas elemental sulfur is insoluble in surface waters. Therefore, these two components did not mix, and their (positive and negative) MIF (D33S ≠0, D36S ≠0) signatures were transmitted to mineral sulfides (FeS2) and sulfates (CaSO4) formed from them in sedimentary environments. H2SO4 + CaCO3 + 2H2O -> (CaSO4 · 2H2O) + CO2 + H2O Gypsum H2SO4 + Ba(OH)2 -> BaSO4 + H2O Barite FeS + S0 -> FeS2 Pyrite Gypsum, Barite, Pyrite with MIF signatures D33S ≠0 D36S ≠0 Conclusions: 2- BIF vs redbeds (…) BIFs are composed of various iron rich minerals: Siderite (FeCO3) with reduced divalent ferrous iron (Fe+2). Magnetite (Fe3O4) with ferrous iron (Fe+2) and oxidized trivalent ferric iron (Fe+3). Hematite (Fe2O3) with oxidized trivalent ferric iron (Fe+3). Notation: reduced divalent ferrous iron (Fe+2) is more soluble than oxidized trivalent ferric iron (Fe+3). soluble Fe2+ accumulates in seawater and at the first fluctuation in O2, it precipitates massively as Fe3+ in hematite forming BIFs The Rajhara Iron Mine is situated approximately 90 km south of Durg in the Indian state of Chhattisgarh and supplies iron ore (haematite). The orebody is a Precambrian banded iron formation. In an oxidizing atmosphere, iron combines with oxygen to from rustlike oxides. This leads to the formation of redbeds instead of BIFs Red beds of the Moenkopi Formation, Utah …an alternative scenario: disappearance of BIFs and deep ocean anoxia… * *) 4FeS2(pyrite) + 14O2 + 4H2O => 4Fe2+ 8SO4 + 8H+ 3- Uraninite - pyrite - siderite BIFs as recorders of incipient oxidation of the atmosphere Frimmel, H.E., 2005. Archaean atmospheric evolution: evidence from the Witwatersrand goldfields, South Africa. Earth-Science Reviews, 70: 37-41. Pyrite grains in conglomerate from the Mississagi Formation (Northern Ontario, Canada) deposited in a braided river at ca. 2.45-2.2 Ga. Note the presence of rounded, detrital pyrite grains and euhedral (sharp edged) pyrite grains (red scale bar is 1 cm). Detrital pyrite has been interpreted to indicate transport in a low oxygen environment (e.g., Frimmel, 2005). Post-GOE mineral species incorporate one or more redoxsensitive elements that can occur in two or more oxidation states. Thus, hundreds of new minerals incorporate iron (Fe2+ vs. Fe3+), copper (Cu1+ vs. Cu2+), and uranium (U4+ vs. U6+), etc. Hazen, mineral evolution 4- The C cycle Organic matter and limestones that accumulate on the present-day ocean floor differ in their ratios of 13C and 12C by about 25 permil, reflecting the fractionation of carbon isotopes by photosynthetic algae and cyanobacteria. A similar difference of 25-30 permil between d13Ccarb e d13Corg existed down to ~2.2 By. Before this time, things were different… Nature 290, 696-699 (23 April 1981) Anomalous 13C depletion in early Precambrian graphites from Superior Province, Canada M. Schoell & F.-W. Wellmer (…) Here we report the results of our measurements of samples of graphite from the Archean of the Superior Province of Canada, one of which being the most depleted in 13C ever found in the Precambrian. Schoell and Wellmer discovered organic matter in lake beds ~2.8 By old that was depleted in 13C by as much as 45 permil, a fractionation too large to be ascribed to photosynthsis alone. We need to invoke additional metabolisms to explain Schoell and Wellmer data. They explain this 13C depletion by organisms in some areas living on CH4 that has been formed by methane-producing bacteria in Precambrian stratified seas. Photosynthesis alone cannot account for Schoell and Wellmer (1981) results. They explained their results by calling into action anaerobic methanogens and methane-eaters. Methanogenic Archaea gain energy by breaking down organic matter to methane.This methane is depleted in 13C. Methane-eating bacteria gain energy from methane (CH4). In doing so, they have preference for 12CH4 over 13CH4 and fractionate methane by 20-25 permil. Then, photosynthetic organisms + methane producing Archaea + methane-eating bacteria can explain the unusual isotopic values of Schoell and Wellmer (1981). This interpretation implies the existence in ~2.8 By old ecosystems of anaerobic metabolisms. No O2-respiring organisms, no sulfate reducers. Therefore, low O2 levels at ~2.8 By (and higher levels by ~2.2 By when dC13 values are ‘normal’. 