School of Engineering and Science - Jacobs University
School of Engineering and Science - Jacobs University
School of Engineering and Science - Jacobs University
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Anoxygenic photoautotrophs <strong>and</strong> the origin <strong>of</strong> b<strong>and</strong>ed iron-formation<br />
Brian W. Alex<strong>and</strong>er * <strong>and</strong> Michael Bau<br />
<strong>Jacobs</strong> <strong>University</strong> Bremen, D28759 Bremen, Germany<br />
*corresponding author<br />
Oxide-facies b<strong>and</strong>ed iron-formation (BIF) comprised <strong>of</strong> alternating layers <strong>of</strong> Fe oxides<br />
<strong>and</strong> quartz formed as a chemical sediment at the Early Precambrian seafloor. Despite<br />
more than 100 years <strong>of</strong> research, however, the oxidative precipitation mechanism is still<br />
a matter <strong>of</strong> debate. Some propose an indirect biogenic origin via oxidation <strong>of</strong> Fe(II) by<br />
molecular oxygen produced photosynthetically by ancient cyanobacteria 1,2 , while others<br />
suggest an abiotic origin via photo-oxidation <strong>of</strong> Fe(II) 3,4 . Although hypothesized 35<br />
years ago 5 , it was the discovery <strong>of</strong> Fe(II) oxidizing photoautotrophs 6 that stipulated<br />
further investigation <strong>of</strong> the potential role <strong>of</strong> direct biogenic Fe(II) oxidation by<br />
anoxygenic photosynthesis (phot<strong>of</strong>errotrophy) in BIF genesis. These studies suggest that<br />
anoxygenic photoautotrophs could theoretically have oxidized enough Fe(II) to form<br />
oxide-facies BIF in the Early Precambrian ocean 7,8 before the onset <strong>of</strong> oxygenic<br />
photosynthesis. However, evidence from the geological record for Fe(II) oxidation in the<br />
absence <strong>of</strong> oxygen is missing. We present chemical data for ferruginous shales deposited<br />
with oxide-facies BIF from the 2.9 Ga old Pongola Supergroup, South Africa, that<br />
indicate that oxide-facies Precambrian BIF formed in the absence <strong>of</strong> photosynthetically<br />
produced oxygen. In contrast to non-ferruginous shales deposited below <strong>and</strong> above the<br />
Pongola BIF, the ferruginous shales are strongly depleted in K, Rb, Cs, Ba <strong>and</strong> Na, due<br />
to alkali metal exchange for ferrous iron, suggesting that oxide-facies BIF formed in<br />
anoxic, ferruginous seawater. While this is fully compatible with Fe(II) oxidation by<br />
anoxygenic photoautotrophs, it renders the presence <strong>of</strong> biogenic molecular oxygen<br />
unlikely <strong>and</strong> suggests that phot<strong>of</strong>errotrophy, not oxygenic photosynthesis, was a<br />
prerequisite for the formation <strong>of</strong> oxide-facies BIF on Early Earth.<br />
Although their abundance appears to have peaked at around 2.5 Ga ago 9 , Fe(III) oxide<br />
bearing chemical sediments such as BIF, jasper, jaspilite or ferruginous chert (here subsumed<br />
under oxide-facies iron-formation, IF) occur in the oldest marine sedimentary sequence on<br />
Earth at Isua, Greenl<strong>and</strong>, in all Archean greenstone belts, <strong>and</strong> in the oldest supracrustal<br />
successions in the Pongola <strong>and</strong> Witwatersr<strong>and</strong> Supergroups, South Africa. Hence, formation<br />
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