Abstract
Ooids (subspherical particles with a laminated cortex growing around a nucleus) are ubiquitous in the geological record since the Archean and have been widely studied for more than two centuries. However, various questions about them remain open, particularly about the role of microbial communities and organic matter in their formation and development. Although ooids typically occur rolling around in agitated waters, here, we describe for the first time aragonite ooids forming statically within microbial mats from hypersaline ponds of Kiritimati (Kiribati, central Pacific). Subspherical particles had been previously observed in these mats and classified as spherulites, but these particles grow around autochthonous micritic nuclei, and many of them have laminated cortices, with alternating radial fibrous laminae and micritic laminae. Thus, they are compatible with the definition of the term ‘ooid’ and are in fact very similar to many modern and fossil examples. Kiritimati ooids are more abundant and developed in some ponds and in some particular layers of the microbial mats, which leads to the discussion and interpretation of their formation processes as product of mat evolution, through a combination of organic and environmental factors. Radial fibrous laminae are formed during periods of increased supersaturation, either by metabolic or environmental processes. Micritic laminae are formed in closer association with the mat exopolymer (EPS) matrix, probably during periods of lower supersaturation and/or stronger EPS degradation. Therefore, this study represents a step forward in the understanding of ooid development as influenced by microbial communities, providing a useful analogue for explaining similar fossil ooids.
Highlights
IntroductionOoids (subspherical particles with a laminated cortex grow‐ ing around a nucleus, cf. Richter 1983a) have fascinated and intrigued humanity for millennia (Burne et al 2012; Weber 2014)
Ooids have fascinated and intrigued humanity for millennia (Burne et al 2012; Weber 2014)
The particles were sputtered with Pt/Pd (14.1 nm for 5 min) and observed in a field-emission scanning electron microscope (FE-SEM) Leica EM QSG100, using a detector of secondary electrons (SE2) at a voltage from 2 to 4 kV, combined with an INCA X-act energy-dispersive X-ray (EDX) spectroscope (Oxford Instruments)
Summary
Ooids (subspherical particles with a laminated cortex grow‐ ing around a nucleus, cf. Richter 1983a) have fascinated and intrigued humanity for millennia (Burne et al 2012; Weber 2014). Siahi et al 2017; Flannery et al 2019) and with the longest history of descriptions and interpretations, since Roman times (Burne et al 2012), they have prompted continuous discussions about their definition, classification, formation processes, mineralogy, diagenesis and evolution throughout Earth history (see some previous reviews in Kalkowsky 1908; Bucher 1918; Bathurst 1968; Teichert 1970; Fabricius 1977; Davies et al 1978; Simone 1981; Krumbein 1983; Richter 1983a; Wilkinson et al 1985). Ooids have an long history of being interpreted as formed by some degree of influence from organic molecules or even microbial communities
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