Understanding the Evolution of Atmospheric Redox State from the Archaean to the Proterozoic

Introduction: Geological differences between the ancient and modern Earth show that there was too little O2 in the early atmosphere to leave traces of oxidation that today are common, such as the reddening of exposed iron-rich rocks [1]. The onset of oxidized paleosols, red beds, and the ensuing absence of detrital pyrite, siderite and uraninite all indicate an increase in atmospheric O2 levels at 2.4-2.3 Ga. Recently, measurements of mass-independently fractionated sulfur isotopes have confirmed the timing of the oxic transition [2]. The fossil record arguably shows that this Palaeoproterozoic oxic transition changed the course of evolution. The oldest macroscopic fossils of possible eukaryotic origin occur in 1.8 Ga rocks [3], 4-5 mm long possibly multicellular fossils appear at 1.7 Ga [4], and acritarchs significantly increase in abundance after 1.7 Ga. However, the cause of the oxic transition remains poorly understood because determining how and why the O2 increase occurred requires an integrated grasp of the redox behaviour of the atmosphere, ocean, biosphere and lithosphere. An important conundrum: Oxygenic photosynthesis is the only plausible source of free O2 that could have caused the oxic transition [5]. However, the oxic transition occurred at ~2.4-2.3 Ga [1,2,6] whereas cyanobacterial oxygenic photosynthesizers existed in the oceans long before that. Isotopic fractionation of carbon between sedimentary organic carbon and carbonates is about 30‰ back to 3.5 Ga and characteristic of photosynthetic fractionation, which suggests an early origin for oxygenic photosynthesis, but is not, on its own, conclusive [7]. What is conclusive is that hydrocarbon biomarkers derived from oxygenic photosynthetic cyanobacteria and from eukaryotic sterols are found at 2.7 Ga [8]. Also localized biological oxygen sources are apparent from 2.7-3.0 Ga [9,10]. This evidence presents a puzzle, given the oft-stated (but perhaps naive) opinion that the rise of O2 could be explained if the rise coincided with the origin of oxygenic photosynthesis. Explanations for the rise of O2: Several ideas have been put forward to account for how oxygenic photosynthesis could have originated long before a detectable rise of O2. One explanation is that large positive carbonate isotope excursions from 2.4-2.1 Ga were due to a massive pulse of organic burial that caused the rise of O2 [11]. However, given the geologically short residence time of O2 (~2-3 million years, even today) a pulse of organic burial would merely cause a parallel pulse in atmospheric O2. Atmospheric O2 would return to its previous low levels once burial and oxidation of previously buried carbon had re-equilibrated. For high O2 to persist, a secular shift in source and sink fluxes of O2 must occur that forces a higher O2 equilibrium level. A second hypothesis attempts to take account of this issue by suggesting that as geothermal heat declined due to the decay of radioactive materials inside the Earth, the flux of volcanic gases dwindled, lessening the sink on O2. However, increased past volcanic outgassing would have also injected proportionately more CO2. Carbon isotopes from 3.5 Ga onwards show that roughly ~20% of the CO2 flux into the biosphere was fixed biologically and buried as organic carbon with the remainder buried as carbonate [7]. Consequently, increased outgassing in the past, on its own, cannot explain the oxic transition because going back in time, O2 production due to organic burial would have risen in parallel with O2 losses. A third explanation of the rise of O2 takes account of the problem with the previous idea and invokes a gradual, irreversible shift of volcanic gases from reduced to oxidized [12,13]. However, studies of redox-sensitive Cr and V abundance in igneous rocks show that the mantle’s oxidation state (i.e., oxygen fugacity), which controls the redox state of volcanic gases, only permits an increase in H2 relative to CO2 by a factor £1.8, which cannot account for a sufficient change in the sink on O2 [14,15]. Atmospheric flip states: Excess hydrogenbearing species vs. excess oxygen: On a geologic timescale, a decrease by a factor of ~3 in the H2/CO2 ratio of fluxes is sufficient to flip the atmosphere from a redox state dominated by hydrogen-bearing species like CH4 to an O2-rich state. This can be understood as follows. Carbon isotopes imply that the ratio of the burial flux of organic carbon to the total flux of carbon input has been ~1:5 since about 3.5 Ga [7]. In photosynthesis, one mole of buried organic carbon generates one mole of O2: CO2 + H2O Æ CH2O + O2 (1) where “CH2O” represents organic matter. The organic carbon has to be buried in sediments beField Forum on Processes on the Early Earth, Johannesburg, July 4-9, 2004