Suarez-Gonzalez, P. | bioRxiv Science

bioRxiv2023

Organic matter influence on ooid formation: New insights into classic examples (Great Salt Lake, USA; Triassic Germanic Basin, Germany)

Ooids are particles composed of a tangential or radial cortex growing around a nucleus. They are common in carbonate deposits of almost any geological age and provide insights into environmental conditions. However, abiotic or biotic factors influencing their formation remain unclear. This study aims to advance our understanding of ooid formation with a multi- analytical approach (e.g., FE-SEM, Raman spectroscopy, -XRF) to classic examples from Great Salt Lake (GSL; USA) and the Lower Triassic Germanic Basin (GB; Germany). Both deposits represent hypersaline shallow-water environments where ooids are closely associated with microbial mats. GSL ooids are dominantly 0.2-1 mm in size, ellipsoidal to subspherical in shape, composed of aragonite, and contain organic matter (OM). GB ooids are mainly [≤]4 mm in size, spherical to subspherical in shape, composed of calcite, and currently contain little OM. Despite the differences, both ooids have the same cortex structures, likely reflecting similar formation processes. Some GSL ooids formed around detrital grains while others exhibit micritic particles in their nuclei. In GB ooids, detrital nuclei are rare, despite the abundance of siliciclastic particles of various sizes in the host rocks. GB deposits also include "compound ooids", i.e., adjacent ooids that coalesced with each other during growth, suggesting static in-situ development, which is supported by the lack of detrital grains as nuclei. GB ooids also grew into laminated microbial crusts with identical microstructures, further indicating a static formation. Such microbial crusts typically form through mineral precipitation associated with OM (e.g., extracellular polymeric substances), suggesting a similar formation pathway for ooids. The inferred key-role of OM is further supported by features in radial ooids from the GSL, which commonly exhibit, from their nuclei towards their surface, increasing OM contents and decreasing calcification.

paleontology↗

bioRxiv2021

Ooids forming in situ within microbial mats (Kiritimati atoll, central Pacific)

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 they 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 ooid and are in fact identical 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 has led 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.

paleontology↗