Isotopic evidence for Mesoarchaean anoxia and changing atmospheric sulphur chemistry

Abstract

The evolution of the Earth's atmosphere is marked by a transition from an early atmosphere with very low oxygen content to one with an oxygen content within a few per cent of the present atmospheric level. Placing time constraints on this transition is of interest because it identifies the time when oxidative weathering became efficient, when ocean chemistry was transformed by delivery of oxygen and sulphate, and when a large part of Earth's ecology changed from anaerobic to aerobic. The observation of non-mass-dependent sulphur isotope ratios in sedimentary rocks more than approximately 2.45 billion years (2.45 Gyr) old and the disappearance of this signal in younger sediments is taken as one of the strongest lines of evidence for the transition from an anoxic to an oxic atmosphere around 2.45 Gyr ago. Detailed examination of the sulphur isotope record before 2.45 Gyr ago also reveals early and late periods of large amplitude non-mass-dependent signals bracketing an intervening period when the signal was attenuated. Until recently, this record has been too sparse to allow interpretation, but collection of new data has prompted some workers to argue that the Mesoarchaean interval (3.2-2.8 Gyr ago) lacks a non-mass-dependent signal, and records the effects of earlier and possibly permanent oxygenation of the Earth's atmosphere. Here we focus on the Mesoarchaean interval, and demonstrate preservation of a non-mass-dependent signal that differs from that of preceding and following periods in the Archaean. Our findings point to the persistence of an anoxic early atmosphere, and identify variability within the isotope record that suggests changes in pre-2.45-Gyr-ago atmospheric pathways for non-mass-dependent chemistry and in the ultraviolet transparency of an evolving early atmosphere.

Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox

“…In contrast, an increase of O 3 or OCS has the opposite effect. Finally, SO 2 self-shielding produces increasing 34 and decreasing 33 E. The previous estimate of isotope effects in SO 2 photolysis from self-shielding found an increasing trend in 33 E (12) that is opposite to our result. The reason for the difference may be because the spectra of the isotopically substituted species were approximated by shifting the absorption peaks of the natural abundance SO 2 spectrum using a set of isotopedependent frequency shifts.…”

“…An important conclusion from the model is that the massindependent isotope effect 33 E for SO 2 photolysis shows a negative sign for most UV-shielding scenarios (Fig. 2).…”

“…3). A tentative O 2 -rise at Ϸ2.9 Ga has been proposed (31), but is now thought to be unlikely due to a small but clear MIF signature in many sulfide deposits (32,33). The ⌬ 33 S value of seawater sulfate after the Pongola glaciation is still poorly understood, although recent analysis of carbonate-associated sulfate (28) suggests that the late Archean sulfate may have possessed the opposite sign of ⌬ 33 S anomaly in contrast to those of early to middle Archean (Fig.…”

Multiple sulphur and iron isotope composition of detrital pyrite in Archaean sedimentary rocks: A new tool for provenance analysis

“…The small range of Δ 33 S values is consistent with that found in some sedimentary and early diagenetic pyrites formed between 2.7 and 3.2 Ga (Farquhar et al, 2000;Ohmoto et al, 2006;Farquhar et al, 2007;Domagal-Goldman et al, 2008). Small, and predominantly positive, Δ 33 S values have also been reported by Ono et al (2006b) for Mozaan Group shales higher up in the stratigraphy.…”

“…Different trends have been observed for different Archaean time intervals and may reflect changes in atmospheric composition or the formation of organic haze in the atmosphere (cf. Farquhar et al, 2007;Domagal-Goldman et al, 2008). Alternatively, the covariation in δ 34 S and Δ 33 S values could reflect a combination of mass-dependent and nonmass dependent isotope effects, as suggested by Ono et al (2003), Ueno et al (2008), and…”