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.