Hamelin Pool Research Articles

Circulation and Stratification Changes in a Hypersaline Estuary Due to Mean Sea Level Rise

Hypersaline Hamelin Pool, with mean salinity >65, is located in Shark Bay, Western Australia. The high salinity has reduced its biodiversity, but it is home to a diverse assemblage of modern marine stromatolites. The limited exchange of water between Hamelin Pool and the rest of Shark Bay, due to the presence of the shallow Faure Sill together with high evaporation and low rainfall-runoff have resulted in a hypersaline environment. With climate-change-induced mean sea level rise (MSLR), hydrodynamic processes that maintain the hypersaline environment may be affected and are the focus of this paper. Oceanographic observations, together with hydrodynamic model results, were used to examine the hydrodynamic processes under present and future MSLR scenarios. A large attenuation in the tidal range, changes in the tidal characteristics, and current speeds together with a strong salinity gradient were observed across the Faure Sill under present-day conditions. Under an MSLR scenario of 1 m, the tidal amplitude decreased by up to 10% to the north, whilst to the south, the tidal range increased by up to 15%. Regions of strong vertical stratification were present on both sides of the Faure Sill. The simulations indicated that, under MSLR scenarios, these regions expanded in area and exhibited higher levels of vertical stratification. The salt flux across the Faure Sill was maintained as a diffusive process under MSLR scenarios.

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

AbstractHamelin Pool, Shark Bay, Western Australia hosts the world's largest and most extensive assemblages of living marine microbialites, comparable in size and shape to ancient structures found throughout the fossil record. Documented here are the internal fabrics of modern microbialites collected throughout Hamelin Pool. Mesoscale and microscale observations of microbialite polished slabs and thin section scans, optical microscopy and scanning electron microscopy coupled with energy‐dispersive X‐ray spectroscopy formed the basis for a fabric classification system that combines accretionary mat type with microfabric. Accretionary mat types included pustular, smooth, colloform, as well as ‘transitional’ mats that are a cross between pustular and smooth mats. Mapping of fabrics in 45 microbialite heads indicated bidirectional evolution. An upward progression of fabrics corresponded to changes in mat type as the head grew upward into shallower water. A downward evolution of microfabrics occurred as surface mats transitioned into the subsurface of the microbialite structure. Downward microfabric evolution occurred as a result of early taphonomic processes, and involved a progression from the original depositional architecture to subsequent stages of “Micritic Thickening”, and finally, “Cement Infilling”. The observed bidirectional evolution of microbialite microfabrics within Hamelin Pool offers a conceptual framework for the study of modern microbialites, not simply as the sole product of accretionary mat types but rather as the combined result of the activity of surface mats and their taphonomic evolution. Early taphonomic processes induce further lithification of the microbialites which may enhance preservation potential in the geological record.

Characteristics of Early Neoproterozoic Stromatolites from Southern Liaoning, North China: Insights into the Formation of Stromatolites

Stromatolites, among the earliest fossils in Earth’s history, are widely distributed on the margins of the North China Precambrian carbonate platform. The formation processes of stromatolites reveal the biomineralization and evolution of early life in the Precambrian. The well-preserved stromatolitic dolostones recorded in the Ganjingzi Formation are developed around Yuanjiagou village, in southern Liaoning Province. The morphology of the Ganjingzi stromatolites manifests in stratiform, columnar, and domal forms. A tripartite lamina structure including light laminae and two types of dark laminae is observed in thin sections. The origins of dark laminae were related to microbial metabolism, while the light laminae were the result of the recrystallization of synsedimentary marine cement. Hardground substrate and carbonate fragments were suitable for microbes to colonize, suggesting that microbes can adapt to various current energy settings. A comparison of the growth environment, morphology, and laminae features between the Ganjingzi stromatolites and modern carbonate stromatolites from Hamelin Pool and Lagoa Vermelha suggest that the Ganjingzi stromatolites may have been formed in a restricted tidal-flat setting with high salinity and evaporation. The role of microbes that form modern stromatolites in inducing precipitation of carbonate or binding sediments, might contribute to the formation of the Ganjingzi stromatolites. The formation process of the Ganjingzi stromatolites indicates that the microbial communities, favorable substrate, and synsedimentary marine cement were the key factors in promoting the development of the Neoproterozoic stromatolites on the northeastern margin of the North China Craton.

