Fossil Stromatolites form some of the earliest evidence for life on Earth, being present in deposits from the Palaeoproterozoic and Archaean, with the oldest known examples currently dated to about 3.48 billion years ago. However, their importance has declined in the Phanerozoic, forming significant proportion of carbonate reefs only for brief periods following the End Ordovician and End Permian extinctions. Stromatolites still exist today, and understanding formation presents us with the possibility of understanding some of the oldest ecosystems on Earth, although modern forms are generally restricted to extreme environments, such as hypersaline marine settings and alkaline lakes, with living Stromatolites only known from two modern open-marine environments, Shark Bay in Western Australia and in the Exuma Islands of the Bahamas.
In a paper published in the journal Geology on 15 February 2024, Volker Vahrenkamp and Viswasanthi Chandra of the Physical Sciences and Engineering Division at King Abdullah University of Science and Technology, Elisa Garuglier and Ramona Marasco of the Biological and Environmental Sciences and Engineering Division at King Abdullah University of Science and Technology, Kai Hachmann, also of the Physical Sciences and Engineering Division at King Abdullah University of Science and Technology, Pankaj Khanna of the Department of Earth Sciences at the Indian Institute of Technology Gandhinagar, Daniele Daffonchio, also of the Biological and Environmental Sciences and Engineering Division at King Abdullah University of Science and Technology, and Alexander Petrovic, again of the Physical Sciences and Engineering Division at King Abdullah University of Science and Technology, and of Carmeuse, describe the discovery of a colony of living Stromatolites in the intertidal zone on Sheybarah Island on the Red Sea coast of Saudi Arabia.
Sheybarah Island forms part of the Al Wajh Carbonate Platform on the northwest coast of Saudi Arabia. The Al Wajh Carbonate Platform is connected to the Arabian mainland, and is enclosed by a 115 km reef-shoal belt. The central part of the platform hosts a lagoon with a maximum depth of 42 m, which is surrounded by 92 islands and patch-reefs. Sheybarah Island is located on the southwest edge of this platform, and has an area of 27 km², with a maximum elevation of 2 m above sealevel. The lagoon-facing rim of the southern slope of the Al Wajh Carbonate Platform is dominated by Mangroves, behind which is a sandy and rocky, then a rocky reef flat facing towards the open sea.
The Red Sea is semi-enclosed, with slow surface-water renewal, creating a low nutrient environment. In the northeast part of the Red Sea, the average surface temperature is typically about 28°C during the summer, falling to about 23°C in winter, and surface salinity can reach 41‰. Prevailing winds come from the north-northwest, with an average windspeed of 4 m per second, although in winter strong southwesterly winds sometimes occur. The prevailing winds bring with them a high load of iron-rich sediment.
The presence of Stromatolites on Sheybarah Island was discovered during a scouting visit made to the island in January 2021. The Stromatolites form a field in the intertidal to shallow subtidal zone, on a flat slope which dips towards the sea, formed from a fossil Coral reef. A core drilled into this reef produced a radiocarbon date of 5264 years before the present, suggesting that it was formed during the Holocene sealevel highstand, between 4000 and 8000 years ago, when sealevels in the area would have been about 2 m higher than today. The surface of this reef is eroded, presumably due to modern wave action lowering the flat upper reef to the modern sealevel. A lithified sand layer beneath the Stromatolites yielded a date of 1640 years before present, which dates obtained from laminations within the Stromatolites ranged from 120 to 325 years before the present. This implies that the onset of Stromatolite growth was no more than 300-400 years ago; it is possible that it was more recent and that sand grains from a now absent upper layer have been incorporated into the Stromatolite structure.
The tidal range in the area where the Stromatolites are growing is typically 50-60 cm, with a maximum of about 1 m, although occasional storm surges can inundate lower lying parts of the island. Sea temperatures measured at a depth of 5 m varied between 21°C and 31°C over the course of a year, though in the intertidal zone the temperature variation was much higher, between 8°C and 48°C, as very shallow seawater was exposed to highs of day time and lows of night time air temperatures. Salinity measured in March was 42‰; at the same time the water pH was 7.8 and dissolved oxygen was 5.9 mg per litre.
