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 ) < or = 1 bar) and nitrogen, and contained minor amounts of hydrogen sulfide (P (sub H 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, transition from pyritic to iron oxide-bearing conglomerates indicates that photosynthetic oxygen production rates began to exceed oxygen consumption rates and that the redox buffers involving hydrogen sulfide and ferrous iron were overwhelmed. Increased oxidation potentials, and increased surface water pH due to declining atmospheric CO 2 partial pressures, led to immobilization of iron and consequently to continental red-bed formation and cessation of banded iron-formation deposition.

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