In the present state of knowledge, the data on the atmospheric oxygen rate, during various geological periods, are full of uncertainties, discussed by the various authors and prone to permanent calling in question, with the researchers works thread. In what follows, we tried, nevertheless, to establish a probable middle increase chronology in the oxygen rate in the Earth's atmosphere. Because of the preceding reserves, one must regard this chronology various stages more as magnitude orders that like precise and rigorous datings (Holland 1984-1998; Mason 1992). It is not excluded, moreover, that the PO2 P.A.L. rate knew fluctuations during geological periods.
Crust/ocean/atmosphere system enrichment in free PO2 P.A.L. intervened, mainly, following photochemical reactions grouped under the photosynthesis generic term (Rybak 1974) and, marginally, of the water vapor photolysis. This process is, presently, primarily, the plants fact. One considers the free molecular oxygen present percentage at 5 % oxygen present total at the earth's crust surface (Schidlowski, Eichman 1977).
We indicate below the earth's atmosphere in PO2 P.A.L. enrichment principal stages since the earth origin (4,6 Billion Years) until today, of which we gave, to Chapter VI, the essential data. The figures indicated are based, either on geological arguments (various radioactive elements isotopic ratios, U 238, K 40, C 14, etc...), or on biological arguments (biological processes physiological thresholds: aerobiosis, collagen production, cutaneous respiration, etc...).
1) Hadean (4,6 to 3,9 B.Y.) the earth paramount atmosphere is made up mainly hydrogen, methane and ammonia (planetologic arguments). This time intense volcanic degazifications do not contain free molecular oxygen. PO2 P.A.L. = 0
2) Archean (3,9 to 2,5 B.Y.) uraninite deposits (UO2.) and pyrite (FeS2) until towards 2,3 B.Y. indicate atmosphere not-oxydative conditions with PO2 P.A.L. rates probably going from 0,0005 to 0,005 (Holland 1998). PO2 P.A.L. = > 0,0005
3) Proterozoic (2,5 to 0,544 B.Y.)
a) Between 2,8 and 1,8 B.Y., B.I.F. deposits (Banded Iron Formations - ribboned iron Formations) alternate layers rich and low in magnetite (FeO4) (Guntflint Chert 1,9 B.Y.), also indicating not-oxydative atmosphere.The photosystème II, appeared towards 2,7-2,5 B.Y. seems to have been dominating and to have caused the increase in the O2 molecules in the atmosphere. Abundance peaks between 2,5 and 2 B.Y., coinciding with the uraninites disappearance indicate an increase in the PO2 P.A.L. rate (Holland 1998). PO2 P.A.L. = > 0,005
b) The Red Beds (Red Formations) hematite (FeO3) deposits appear towards 2,00 B.Y., indicating oxydative atmosphere conditions and lead to the B.I.F. disappearance towards 1,8/1,7 B.Y. with the ozone layer O3 development (Levin). The increase in the molecules O2 in the atmosphere allows the emergence of the oldest known aerobic eucaryote cells, approximately 1,9 B.Y. old (Gryptania spiralis - Pan Terra 1996); 0,01 threshold for the protists aerobic breathing (Holland 1998). PO2 P.A.L. = > 0,01
c) Vendian (0,565 B.Y.) Between 0,750 and 0,550 B.Y., the carbon isotopic composition suggest an appreciable increase in the PO2 P.A.L. rate before the cambrian era (Hoffmann, Kaufman, Halverson 1998). The geochemical data clearly indicate a rise in the PO2 P.A.L. rate right before the Vendian macroscopic animals appearance. They also show a phytoplankton dynamic evolution at the Precambrian/Cambrian (Knoll 1996) border. In addition, the apparition of the ediacarian fauna of soft body metazoa requires, for the collagen, muscles production and the cutaneous respiration (Dickinsonia: 1 meter length for a maximum thickness of 6 mm), a 0,07 PO2 P.A.L. minimum (Towe 1970 - Bruce Runnegar 1982 - Rudolf, Elizabeth Raff). PO2 P.A.L. = > 0,07
4) Paleozoic (0,544 to 0,250 B.Y.)