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

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.

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

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.

A Cyanobacteria Enriched Layer of Shark Bay Stromatolites Reveals a New Acaryochloris Strain Living in Near Infrared Light.

The genus Acaryochloris is unique among phototrophic organisms due to the dominance of chlorophyll d in its photosynthetic reaction centres and light-harvesting proteins. This allows Acaryochloris to capture light energy for photosynthesis over an extended spectrum of up to ~760 nm in the near infra-red (NIR) spectrum. Acaryochloris sp. has been reported in a variety of ecological niches, ranging from polar to tropical shallow aquatic sites. Here, we report a new Acarychloris strain isolated from an NIR-enriched stratified microbial layer 4–6 mm under the surface of stromatolite mats located in the Hamelin Pool of Shark Bay, Western Australia. Pigment analysis by spectrometry/fluorometry, flow cytometry and spectral confocal microscopy identifies unique patterns in pigment content that likely reflect niche adaption. For example, unlike the original A. marina species (type strain MBIC11017), this new strain, Acarychloris LARK001, shows little change in the chlorophyll d/a ratio in response to changes in light wavelength, displays a different Fv/Fm response and lacks detectable levels of phycocyanin. Indeed, 16S rRNA analysis supports the identity of the A. marina LARK001 strain as close to but distinct from from the A. marina HICR111A strain first isolated from Heron Island and previously found on the Great Barrier Reef under coral rubble on the reef flat. Taken together, A. marina LARK001 is a new cyanobacterial strain adapted to the stromatolite mats in Shark Bay.

Impacts of Severe Tropical Cyclone Olwyn and the biogeomorphic response, Hamelin Pool, Shark Bay, Western Australia

AbstractStorm disturbance and recovery of the peritidal benthic microbial ecosystem occurs as part of the natural climate regime in Shark Bay. However, tropical cyclone and winter storm frequency and intensity are known to be changing due to climate forcing. Presented here is an analysis of the biogeomorphic response of the benthic microbial ecosystem within the intertidal to upper subtidal zone and the beach face coquina deposits of Hamelin Pool, to the passage of Category 3 Severe Tropical Cyclone Olwyn (13 March 2015). Storm effects (initial response to 40 days post‐event) include: erosional sculpting of sediments, mats and structures; deposition and winnowing of sediments; accumulation of mucilaginous products into flocs, slurries and sludges; along with limited development of new coquina deposits in the beach face. Medium (15 months) term observations include: mat recovery and changes with transformation of mucilage deposits into new subtidal gelatinous mats, intertidal transitional mats and low‐elevation microbial structures. Observations suggest that this disturbance had both positive (the floc‐to‐mat biogeomorphic storm response) and negative feedbacks (enhanced bioturbation), which impact the development of stromatolite forming microbial mats and microbialite structures.

Microbial contribution to early marine cementation

AbstractThe origin of early marine cements is still being debated as either abiotic, triggered by evaporation and CO2 degassing, or microbially‐mediated. In particular, micritic envelopes and micritic meniscus cements have been proposed to be microbially‐mediated but the microbial community and the resulting organomineralization processes have rarely been documented. Herein, scanning electron microscopy and thin section analyses of samples from the Bahamas and Hamelin Pool, Australia, provide evidence that early cements in intergranular areas are the result of organomineralization processes driven by both active (microbial metabolism) and passive mechanisms influenced by mucilaginous extracellular polymeric substances and/or microbial cell walls acting as matrices for crystal nucleation and growth. Signatures of biogenicity are supported by the conspicuous presence of early micritic cements in association with dense filamentous fabrics and entombed microbial forms. Most importantly, the prolific colonization of metabolically active communities, many of which assist in carbonate precipitation, trapping, binding and encrustation of the grains, testifies to the contribution of microbes in early cementation processes. The extent and ubiquity of mucilaginous extracellular polymeric substances and its concomitant association with amorphous calcium carbonate nanograins indicates that passive organomineralization via an intermediary amorphous phase is the other leading process for the generation of micrite and fine mesh of aragonite needle‐like crystals. Furthermore, the conspicuous occurrence of microbially‐mediated micrite, meniscus and bridging cements, as well as microcrystalline Mg‐calcite cement in the marine phreatic realm challenges the widespread notion that these fabrics are indicative of marine vadose or quiet marine phreatic environments. All analyzed samples show a remarkable similarity in early cement fabrics and a consistent superposition of prismatic cements on the micrite meniscus and bridging cements. These microbially‐mediated micritic cements represent the first generation of marine cements upon which aragonite and high‐Mg calcite develop.

