Initial Accretion in Hamelin Pool Microbialites: The Role of Entophysalis in Precipitation of Microbial Micrite

Abstract

One of the largest assemblages of living marine microbialites, with shapes and sizes analogous to ancient structures, is found along the margins of Hamelin Pool, Shark Bay, Western Australia. An investigation of microbial mats on the surfaces of these structures using petrographic analysis, light, and scanning electron microscopy identified the in situ precipitation of micrite as an important accretion mechanism in all major mat types (pustular, smooth, and colloform). Within each mat type, peloidal micrite, composed of nano-bulbous spheres to tabular and rod-shaped crystals, was closely linked with cells of the coccoid cyanobacterium Entophysalis, and microtextures of the micrite reflected the size and distribution of Entophysalis colonies. In pustular surface mats, where large colonies of Entophysalis were common, large clots of micrite were distributed randomly throughout the mat. In contrast, in smooth and colloform mats, where smaller colonies of Entophysalis were distributed along horizons, micrite formed fine laminae. In all surface mat types, micrite associated with Entophysalis had a characteristic honeycomb appearance, resulting from cell and/or colony entombment. These findings redefine our understanding of microbialite accretion in Hamelin Pool, recognizing the importance of microbial micrite in microbialite growth and showing that coccoid cyanobacteria are capable of building laminated structures. Moreover, Entophysalis, the dominant visible microbe associated with the precipitation of micrite in Hamelin Pool, has a lineage to Eoentophysalis, found throughout early and middle Proterozoic microbialites assemblages. These findings reinforce the importance of Hamelin Pool as a window to the past.

Bidirectional fabric evolution in Hamelin Pool microbialites, Shark Bay, Western Australia

“…Continuity of laminations between columns in colloform mats is low, and sediment is often observed in the spaces between columns. Proportions of sediment in colloform mats ranged between ca 0 and 50%, and averaged ca 20% (n = 10; Vitek et al, 2022).…”

“…(B) Depth transect showing transitional mats at mean sea level, between pustular mats of the upper intertidal zone and smooth mats of the lower intertidal zone. (C) Box and whisker plot showing the mean (x), median (horizontal line) and standard deviation of % micrite in comparison to sediment grains within each surface mat type (modified from Vitek et al, 2022).…”

“…Smooth mats form fine, continuous horizons of micrite, typically less than 250 μm thick, with abundant sediment grains (Figure 5E,F). Proportions of sediment in smooth mats ranged between ca 30 and 95%, and averaged ca 65% (n = 10; Vitek et al, 2022). Colloform mats form finely laminated fabrics composed of gently to steeply convex micritic horizons, usually less than 500 μm thick, often stacked into columns.…”

“…In all three of these previously described structurebuilding mat types, in situ precipitation of microbial micrite was recognised as an important accretion mechanism. Micrite comprises on average 36% to 81% of accreted products, with up to 99% micrite in some pustular mats and 98% micrite in some colloform mats (Figure S1; Vitek et al, 2022); energy-dispersive X-ray spectroscopy analyses indicated that this micrite is aragonite (Vitek F I G U R E 1 Hamelin Pool, Shark Bay, Western Australia. Boundaries of Stromatolite/Microbialite Provinces described by Suosaari et al (2016aSuosaari et al ( , 2016bSuosaari et al ( , 2019aSuosaari et al ( , 2019b are indicated by red lines.…”

Stromatolites: Linking the past to the future

“…The pustular mats, dominated by Entophysalis major , and the gelatinous pavement, comprised of a diverse community of small unicellular cyanobacteria (i.e., Aphanothece sp., Aphanocapsa sp., Entophysalis spp.) were found to produce ~26% of total carbonate sediment in the pool amounting to a potential 24,000 metric tons per year (Suosaari et al, 2022; Vitek et al, 2022).…”