Evidence for Low Late Archean Atmospheric Oxygen from Oceanic Depth Gradients in Iron Concentration
Dawn Y. Sumner
1996, GSA Annual Meeting, Abstracts, v. 28, p. A218
(The links "*" connect to definitions of the preceeding word.)
The 2521+/-3 Ma* Gamohaan Formation and basinal equivalents, Transvaal Supergroup, South Africa, provide a Late Archean* record of subtidal* oceanic chemical processes. Stratigraphic and facies* analysis of these deposits demonstrates a change from calcite* to shale* and then siderite* and oxide facies iron-formation* deposition with depth suggesting an increase in ferrous iron concentration with depth. This transition in mineralogy is seen both upwards through a carbonate platform drowning sequence and laterally between contemporaneous platform and basinal deposits. In addition to these mineralogical changes, the texture of synsedimentary calcite precipitates (specifically herringbone calcite) within subwave-base* lithofacies* in the Gamohaan Formation implies the presence of an inhibitor to calcite precipitation. Experimental evidence suggests that ferrous iron is a strong inhibitor to calcite precipitation at 20 Ámol/l at reasonable supersaturation (2-9 times, Dromgoole and Walter 1990). Thus, the texture of the carbonates also suggests the presence of aqueous iron.
Although absolute depositional depths for the Gamohaan Formation are poorly known, stratigraphic data constrain maximum depths to less than a few hundred meters, suggesting the presence of significant concentrations of ferrous iron in relatively shallow sea water. Similar calcite textures also are present, although less abundant, in peritidal* carbonates deposited at the base of and below the Gamohaan Formation. Since the surface of the oceans is well mixed to depths of 20-100 m on a monthly to yearly time scale, the presence of ferrous iron within this depth range implies that surface sea water was anaerobic*. Since atmospheric chemistry is buffered largely by the composition of surface ocean water, it is unlikely that the lower atmosphere contained free oxygen at 2520 Ma, and oxygenic environments in latest Archean* time probably were limited to microenvironments within mats of photosynthetic cyanobacteria.
This abstract is also available at the GSA web site where some abstracts from the special session Geochemical Constraints on Seawater Composition
and the Coupled Ocean-Atmosphere System: The Precambrian Revisited can also be viewed.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616