The early Precambrian atmosphere and hydrosphere; thermodynamic constraints from mineral deposits

Abstract

A chemical equilibrium model for the early Precambrian atmosphere and hydrosphere is presented. Thermodynamic calculations based on observed stable minerals and mineral assemblages in banded iron-formation (BIF), pyritic, uraniferous, and auriferous quartz-pebble conglomerates, gold-bearing conglomerates of the Tarkwaian in Ghana, and red-bed sediments, yield the following conditions.Before approx 2350 Ma, the atmosphere was oxygen free (P (sub O 2 ) [leftpointingdoubleanglequotationmark] 10 (super -10) bars), consisted mainly of CO 2 (P (sub CO 2 ) 2 S) = 10 (super -7.5) bars over continents to 10 (super -9) bars over oceans) and hydrogen (P (sub H 2 ) = 10 (super -9) bars over continents to 10 (super -7.5) bars over oceans). The high CO 2 partial pressures generated acid rain and surface waters (pH > or = 3.9) and buffered the ocean water to values around pH = 6.5 to 7. In the oceans, the iron minerals of the various banded iron-formation facies (goethite, pyrite, siderite) in conjunction with the atmospheric CO 2 partial pressure acted as a mineral buffer for P (sub H 2 S) , and for the redox state (P (sub H 2 ) ). In continental weathering environments, pyrite formation from ferrous iron and atmospheric H 2 S, and oxidation of H 2 S to SO 4 (super 2-) are considered the prime redox-controlling mechanisms. River waters which previously equilibrated with pyrite precipitated banded iron-formation upon mixing with seawater, mainly due to pH changes.Between 2350 and 2100 Ma, t…