Seasonal changes in sediment erodibility in a sandy carbonate environment detected from turbidity time series

Sediment resuspension and sediment erodibility were investigated in Hamelin Pool (Western Australia), which is one of the few modern carbonate settings where stromatolites and ooids are actively forming. For this purpose, the first time series of wave height and water turbidity were recorded at a ~2.5 m deep subtidal location in Hamelin Pool from July 2017 to March 2018. Sediment grain size analyses indicate that this environment is nearly free of mud, so the optical turbidity sensor was able to detect the intra-wave modulation of sand resuspension. This allowed us to use the standard deviation of the turbidity within a burst – as opposed to the mean turbidity – as a proxy for sand resuspension. This approach was further validated by wave tank experiments, which also helped to identify the occurrence of biofouling and discard the affected data. Although the wave regime was similar throughout the year, sand resuspension changed with seasons. Sediment erodibility, defined as the sand resuspension normalized by the bed shear stress, was higher during the Austral winter and spring, and lower in summer and autumn. This decrease in erodibility coincided with an increase in temperature and solar irradiance. These two environmental factors likely modulate the growth of benthic microbes, which in turn affects sediment dynamics. This seasonal modulation of sediment dynamics may influence additional processes, including the delivery of sand grains to stromatolites and the precipitation of aragonite around ooids and in stromatolite laminae.

Microbialite fields developed in a protected rocky coastline: The shallow carbonate ramp of the Aptian Romualdo Formation (Araripe Basin, NE Brazil)

The Aptian Romualdo Formation is a siliciclastic-dominated sedimentary unit recording the last marine ingression within the Cretaceous interior basins of northeastern Brazil. At the western margin of the Araripe Basin, rocks of the Romualdo Formation are mainly represented by carbonates resting abruptly over the Proterozoic crystalline basement. Detailed mapping and stratigraphic analysis revealed two stromatolite fields that were described and discussed for the first time. Several bioherms, biostromes and isolated stromatolites characterized by distinct microbialite morphologies associated with echinoid-rich strata have been identified, suggesting that hypersalinity, water depth and hydraulic conditions were the main factors controlling stromatolite morphogenesis. A cm-thick amalgamated bivalve rudstone, resting directly on the basement and representing a shell concentration formed above the fair-weather wave base was also recorded. Based on the regional distribution of the stromatolite types and associated sedimentary facies, we interpret the depositional environment as a local low-gradient carbonate ramp deepening to the east. Our data robustly indicated that the western rocky shorelines of the Araripe Basin during the Aptian were populated by microbial mats and stromatolites in a condition analogous to the modern world-famous Hamelin Pool, Shark Bay, Western Australia. Finally, the development of stromatolites and echinoid-bearing microbialites at the western margin of the basin may be correlated with the formation of bakevelliid- and cassiopid-rich shell beds in the upper part of the Romualdo Formation at the eastern margin.

