The microbial carbonate factory of Hamelin Pool, Shark Bay, Western Australia

Abstract

Microbialites and peloids are commonly associated throughout the geologic record. Proterozoic carbonate megafacies are composed predominantly of micritic and peloidal limestones often interbedded with stromatolitic textures. The association is also common throughout carbonate ramps and platforms during the Phanerozoic. Recent investigations reveal that Hamelin Pool, located in Shark Bay, Western Australia, is a microbial carbonate factory that provides a modern analog for the microbialite-micritic sediment facies associations that are so prevalent in the geologic record. Hamelin Pool contains the largest known living marine stromatolite system in the world. Although best known for the constructive microbial processes that lead to formation of these stromatolites, our comprehensive mapping has revealed that erosion and degradation of weakly lithified microbial mats in Hamelin Pool leads to the extensive production and accumulation of sand-sized micritic grains. Over 40 km² of upper intertidal shoreline in the pool contain unlithified to weakly lithified microbial pustular sheet mats, which erode to release irregular peloidal grains. In addition, over 20 km² of gelatinous microbial mats, with thin brittle layers of micrite, colonize subtidal pavements. When these gelatinous mats erode, the micritic layers break down to form platey, micritic intraclasts with irregular boundaries. Together, the irregular micritic grains from pustular sheet mats and gelatinous pavement mats make up nearly 26% of the total sediment in the pool, plausibly producing ~ 24,000 metric tons of microbial sediment per year. As such, Hamelin Pool can be seen as a microbial carbonate factory, with construction by lithifying microbial mats forming microbialites, and erosion and degradation of weakly lithified microbial mats resulting in extensive production of sand-sized micritic sediments. Insight from these modern examples may have direct applicability for recognition of sedimentary deposits of microbial origin in the geologic record.

Figure 1

(a) Hamelin Pool map showing the Provinces of Hamelin Pool, the location of collected sediment samples, and the percentage of irregular micritic grains in collected sediment samples (see Supplemental Fig. S1 for comparison to peloid percentage as shown in Fig. 10c in), basemap sources: Esri, DigitalGlobe, GeoEye, i-cubed, USDA FSA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community, created in ArcMap 10.6 https://support.esri.com/en/products/desktop/arcgis-desktop/arcmap/10-6-1; (b) pie chart showing sediment composition of all samples collected in Hamelin Pool. Dominant components are peloids (red) and irregular micritic grains (yellow), which make up almost half of sediments; and (c) the classic diagram of Hamelin Pool stromatolites which make up less than 2% of the total area in Hamelin Pool, with the addition of intertidal unlithified sheet mats, which make up over 3% of the total area of Hamelin Pool, and lithified pavements, which make up 9% of the total area in Hamelin Pool (percentages taken from). Diagram modified from Suosaari et al..

Figure 2

Unlithified pustular sheet mats dominated by Entophysalis. (a) Pustular sheet mats in the upper intertidal zone around the margin of Hamelin Pool in the Flagpole Province, scale bar applies to foreground; (b) hand sample of pustular sheet mat (HP14-JS21); (c) confocal image showing healthy live Entophysalis major cells within EPS (HP13-JS13).

Figure 3

Rounded irregular micritic grain production in unlithified pustular sheet mats. (a) eroded pustules from pustular sheet mats; (b), degrading eroded Entophysalis pustule (HP13_T4_EP), the gel is organic matter, the white is micritic precipitate (image from Suosaari et al. supplemental material); (c) thin section photomicrograph of pustule shown in (b), Entophysalis cells and surrounding organics are stained purple with crystal violet, boxed area shown in higher resolution in (d); (d) Entophysalis cells are being replaced by microcrystalline carbonate, micrite (m, arrows), boxed area shown in higher resolution in (e); (e) Entophysalis cells are being replaced by microcrystalline carbonate, micrite (m, arrows); (f) photomicrograph of wet thin section of an eroded pustule collected after cyclone Olwyn (4_15EPS_1) showing clumps of micrite (arrows) in a matrix of E. major cells throughout the sample.

Figure 4

Rounded irregular micritic grain production in core beneath unlithified pustular sheet mats. (a) one inch diameter core taken through pustular sheet mat in Nilemah embayment; (b) photomicrograph of a thin section made from the core of sediment underlying the pustular sheet mat showing sediment comprised of abundant irregular micritic grains released from degrading Entophysalis and fresh foraminifera; and (c) shows a high resolution of the boxed area from (b) showing micritic grains formed within the pustules easily recognizable by their irregular shapes and peloidal textures.

Figure 5

Irregular micritic grain production in gel mats colonizing the surface of low-relief microbial pavements. (a) Gel mats in the shallow subtidal zone of Hamelin Pool in the Booldah Province; (b) gel mat showing thin laminae of micritic calcium carbonate on the surface and as horizons within the mat (arrows), as well as within the mat; (c) phase contrast micrograph of the gel mat showing cells of the 10 µm in diameter microalga with conspicuous pyrenoid (phase bright spheres); (d) TEM image of ultrathin section of the microalga through the pyrenoid, revealing the starch granules (arrow).

Figure 6

Erosion of gel mats colonizing the surface of low-relief microbial pavements showing irregular micritic grain production. (a) micritic crust (arrow) with gelatinous mat underneath; (b) platy micritic crusts (arrow) under partially eroded gel mats on top of subtidal low-relief microbial pavement in the Booldah Province; (c) detached, eroded globs of gel mat, some with attached crusts (arrows); (d) platy crusts from eroded globs of gel mat after organics were removed with bleach.

Figure 7

Irregular micritic grain production in gel mats. (a) thin section photomicrograph of showing a gel mat intersected by a micritic carbonate laminae and with homogeneous to clotted micritic textures shown in (b); (c) bulk sediment sample collected from the Booldah Province where irregular micritic grains can make up more than 75% of the total sediment; (d) high resolution image of micritic grains from box in (c) showing the characteristic irregular, often platy morphologies.

Figure 8

Modification of the Schlager and Reijmer carbonate factory diagram with the inclusion of a microbial pathway informed by the microbialite-peloidal system observed in Hamelin Pool with construction by lithifying microbial mats forming microbialite buildups and erosion of unlithified microbial mats forming sand-sized micritic sediments.