5- Paleosols Especially relevant are the Hekpoort, Wolhaakop and Drakenstein paleosols from South Africa Rye and Holland found in soils older than 2.4-2.2 By that the iron originally present in the underlying rocks was removed as the soils formed. In contrast, iron in younger soils is retained. When parent rocks weather under low O2 conditions, iron is released as reduced soluble ferrous ions (Fe2+) and carried away by in solution by oxygen-poor groundwaters. In contrast, once oxygen increased, iron was immediately converted into insoluble iron oxides (Fe3O4, Fe2O3) and remained in place (retained). [A.H. Knoll, Life on a young planet, 2005] 2.24-2.20 By Hekpoort Paleosol: less than 8x10-4 pO2 GOE Notation: Today’s pO2 is 2x10-1 atm 2.2-2.0 By Drakenstein Paleosol: 3x10-3 pO2 Summary G.O.E. Pre-G.O.E. ocean&atmosphere was a place where 1) Sulphur was fractionated mass-independently, 2) oceans were highly ferrous and precipitated en-masse BIFs at minimum O2 fluctuations, 3) U, Fe and other minerals could exist only in reduced forms (Uraninite, Pyrite etc) and could not oxidize, 4) the d13C of organic matter was highly negative (-45 per mil) and reflective of anaerobic metabolic processes involving mathanogens and methane eaters, 5) paleosols were highly depleted in soluble Fe2+ Post-G.O.E. ocean&atmosphere was a place where 1) Sulphur was fractionated mass-dependently, 2) no Fe2+ dissolved in oceans, 3) all Fe2+ readily oxidized to Fe3+ forming hematitic rust in redbeds, 4) U, Fe and other minerals could oxidize, forming a variety of new minerals, 5) the d13C of organic matter reflected fractionation by photosynthesis, 6) paleosols retained Fe2+ in magnetite and contained also Fe3+ in hematite Open questions: 1- when did the G.O.E. ‘exactly’ occur (within the broad time window comprised between 2.5 and 2.0 Ga)? Last results from the S cycle 2- how large was it with respect to present-day Oxygen level (0.2 atm)? 3- what caused it? Time is everything. When did the G.O.E. occur? The youngest sediments with a strong MIF signal is 2.47-Gyr-old Dales Gorge Member, Western Australia (Farqhar et al., 2000; see previous slides). The oldest sediments with a small or no MIF signal is 2.32-Gyr-old Rooihoogte Formation (Bekker et al., 2004). G.O.E. between 2.47 and 2.32 Gy …possibly close to 2.32 Gy… How large was the G.O.E.? The G.O.E. produced apparently a very rapid rise of oxygen from less than 0.001% of PAL to 1-40% PAL between 2.47 and 2.32 Gy (early Paleoproterozoic). The rest of O2 was added later, at ~0.635-540 Gy (Ediacara-early Phanerozoic time) Notation: PAL is 2x10-1 atm A two step or multi-step model? What caused the G.O.E.? A change in style of volcanism or the rise of photosynthesis? …or perhaps an increase in organic carbon burial? A change in style of volcanism A P A change in style of volcanism A change in style of volcanism CH4 + 2 O2 = CO2 + 2 H2O 2 H2S + 3 O2 = 2 SO2 + 2 H2O A change in style of volcanism Photosynthesis Recall that G.O.E. seems to have occurred between 2.47 and 2.32 Gy …possibly close to 2.32 Gy, which is the age of the Makganyene SE Kirschvink & Kopp 2008 CAUSE OF GOE Burial of Organic Matter? The anomalous carbon isotope behaviour of the Lomagundi excursion is tied to intense burial of organic matter Links btw GOE, LIPs & Snowball Earth ~2.45 Gy supercontinent and Ongeluk LIP Gumsley et al. 2017 Paleoproterozoico GOE, LIPs & Snowball Earth Large juvenile volcanic provinces on extensive low-latitude continental landmasses are likely to have enhanced chemical weathering of aerially extensive, nutrient-rich continental flood basalts. This weathering resulted in enhanced