Were Archean calcium carbonate precipitation kinetics related to
oxygen concentration?
by
Dawn Y. Sumner and John P. Grotzinger
1996, Geology, v. 24, p. 199-122
(The links "*" connect to definitions of the preceeding word.)
Archean* carbonates* commonly contain decimetre- to metre-thick beds
consisting entirely of fibrous calcite* and neomorphosed fibrous aragonite* (Figures) that
precipitated in situ on the sea floor. Such thick accumulations of precipitated
carbonate are rare in younger marine carbonates suggesting an important
change in the modes of calcium carbonate precipitation through time.
Kinetics of carbonate precipitation depend on the concentration of inhibitors
to precipitation which reduce crystallization rates and crystal nuclei
formation, leading to kinetic maintenance of supersaturated solutions.
Inhibitors also affect carbonate textures by limiting micrite* precipitation and
promoting growth of older carbonate crystals on the sea floor. Fe2+, a strong
calcite precipitation inhibitor, is thought to have been present at relatively
high concentrations in Archean seawater because oxygen concentrations were
low. The rise in oxygen concentration at 2.2-1.9 Ga* led to the removal of Fe2+
from seawater and resulted in a shift from Archean facies*, which commonly
include precipitated beds, to Proterozoic* facies which contain more micritic
sediment and only rare precipitated beds.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616
sumner@geology.ucdavis.edu