The Stromatolites are found over an area of about 50 000 m³, which could be divided into three zones, upper intertidal or beach-adjacent, mid-intertidal, and shallow subtidal, each of which was dominated by Stromatolites of a different morphotype. Stromatolites in the beach-adjacent zone, referred to as Type 1 Stromatolites, tend to be grey-green to dark brown in colour, and elongated-sinusoidal to rhomboidal in shape, aligned so that their long axis is perpendicular to the predominant wave crest direction. These tend to be less than 15 cm high, 5-50 cm wide, and 10-100 cm long, although they often coalesce into larger structures, which can be as much as 10 m long. The surface of these Stromatolites tends to be pustular in texture, and their interiors fairly well lithified. The Stromatolites of the mid-to-lower intertidal zones, referred to as Type 2 Stromatolites, are flatter, reaching a maximum of about 5 cm in height, forming irregularly shaped, ovoid to tabular clusters which can cover as much as 100 m³. The base of these Stromatolites is often raised above the platform, on a small column of eroded Holocene Coral. In the lower intertidal to shallow subtidal zones Type 3 Stromatolites are low relief and poorly lithified, and often covered by a thin layer of carbonate sand.
The internal structure of Type 1 Stromatolites was found to be laminated, with undulating layers of sediment interspersed with layers with clotted fabrics and vugs (cavities lined with mineral crystals), which in the fossil record would be interpreted as Thrombolitic Stromatolites. When sections of this material were cut and washed, dense lithified layers stood out in relief. Grazing organisms such as Gastropods were often trapped in the matrix. Millimetre scale microlitic crusts (microbially derived calcium carbonate crusts) alternated with millimetre scale sediment layers, within which lithification was beginning to break down grain boundaries. These grain layers often showed high levels of microboring, suggesting ongoing micritization even after sediment accretion. Rim cements contained numerous aragonite needles, while microlitic crusts were predominantly aragonite (85%), with significant proportions of high magnesium calcite (9%) and low magnesium calcite (5%), and small amounts of quartz and clay minerals.
Examined through a scanning electron microscope, filamentous Cyanobacteria appeared to be the most abundant organisms within the structure of the Stromatolites, enveloping sediment grains in single strands of bundles, covered with mucous sheaths made up of excreted biological polymers. These filament and biopolymer masses also contained large numbers of sub-micron sized calcium and magnesium carbonate crystals. Also present were biofilm structures with Bacterial cells, and Navicula-like Diatoms. The upper and lower surfaces of the topmost microbial mat included numerous reticulated filament structures. An investigation into the biodiversity of the mats using 16S rRNA gene metabarcoding found that the most abundant micro-organisms were Proteobacteria, which made up 49% of the total (30% Alphaproteobacteria, 12% Gammaproteobacteria, and 7% Deltaproteobacteria), with Cyanobacteria making up 16% of the total, and Bacteroidetes 11%.
The presence of Stromatolites on the intertidal platform of Sheybarah Island appears to be driven by environmental factors. The platform surface here is exposed to frequent wetting and drying cycles, as well as extreme temperature fluctuations, with generally low current conditions, apart from the occasional storm event. Similar conditions are found on the other islands of the Al Wajh Carbonate Platform, making it likely that these to are home to Stromatolite colonies. The conditions here are similar to those found in the Exuma Islands of the Bahamas, where Stromatolites are also found; the much lower profile of the Sheybarah Island Stromatolites (never more than 15 cm high) probably reflect the limited tidal range of the Red Sea.
Growth of the Sheybarah Island Stromatolites appears to be driven by microbial activity, which leads to the accretion and differential lithification of sediment grains. The range of structures observed appears to be driven by a cycle of grain-entrapment followed by sedimentation, similar to that which has been documented in the Bahamas. The microbial community within the Stromatolites appears to be made up of a combination of photoautotrophic organisms (Cyanobacteria), and heterotrophic organisms, including ones capable of reducing sulphates.
The reticulated filaments seen in the Sheybarah Island Stromatolites are a surprising structure. Such filaments have previously been observed in microbial mats from aphotic environments, such as caves. At Sheybarah Island they appear to be ubiquitous in the upper layer of Stromatolites, and have a variety of morphologies, including horizontal ridges supported by vertical columnar structures. The nature and composition of these filaments is unclear, and will be the subject of future research.
Vahrenkamp et al. believe the Sheybarah Island Stromatolites to be the first open marine Stromatolites discovered in the Middle East, providing a new opportunity to study structures sparsely distributed on the modern Earth, but which were an important part of the Earth's earliest ecosystems. To date, the Stromatolites of the Bahamas have been considered the best analogue for the shallow-marine Stromatolites which formed throughout the Proterozoic, making the similar, but not identical, Stromatolites from Sheybarah Island a significant discovery with the potential to greatly enhance our understanding of Proterozoic ecosystems.
See also...
