1. Introduction
Ooids are common in the modern and ancient carbonate sediments of both marine and terrigenous environments. They were regarded as chiefly ‘inorganic grains’ in traditional view (Davies, Bubela, & Ferguson, 1978; Morse & Mackenzie, 1990), but recent studies inferred that microbes played important role in ooid construction and destruction (Summons et al., 2013; Diaz et al., 2015; Mariotti, Pruss, Summons, Newman, & Bosak, 2018). Two pathways controlling the organomineralization in ooids are biologically induced and biologically influenced mechanisms (Diaz & Eberli, 2018). Biologically influenced mechanism is passive mineralization, through which mucilaginous material or extracellular polymeric substances (EPS) control mineral nucleation (Diaz, Piggot, Eberli, & Klaus, 2013; Diaz et al., 2014; Dupraz et al., 2009). Biologically induced mechanism is active mineralization, which generates minerals along metabolic activities (Diaz & Eberli, 2018). Several metabolic processes have been identified throughout molecular techniques. Denitrification oxidizes organic carbon and could produce up to 19g CaCO3/g NO3-N in 2 days (Erşan, De Belie, & Boon, 2015), which is one of the most significant nitrogen transforming processes in ooids (Diaz, Piggot, Eberli, & Klaus, 2013; Diaz et al., 2015). CO2 fixation by photoautotrophs has been confirmed through studies on modern (Summons et al., 2013; O’Reilly et al., 2016) and ancient ooids (Pacton et al., 2012; Li et al., 2017). It seems to be the most productive microbes in the formation of ooids (Pacton et al., 2012; Summons et al., 2013). The presence of sulfate reduction, ammonification (Diaz et al., 2014) and sulfate oxidation (Summons et al., 2013; Diaz, Piggot, Eberli, & Klaus, 2013; Diaz et al., 2014; O’Reilly et al., 2016) bacteria in ooids has been detected through biomarker studies. However, their contributions to the formation of ooids were less investigated.
Ooids would be modified by microbes throughout their life cycle (Diaz & Eberli, 2018) as evidenced by isotopic geochemistry studies. The precipitation and/or alteration of ooids by microbial photosynthesis would elevate δ11B signal (Zhang et al., 2016). Denitrification may bring positive δ15N and δ18O of the NO3- in leachates of ooids (Diaz et al., 2015). Weak sulfate reduction is conjected through leachates of ooids (Diaz et al., 2015). However, the research about the modification of ooids by microbes mainly focuses on geochemical analysis rather than minerals, since carbonate minerals generated biologically (such as amorphous calcium carbonate (ACC), Duguid, Kyser, James, & Rankey, 2010) are thought to be primary minerals during the accretion of ooids. Ooids containing iron minerals are supposed to play a significant role in tracing microbial activities since iron is an important element for the growth and evolution of organisms. For instance, the formation of many banded iron formations (BIFs) has strong connection with bacteria (Posth, Konhauser, & Kappler, 2013; Chi Fru et al., 2013). Reddish pigment of rounded ferruginous oncoidal nucleus in European Phanerozoic red limestones is thought to be caused by iron bacteria mediated hematite (Préat, Mamet, De Ridder, Boulvain, & Gillan, 2000; Mamet & Préat, 2006). However, to date there are very few studies associating iron bacteria mediated organomineralization with carbonate ooids.
The Lower Jurassic Nieniexionala Formation in the Tethyan Himalayas of the southern Tibet contains thin- to medium-bedded red oolitic limestones. Although the lithology, microfacies, age and depositional environments were well studied (Han, Hu, Li, & Garzanti, 2016; Han, Hu, Kemp, & Li, 2018), the formation process of the red ooids has not been studied to date. Petrographic analysis shows the identified ferruginous minerals are responsible for the red color of the ooids. Iron minerals in sedimentary rocks are always thought to be related to microorganism activities (pyrites: Berner, De Leeuw, Spiro, Murchison, & Eglinton, 1985; Raiswell & Berner, 1986; Rickard, 2012; Wei, Chen, Wang, Yu, & Tucker, 2012; and hematites: Préat, Mamet, De Ridder, Boulvain, & Gillan, 2000; Mamet & Préat, 2006) and/or ambient redox conditions (e.g., Cornell & Schwertmann, 2003). In this paper, we studied the mineralogy composition and the forming process of the Nieniexiongla red ooids in order to identify the metabolic process of sulfate-reducing in these ooids.