flux of phosphorus and other essential nutrients onto extensive continental margins and into intracratonic basins. An enhanced nutrient flux would have greatly increased photosynthetic activity and oxygen production, temporally linked to higher net burial of organic carbon in accumulating sediments. An incipient rise of free atmospheric oxygen would have led to rapid oxidation of atmospheric methane forcing catastrophic climate change and plunging Earth into a global glaciation (Huronian/Makganyene Paleoproterozoic Snowball Earth). Extra nutrients delivered to oceans during Snowball Earth deglaciation would have sustained photosynthetic activity and oxygen production ->GOE. The Neoproterozoic Oxygenation Event (NOE) The Neoproterozoic Oxygenation Event (NOE) The Neoproterozoic Oxygenation Event (NOE) Evidence from Molibdenum (Mo) and Vanadium (V) Sahoo et al. (2012) In today’s predominantly oxygenated oceans, Mo and V are abundant and have long residence times (Mo = 800–440 kyr; V = 50 kyr), much longer than the ocean mixing time (1.5 kyr), and hence at any local place they track global seawater composition. Mo and V exit the water column in black shales forming in reducing marine environments (e.g., Black Sea). The magnitude of Mo and V enrichments in black shales reflect dissolved Mo and V concentrations in seawater The observed Mo and V enrichments in lower Doushantuo Formation black shales suggest that oxic waters bathed the vast majority of the ocean. Oxygen-deficient conditions must have been spatially limited and only located where high organic matter loading caused oxygen depletion along ocean margins CAUSE OF NOE: Burial of Organic Matter? Extreme variations in the carbon isotopic record are characteristic of later Neoproterozoic times: (1) an exceptionally long lived positive δ13Ccarb excursions after c. 800 Ma has been interpreted to represent increased organic carbon burial and hence, a release of oxidizing power into the Earth's surface environment; while (2), the extremely negative δ13Ccarb excursion in the mid-Ediacaran suggests the oxidation of a very large organic carbon pool, possibly due to enhanced ocean cycling and resultant deep ocean ventilation. The extreme mid-Ediacaran excursion is consistent with the hypothesis that atmospheric oxygen rose significantly after ca. 550 Ma, and contemporaneous with diversification of macroscopic metazoans at the end of the Ediacaran Period. In addition, the overall increase in 87Sr/86Sr ratio in marine sediments through the Neoproterozoic and into the Cambrian indicates a rise in chemical weathering and nutrient supply to the ocean which might have increased primary production including photosynthesizing cyanobacteria and enhanced the rates of organic carbon burial. Evolution of seawater 87 Sr/ 86 Sr since the last 70 My. Intense chemical weathering in the Himalaya is suggested as a potential cause for the steady increase in 87 Sr/ 86 Sr since 40 My (Raymo and Ruddiman 1992). Links btw NOE, LIPs & Snowball Earth NOE, LIPs & Snowball Earth ~0.715 Gy Rodinia supercontinent and Franklin LIP Neoproterozoico NOE, LIPs & Snowball Earth Large juvenile volcanic provinces on extensive low-latitude continental landmasses are likely to have triggered near-equatorial glaciations via enhanced chemical weathering of aerially extensive, nutrient-rich continental flood basalts. This weathering resulted in increased carbon dioxide drawdown and onset of glaciation (Sturtian-Marinoan Neoproterozoic Snowball Earth), and an enhanced flux of phosphorus and other essential nutrients onto extensive continental margins and into intracratonic basins. An enhanced nutrient flux would have greatly increased photosynthetic activity and oxygen production, temporally linked to higher net burial of organic carbon in accumulating sediments. Extra nutrients delivered to oceans during Snowball Earth deglaciation would have sustained photosynthetic activity and oxygen production ->NOE. Ediacara !