Centimeter to meter tall aragonite pseudomorphs are abundant in Neoarchean marine carbonates and less common, but present in some Proterozoic carbonates. Neoarchean carbonates also contain substantial proportions of decimeter-thick calcite encrustations on the sea floor, whereas their abundance declines through time. In contrast, the relative proportion of micritic sediment increases during Proterozoic time. The precipitation of large aragonite crystals and thick layers of calcite on the sea floor implies that crystal growth rates were very high relative to sedimentation rates. For meter-tall Neoarchean fans in shallow subtidal depositional environments, crystal growth rates must have substantially exceeded estimates of modern aragonite cement precipitation rates. High aragonite saturation states in seawater could produce rapid precipitation rates. The paucity of micrite suggests that nucleation of carbonate in the water column was limited. These conditions need to be maintained for millions years and may be due to the presence of chemical inhibitors that slow crystal nucleation and precipitation rates. Rapid local crystal growth may reflect the local absence of inhibitors and globally low carbonate accumulation rates relative to calcium influx to the oceans.
Some Neoproterozoic "cap carbonates", which immediately overly glacial deposits, contain large aragonite pseudomorphs, similar to those common in Neoarchean carbonates. Unlike Neoarchean carbonates, however, sea floor calcite encrustations are rare, and micrite precipitation was abundant. These differences suggest that the circumstances leading to growth of Neoproterozoic large aragonite fans were different. The limited stratigraphic distribution of aragonite pseudomorphs also suggests that changes in ocean dynamics may have produced a temporary increase in carbonate saturation states, in contrast to the long term maintenance of high supersaturation required by the distribution of Neoarchean aragonite pseudomorphs.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616