Stromatolites, so what?! A tribute to Robert N. Ginsburg

AbstractIn tribute to Dr. Robert N. Ginsburg (1925–2017), we examine his 1991 seminal paper Controversies about Stromatolites: Vices and Virtues, which summarized current ideas about stromatolites including controversies of definition, whether ancient stromatolites should be interpreted as sedimentary structures and mechanisms of carbonate fixation. The accepted model of stromatolite morphogenesis in 1991 was that biology controls microscale internal structure whereas environment controls macroscale morphology. Ginsburg, however, predicted that biology and environmental influences on stromatolite growth were closely coupled at macro, meso and micro scales. Recent research in Hamelin Pool and the Bahamas has advanced our understanding of the inherent duality of stromatolites and associated controversies. These studies suggest that at the macroscale, when physical forces are strong, the environment is the main control on the morphology; however, when physical forces are weak, biological communities become the main drivers of morphology. Therefore, stromatolites can be considered both as fossils and as sedimentary structures dependent on the energy in the environment of deposition. At the mesoscale, as predicted by Trompette, stromatolite fabrics are influenced equally by environment and biology. As the degree of lamination is often unknown or heterogeneous, a generic genetic term such as ‘microbialite’ is considered the most appropriate terminology for structures of probable microbial origin. At the microscale, stromatolite microfabrics reflect environmentally driven cycling of microbial communities, reflecting both biology and environment. With respect to carbonate fixation, research on modern stromatolites provides a model for biofilm precipitation of micritic laminae lacking microfossils in ancient stromatolites. As pointed out by Ginsburg, the inherent duality of environmental and biological controls of morphogenesis at all scales is at the root of many long‐standing controversies. Recent investigations corroborate the foresight of Ginsburg nearly 30 years ago, further confirming well‐preserved stromatolites can provide insight into both biology and environmental factors in ancient ecosystems.

Stromatolite Provinces of Hamelin Pool: Physiographic Controls On Stromatolites and Associated Lithofacies

Recent studies recognized distinct stromatolite provinces in Hamelin Pool, Western Australia, each with morphologically distinct stromatolite structures paired with characteristic shelf physiography. In the present paper, we couple detailed lithofacies mapping with Hamelin Pool bathymetry and consider physiography as a control of sedimentation processes, including stromatolite development. Bathymetric transects, derived from a high-resolution bathymetry map with depths from 0 to 11 meters, allow calculation of slope gradients in the provinces. As in other settings, bathymetry is linked to energy regimes, which in turn appear to be coupled with variations in stromatolite morphologies and associated lithofacies as follows: (1) low-gradient ramps with low-energy settings are associated with sheet mats and elongate-clustered stromatolites that exhibit regular spatial patterns, possibly indicative of self-organization; (2) low gradients coupled with high-energy settings resulting from strong winds result in seif stromatolites with pronounced directional bands; (3) medium to steep gradients coupled with medium to high energy are associated with individual and merged stromatolites, often with thin basal necks; (4) headlands and promontories where the topography deflects currents are associated with elongate-nested stromatolites; and (5) medium- to high-energy slopes typically found at promontory edges and shelf margins are dominated by blocky pavement. Observations linking stromatolite morphology to physiography in a modern microbial system provide insight into the long-lived debate about biology versus environment in controlling stromatolite morphology. When physiography leads to a high-energy regime, environmental controls are the main factor determining stromatolite morphology. In contrast, when physiography promotes a low-energy environment, the response of biological communities becomes the main driver of macroscale stromatolite morphology.

Correlation of bio-optical properties with photosynthetic pigment and microorganism distribution in microbial mats from Hamelin Pool, Australia.

Microbial mats and stromatolites are widespread in Hamelin Pool, Shark Bay, however the phototrophic capacity of these systems is unknown. This study has determined the optical properties and light-harvesting potential of these mats with light microsensors. These characteristics were linked via a combination of 16S rDNA sequencing, pigment analyses and hyperspectral imaging. Local scalar irradiance was elevated over the incident downwelling irradiance by 1.5-fold, suggesting light trapping and strong scattering by the mats. Visible light (400-700 nm) penetrated to a depth of 2 mm, whereas near-infrared light (700-800 nm) penetrated to at least 6 mm. Chlorophyll a and bacteriochlorophyll a (Bchl a) were found to be the dominant photosynthetic pigments present, with BChl a peaking at the subsurface (2-4 mm). Detailed 16S rDNA analyses revealed the presence of putative Chl f-containing Halomicronema sp. and photosynthetic members primarily decreased from the mat surface down to a depth of 6 mm. Data indicated high abundances of some pigments and phototrophic organisms in deeper layers of the mats (6-16 mm). It is proposed that the photosynthetic bacteria present in this system undergo unique adaptations to lower light conditions below the mat surface, and that phototrophic metabolisms are major contributors to ecosystem function.

Comparative Metagenomics Provides Insight Into the Ecosystem Functioning of the Shark Bay Stromatolites, Western Australia.

