Download Importance of Atmospheric H2 in Surficial Environments of the

Importance of Atmospheric H2 in Surficial Environments of the Archean
Jihua Hao (Johns Hopkins University)
Additional Authors: D.A. Sverjensky, R.M. Hazen
The Archean atmosphere is widely recognized as anoxic with negligible PO2,g , but moderately
high PH2,g , despite recent studies suggesting occasional whiffs of oxygen and haze formations.
This report emphasizes that PO2,g in the Archean atmosphere is too low to control the redox state
of the surface environment; however, PH2,g is high enough to control the redox state and plays
important roles in various biogeochemical processes during the Archean.
We firstly calculated the stability of detrital pyrite, siderite, and hematite as functions of PS2,g and
PO2,g or PH2,g . The results showed that hematite is a thermodynamically stable mineral within the
range of PO2,g provided by Archean atmospheric models, inconsistent with detrital mineral
records. In contrast, siderite and pyrite are predicted to be thermodynamically stable within the
range of Archean PH2,g . Therefore, PH2,g rather than PO2,g is the more useful redox indicator for
Archean surface conditions. Then we examined previous methods for the estimation of PH2,g in
the Archean atmosphere, and found that PH2,g in volcanic gases varied greatly with different
temperature and tectonic setting of the volcanoes sampled. Thus, for a better estimation of PH2,g
in the Archean atmosphere, temperature and tectonic setting should be considered. Moreover,
early biological activity could affect greatly PH2,g . Therefore, it is necessary to build a coupled
model including all these considerations to study fluctuations of PH2,g , implications for the redox
state during the Archean.
With redox controlled by PH2,g , we calculated the solubility of the trace elements Mo and Re. The
results showed that oxidation and therefore enhanced mobility of Mo and Re could be a
consequence of fluctuations of PH2,g and/or changes of pH without any role for O2,g. In contrast to
the potential mobilization of trace elements at low PH2,g , modeling of Archean rainwater at higher
PH2,g (~10-4 or greater) showed that there is no thermodynamic driving force to destroy simple
aqueous organic compounds such as formate or acetate when methane formation is kinetically
inhibited. This implies a possible riverine transport of aqueous organic C species derived from
fermentation to the Archean ocean, and may contribute to haze formation by feeding
methanogens in marginal marine systems.