Modern Marine Stromatolites discovered in the NE Red Sea - Al Wajh carbonate platform, KSA

Stromatolites are the vestige of first life on earth and were the dominating carbonate forming marine biota in the Archean and Proterozoic.  During the course of the Phanerozoic their importance in producing carbonates has been reduced to niche occurrences usually found in challenging environments, such as hypersaline marine settings and alkaline lakes.  Most recently, the discovery in 2010 of a new chlorophyll type - chlorophyll f - from stromatolites in Hamelin Pool in Shark Bay, Western Australia has sparked much additional interest in the genesis and composition of modern stromatolites.  

We report the discovery of stromatolites in the NE Red Sea on Sheybara Island, Al Wajh carbonate platform, KSA.  Based on satellite and drone surveys calibrated by site surveys, the Red Sea stromatolites are distributed over an area of about 50,000 m² in an intertidal to very shallow subtidal setting on a paleo-reef flat facing the open sea.  Two principal growth shapes are recognized: (i) elongated rhomboidal structures 10-100 cm in length, up to 5-50 cm in width and up to 10 cm in height and (ii) low relief (height <3 cm) irregular shaped tabular sheets in the shallow subtidal environment.  The rhomboidal intertidal stromatolites are pustular on the outside and laminated internally. X-ray CT scanning of the stromatolite samples showed moderately well laminated, millimeter scale, lithified layers potentially representing alternating modes of sedimentation and growth. Scanning Electron Microscopy (SEM) revealed that laminae consist of heavily bored carbonate grains, calcified tubes of filamentous cyanobacteria, mucoid sheets and spider-web like organic matter of likely dehydrated extracellular polymeric substance (EPS).  Carbonate precipitates of sub-micron size equant crystals and elongated aragonite needles, either occurring as single rods or in mashes, were also apparent from SEM. Molecular analysis of bacteria diversity show that cyanobacteria dominate the stromatolite surface, while heterotrophic bacteria are the main component in deeper layers.

During a sampling campaign in March 2021 salinity, pH and dissolved oxygen have been measured with average values at 42ppt, 7.8±0.1 and 5.9±0.5mg/L, respectively, typical for coastal Red Sea surface marine waters.  Water temperatures range from 18°C in the winter to 29°C in the summer.  During exposure at low tides surface temperatures over the tidal flats may fall as low as 12°C in the winter exceeding 43°C in the summer.  Large numbers of cerithid gastropods were found grazing on the stromatolite surfaces apparently not affecting their growth.

Hence, the setting and conditions are overall similar to some of the stromatolites found on the Exuma Islands in the Bahamas, the only other known occurrence of stromatolites in normal marine waters.  Research is continuing on the environmental conditions, the aerial distribution, the microbial diversity and chemical composition of these modern stromatolites to determine why they form in this particular location and if they are similar or not to other reported occurrences of stromatolites.