Botryoidal and Spherulitic Aragonite in Carbonates Associated with Microbial Mats: Precipitation or Diagenetic Replacement Product?

Abstract

Similar carbonate fabrics may result from different pathways of precipitation and diagenetic replacement. Distinguishing the underlying mechanisms leading to a given carbonate fabric is relevant, both in terms of an environmental and diagenetic interpretation. Prominent among carbonate fabrics are aragonite botryoids and spherulites, typically interpreted as direct seawater precipitates and used as proxies for fluid properties and depositional environments. This study investigated μm to mm-scale Holocene botryoidal and spherulitic aragonite from marine and non-marine carbonate settings associated with microbial mats, and reports two distinct formation mechanisms: 1) early diagenetic replacement, and 2) primary precipitation via nanocrystal aggregation. In the intertidal microbial mats of Khawr Qantur (Abu Dhabi), botryoidal and spherulitic aragonite are replacement products of heavily micritized bioclasts. To form the botryoidal and spherulitic aragonite, skeletal rods and needles, resulting from disintegration of micritized bioclasts, recrystallize into nanocrystals during early marine diagenesis. These nanocrystals then grow into fibrous crystals, forming botryoidal and spherulitic aragonite. In the lacustrine microbial bioherms of the hypersaline Great Salt Lake (United States) and in the hydrothermal travertines of Bagni San Filippo (Italy), botryoidal and spherulitic aragonite evolve from nanocrystals via precipitation. The nanocrystals are closely associated with extracellular polymeric substances in microbial biofilms and aggregate to form fibrous crystals of botryoidal and spherulitic aragonite. The studied fabrics form a portion of the bulk sediment and show differences in terms of their formation processes and petrological features compared to the often larger (few mm to over 1 m) botryoidal and spherulitic aragonite described from open-marine reefal cavities. Features shown here may represent modern analogues for ancient examples of carbonate depositional environments associated with microbialites. The implication of this research is that botryoidal and spherulitic aragonite associated with microbial mats are relevant in paleoenvironmental interpretations, but must be combined with a detailed evaluation of their formation process. Care must be taken as the term “botryoidal and spherulitic aragonite” may in fact include, from the viewpoint of their nucleation and formation mechanism, similar fabrics originated from different pathways. At present, it seems unclear to which degree the μm to mm-scale botryoids and spherulites described here are comparable to their cm-to dm-size counterparts precipitated as cements in the open pore space of reefal environments. However, it is clear that the investigation of ancient botryoidal and spherulitic aragonite must consider the possibility of an early diagenetic replacement origin of these precipitates.

Characteristics and controls on the distribution of sublittoral microbial bioherms in Great Salt Lake, Utah: Implications for understanding microbialite development

“…The mineralogical composition of the sublittoral bioherms, the focus of this study, does not differ from the bioherms found adjacent to the present‐day shoreline, implying that mineralogy cannot be used to differentiate environments. The predominance of aragonite precipitates associated with calcite and dolomite in GSL microbial bioherms and ooids as reported in this and previous studies (Della Porta, 2015; Dunham et al, 2020; Eardley, 1938; Ge et al, 2021; Pace et al, 2016; Pedone & Folk, 1996; Sandberg, 1975) is explained by the high carbonate supersaturation and Mg/Ca ratio of lake water, which is between 9.6 and 115.7 (cf. Müller et al, 1972).…”

“…The clotted peloidal and laminated micrite-grade fabric observed petrographically resemble the textures interpreted as microbial boundstone in several studies about recent and fossil microbialites. Nanometrescale sub-spherical carbonate structures eventually coalesce to form larger crystals with rational/euhedral crystal faces as similarly described in studies about microbially mediated carbonate precipitation (Bontognali et al, 2010;Ge et al, 2021;Jones, 2017;Jones & Peng, 2014;Pedley, 2014;Pedley et al, 2009;Perri et al, 2012Perri et al, , 2018. Meldrum and Cölfen (2008) have shown that several biominerals of skeletal organisms can form from amorphous precursors represented by nanoparticles, which are favoured by supersaturation.…”

Dissolved silica‐driven dolomite precipitation in the Great Salt Lake, Utah, and its implication for dolomite formation environments

“…The South Arm sediments (carbonate mud) are composed of dolomite, quartz and calcite with a minor amount of aragonite, albite, illite and poorly crystalline Mg‐bearing clay (Table 2). Stromatolite samples collected from the North Arm and South Arm also consist of predominately aragonite with a trace amount of calcite, which agrees with previous observations (Dunham et al ., 2020; Ge et al ., 2021). Carbonate mud samples collected from ridges between ponds in the North Arm have a similar mineral assemblage as carbonate mud from the South Arm with dolomite, quartz and calcite constituting major components and minor amounts of albite, illite and poorly crystalline Mg‐bearing clay.…”

Disordered dolomite as an unusual biomineralization product found in the center of a natural Cassis pearl

“…The precipitation of disordered dolomite could be triggered by the oversaturation of carbonate in this enclosed region. The spheroidal shape (commonly called spherulite) is very common for carbonate precipitates [38][39][40], which indicates fast precipitation from highly non-equilibrium conditions [41]. The eccentricity of the brown granular core is probably controlled by gravity (relative to the original orientation of the pearl during its formation).…”