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.