Stromatolites are organosedimentary build-ups that have formed as a result of the sediment trapping, binding and precipitating activities of microbes. Today, extant systems provide an ideal platform for understanding the structure, composition, and interactions between stromatolite-forming microbial communities and their respective environments. In this study, we compared the metagenomes of three prevalent stromatolite-forming microbial mat types in the Spaven Province of Hamelin Pool, Shark Bay located in Western Australia. These stromatolite-forming mat types included an intertidal pustular mat as well as a smooth and colloform mat types located in the subtidal zone. Additionally, the metagenomes of an adjacent, non-lithifying mat located in the upper intertidal zone were also sequenced for comparative purposes. Taxonomic and functional gene analyses revealed distinctive differences between the lithifying and non-lithifying mat types, which strongly correlated with water depth. Three distinct populations emerged including the upper intertidal non-lithifying mats, the intertidal pustular mats associated with unlaminated carbonate build-ups, and the subtidal colloform and smooth mat types associated with laminated structures. Functional analysis of metagenomes revealed that amongst stromatolite-forming mats there was an enrichment of photosynthesis pathways in the pustular stromatolite-forming mats. In the colloform and smooth stromatolite-forming mats, however, there was an increase in the abundance of genes associated with those heterotrophic metabolisms typically associated with carbonate mineralization, such as sulfate reduction. The comparative metagenomic analyses suggest that stromatolites of Hamelin Pool may form by two distinctive processes that are highly dependent on water depth. These results provide key insight into the potential adaptive strategies and synergistic interactions between microbes and their environments that may lead to stromatolite formation and accretion.

From supratidal to subtidal, an integrated characterisation of Carbla Beach shallow microbial mats (Hamelin Pool, Shark Bay, WA): Lipid biomarkers, stable carbon isotopes and microfabrics

Modern microbial mats from Shark Bay are commonly regarded as robust analogues for Precambrian stromatolites. These microbial mats are complex ecosystems that exhibit intense biogeochemical recycling. In this study, a multi-proxy approach (including lipids, compound-specific carbon isotope analysis and petrography) is used to characterise microbial communities in three different types of mats (tufted, pustular and smooth) along the shallowest section of a tidal flat gradient. Cyanobacterial lipids were present in all three mats. Petrographical (optical and scanning electron microscopy) investigations also revealed that ooids in the tufted mat were larger and more common compared to the pustular and smooth mats. Biomarkers specific to sulfate reducing bacteria were detected in all mats. The diatom-specific C25:1 highly branched isoprenoid (HBI) alkene was most abundant in the smooth mat. However, imaging revealed that the smooth mat only contained rare diatoms of small size (~10 μm), whereas the pustular mat contained a variety of larger diatoms (~50 μm). The C25:1 HBI alkene marker is only produced by four diatom genera, which were most likely more represented in the smooth mat. Additionally, the smooth mat contained a greater contribution from aquatic macrophytes (Paq = 0.38) compared to the shallower mats, which is corroborated by the presence of 13C-enriched seagrass lipids (i.e. C29 steradiene). In all the mats, a major eukaryotic contribution was revealed via imaging techniques and supported by a high sterol content. This eukaryotic component (e.g. benthic foraminifera, diatoms) can impact the cohesive structure of the mats, the lithification processes and the lipid distribution, potentially complicating comparisons with Precambrian microbialites that were not affected by eukaryotic activity. This study reemphasizes the complexity of microbial ecosystems, and therefore highlights the benefit of multi-proxy approaches to characterise these biological systems.

Living Dendrolitic Microbial Mats in Hamelin Pool, Shark Bay, Western Australia

Hamelin Pool, Shark Bay, Western Australia, is home to the largest and most diverse assemblage of living marine stromatolites, with shapes and sizes comparable to ancient structures. A recent field-intensive program revealed seasonally ephemeral occurrences of modern dendrolitic microbial mats forming in intertidal, low energy settings. Dominated by filamentous cyanobacteria, dendrolitic microbial mats are formed when filaments provide a supporting framework as a result of gliding mobility, to build a shrubby morphology. Dendrolites, known throughout the rock record, refer to macroscopic microbialites with mesostuctures composed of unlaminated arborescent structures called shrubs. In these modern examples, thick filaments of Lyngbya aestuarii form the “trunk” of the bush, with finer filaments of Lyngbya fragilis, Phormidium sp. and Schizothrix sp. forming the “branches” These biologically-influenced dendrolitic structures provide insight into the complex interplay of microbial communities and the environment, broadening our understanding of shrub and dendrolite formation throughout the rock record.

