2:45 PM - 3:00 PM
[BCG09-11] The spatial distribution of transitional elements of the BIFs and the manganese rocks in the Paleoproterozoic Hotazel Formation
Keywords:banded iron formations, manganese deposits, bio-essential elements
The compositions of chemical sedimentary rocks such as the banded iron formations (BIFs) could be proxies for the paleoseawater chemistry because they adsorbed the dissolved elements of the seawater into their constituent minerals at their depositions. For estimate of the bioavailability of the metal elements in the Paleoproterozoic ocean, we have analyzed the trace-element abundance of the geochemistry of the BIFs and interlayering manganese rocks in the ca. 2.4 Ga Hotazel Formation of the Transvaal Supergroup, South Africa.
Whole-rock chemical compositions of the BIFs and the manganese rocks show that Co, Ni and Zn are concentrated in Mn minerals in them, suggesting that those three elements were primarily adsorbed on Mn-oxyhydroxide phases like modern manganiferous deposits. Especially, Ni and Zn contents in them show a positive correlation with Mn contents. However, Co contents show an unclear-positive correlation with Mn contents over 20 wt% of Mn, and Mn-rich rocks (Mn > 20 wt%) have lower Co/Mn ratios than Mn-poor ones. The lower Co/Mn ratios in the manganese rocks suggest that dissolved Co contents in seawater were decreased at their depositions.
Moreover, the distribution mapping of the BIFs without Mn minerals shows that much of Co does not exist in Fe-oxide bands, suggesting that dissolved Co adsorbed on Fe-oxyhydroxide was limited in the Paleoproterozoic seawater. On the other hand, much of Co in the Archean BIFs analyzed for comparison distributes in the Fe-oxide bands.
The low Co/Mn ratios in the manganese rocks and the Co distribution of the BIFs suggest that there was limited Co reservoir in the Paleoproterozoic ocean, possibly due to Co-fixation by Mn oxyhydroxide minerals under oxic conditions.
This study suggest that the Paleoproterozoic ocean proliferated the Co-independent prokaryote, which can use Zn under Co-depleted environments. They became the direct ancestor of the primitive eukaryote, as shown by the molecular phylogenetic study of methionine-synthesizing enzyme (Helliwell et al., 2011).