5.5. The modification of ooids by sulfate-reducing bacteria
The biosignature examination of modern Bahamian ooids revealed that
there are diverse microbes including photoautotrophs, heterotrophs, and
sulfate-reducing bacteria contained in ooids (Summons et al., 2013;
Diaz, Piggot, Eberli, & Klaus, 2013; Diaz et al., 2014; O’Reilly et
al., 2016). Although sulfate-reducing bacteria may not contribute
significantly in carbonate biomineralization or only play as an
unimportant part in cyanobacteria dominated biofilm (Pacton et al.,
2012), but we found that sulfate-reducing bacteria would change the
mineral composition of ooids after deposition under suitable conditions.
However, most ooids would not be modified by sulfate-reducing bacteria
without proper environment with sufficient sulfur and active iron
influx. Iron ooids are generally red, their cortex may consist of iron
minerals (goethite, hematite, chamosite and so on, Sturesson, Dronov, &
Saadre, 1999; Mücke, 2006), or iron minerals mixed with other minerals
(phosphate-rich layers+Fe-oxide-rich layers; Barale, d’Atri, & Martire,
2013). However, most researches about iron ooid formation indicated that
these ooids formed under sufficient iron supplement and agitated
hydrodynamic environment (Di Bella et al., 2019), or associated with
volcanism activities (basalt extrusions, hydrothermal fluids, volcanic
ash) (Sturesson, Dronov, & Saadre, 1999). We speculate that this kind
of red ooids are lack of organic matter and sulfate-reducing bacteria,
while Bahamian ooids are lack of iron supplement in proper time.