Complete Genome Sequence of Methanohalophilus halophilus DSM 3094T, Isolated from a Cyanobacterial Mat and Bottom Deposits at Hamelin Pool, Shark Bay, Northwestern Australia.

ABSTRACT The complete genome sequence of Methanohalophilus halophilus DSM 3094T, a member of the Methanosarcinaceae family and the Methanosarcianales order, consists of 2,022,959 bp in one contig and contains 2,137 predicted genes. The genome is consistent with a halophilic methylotrophic anaerobic lifestyle, including the methylotrophic and CO2-H2 methanogensis pathways.

ENVIRONMENTAL PRESSURES INFLUENCING LIVING STROMATOLITES IN HAMELIN POOL, SHARK BAY, WESTERN AUSTRALIA

Environmental parameters in Hamelin Pool, Shark Bay were investigated to characterize extrinsic factors that may be affecting stromatolite morphogenesis. Hamelin Pool, which evolved into a restricted environment during the last few millennia, sustains the world's most extensive and diverse assemblage of modern marine stromatolites. These stromatolites occur in a shallow nearshore facies belt covering over 100 km of coast. Temperature, salinity, water level, and current data collected between 2012 and 2014 have revealed previously undocumented regional and seasonal trends. Regional trends include increasing salinity, greater temperature range, and decreasing energy moving southward from Faure Sill and to the Nilemah Embayment. Seasonal trends reveal paradoxically increased salinities in wet winter months and decreased salinity in dry summer months. When paired with annual tidal cycles, these trends suggest the influx of low salinity groundwater along the Hamelin Pool shelf. Speculation on how the documented environmental parameters may affect stromatolite growth suggests potential impact on morphology, internal fabric, and stromatolite-building microbial communities. These insights into environmental pressures within a living stromatolite system provide a framework for understanding extrinsic factors affecting microbial communities and stromatolite development throughout Earth history.

Short-stop visitation in Shark Bay World Heritage Area: an importance–performance analysis

ABSTRACTAn importance–performance analysis (IPA) investigated levels of satisfaction with natural attributes and facilities in place at a short-stop nature-based tourism site in the Shark Bay World Heritage Area (WHA). Over 700 on-site questionnaires were collected from visitors who visited the stromatolite viewing-boardwalk, an attraction with little managerial presence in the Hamelin Pool Marine Nature Reserve. Results indicate that, although an attraction in its own right, visitors use the Reserve as a ‘convenient break’ en route to other destinations in the WHA. Visitors indicated high interest in the natural attributes and the interpretive educational facilities at the site and are overall satisfied with their experience, despite low levels of visitor infrastructure and facilities. This study found that acceptable levels of visitor satisfaction can occur at short-stop sites with minimal levels of infrastructure. Visitors identified the need for additional educational facilities, with a preference for updating interpretative signs or developing an educational mobile phone app. In addition, this research emphasises the IPA approach as a useful tool for identification of management priorities for infrastructure and programme development, prior to implementing management change of a site.

New multi-scale perspectives on the stromatolites of Shark Bay, Western Australia.

A recent field-intensive program in Shark Bay, Western Australia provides new multi-scale perspectives on the world’s most extensive modern stromatolite system. Mapping revealed a unique geographic distribution of morphologically distinct stromatolite structures, many of them previously undocumented. These distinctive structures combined with characteristic shelf physiography define eight ‘Stromatolite Provinces’. Morphological and molecular studies of microbial mat composition resulted in a revised growth model where coccoid cyanobacteria predominate in mat communities forming lithified discrete stromatolite buildups. This contradicts traditional views that stromatolites with the best lamination in Hamelin Pool are formed by filamentous cyanobacterial mats. Finally, analysis of internal fabrics of stromatolites revealed pervasive precipitation of microcrystalline carbonate (i.e. micrite) in microbial mats forming framework and cement that may be analogous to the micritic microstructures typical of Precambrian stromatolites. These discoveries represent fundamental advances in our knowledge of the Shark Bay microbial system, laying a foundation for detailed studies of stromatolite morphogenesis that will advance our understanding of benthic ecosystems on the early Earth.