a) Cambrian (0,544 to 0,505 B.Y.) "the cambrian explosion", with its various episodes, S.S.F. (Small Shelly Fossils), tommotian and atdabanian radiations, the Burgess Shale fauna, implies an increase in PO2 P.A.L. necessary to the organisms complexification and their biomineralization. The correlation between PO2 P.A.L. and the living organisms evolution is corroborated by many data compilation which we indicated in Chapter VI and which we point out here. The analysis of these data (Rhoads, Morse 1971) made it possible to show the relation between the dissolved oxygen level and the fauna benthic presence in the basins on low oxygen level, in the Black Sea (Bacescu 1963), the Gulf of California (Parker 1964), the Basin of Santa-Barbara (Emery, Hulsemann 1961) and the Basin of San Pedro (Hartman 1955, 1966). They established that faunas can be classified in three facies correlated at different PO2 P.A.L. rates. With a value < 0,1 ml/l (approximately 0,01 PO2 P.A.L.), the marine sediments are primarily metazoa benthic deprived; for a value ranging between 0,3 ml/l and 1 ml/l (between approximately 0,03 and 0,10 PO2 P.A.L.), benthic faunas are made up small species mainly with soft body; when the level is higher than 1 ml/l (approximately 0,10 PO2 P.A.L.), faunas are relatively varied and made up many species which secrete skeletons limestones. Similar relations were observed in the Saanich split in the Vancouver Island (Tunnicliffe 1981). Rhoads and Morse proposed that these relations between the faunas emergence and the oxygen rates dissolved in these basins are regarded as analogues with the metazoa groups development during the Proterozoic last stages and the Phanerozoic first stages. PO2 P.A.L. = > 0,10
b) Ordovician (0,505 to 0,438 B.Y.) The first appearance of terrestrial life would go up in middle Ordovician where terrestrial spores traces are found (0,449/0,458 M.A.- Jane Gray) but not from vascular plants.The first spores would be perhaps dated from Cambrian. The PCO2 P.A.L. level before 0,440 B.Y. would have been higher 16 to 18 times on its present level (Crayton J.Yapp 1998). PO2 P.A.L. = > 0,10
c) Silurian (0,438 to 0,408 B.Y.) If the possibility of plants and terrestrial animals is probable in Ordovician, their obviousness is established in Silurian with the plants vascular fossils (Cooksonia), perhaps of Lycophytes (Baragwanathia?), mushrooms (ascomycetes) and Arachnida and millipede first fossils (Taylor and Taylor 1993). The analysis, by stable isotopes geochemical methods, makes it possible to provide the oxygen lower level rate, during the last 440 million years. This rate would not be lower than 0,13 (Crayton J.Yapp 1998). PO2 P.A.L. > 0,13
d) Devonian (0,408 to 0,360 B.Y.) From upper Silurian to higher Devonian, one attends a considerable development of the vegetable cover by terrestrial vascular plants with the sheets, roots, secondary tissues and seeds acquisition. Concomitantly, the PCO2 P.A.L.. rate falls quickly and in a very significant way (Algeo and Scheckler 1998). An imbalance between the oxygen production by the carbon cycle and its consumption by the sulphur cycle, by only 5 %, can increase the PO2 P.A.L. rate of 50 % in 40 million years. The iron cycle can also intervene in this imbalance. On the long term, a balance is roughly established between the sulphur cycle and the carbon cycle which is an atmosphere/ocean/terrestrial crust system redox state major element (Holland 1984). During middle and higher Devonian, the increase in the vegetable biomass accelerates the atmospheric CO2 "pumping" level towards the ground and establishes a carbon cycle new long-run equilibrium, between its production and its consumption, which is maintained until our days (Algeo, Scheckler and Maynard 1998). It is estimated that the atmospheric PCO2 P.A.L. level between Silurian and higher Devonian is divided by 5 or 6 (Berner 1994). One can suppose, reciprocally, an increase in atmospheric PO2 P.A.L. of the same magnitude order. This assumption is corroborated by Heinzinger, Schidlowski and Junge work (1974). These authors, by comparing the isotope § 18 O value (11,4 o/oo SMOW), in meteorite magnetite spangles dating from higher Devonian, found values around 0,65 (11,4/17,6) of those of the recent samples § 18 O (17,6 o/oo SMOW). Other magnetite spherules, dating from Oligocene gave values appreciably equivalent to those of today (17,4 o/oo SMOW). The atmospheric isotope § 18 O respective values give, for superior Devonian 17,3 o/oo SMOW, for the Oligocene one, 23,3 o/oo SMOW and 23,5 o/oo SMOW for today is a value of 17,3/23,5 = 0,74 PO2 P.A.L. for Devonian. PO2 P.A.L. = 0,65 to 0,74
e) Carboniferous/Permian (0,360-0,250 B.Y.)
5) Mesozoic (0,250 to 0,065 B.Y.)
The carboniferous flora and the terrestrial vascular plants radiations during the Permian and the Triassic, with the PCO2 P.A.L. rate decrease (Berner 1994), could carry the PO2 P.A.L. oxygen rate which one estimates at 0,93 with the Gymnosperms blooming (Coniferals) until lower Cretaceous. PO2 P.A.L. = 0,93
6) Cenozoic (0,065 B.Y. - present)
Of the Cretaceous until our days, develop Angiospermae. A last increase in PO2 P.A.L. partial pressure had to occur during the Cretaceous various oceanic anoxic events OAEs (Holland 1984), carrying it to its present value with the Cenozoic one. PO2 P.A.L. = 1
