Lagoon microbialites on Isla Angel de la Guarda and associated peninsular shores, Gulf of California (Mexico)

Examples of two closed lagoons with extensive growth of Recent microbialites showing variable surface morphology and internal structure are found on Isla Angel de la Guarda in the Gulf of California. Comparable lagoonal microbialites also occur ashore from Ensenada El Quemado on the adjacent peninsular mainland of Baja California. The perimeters of all three lagoons feature crusted structures indicative of thrombolites with a knobby surface morphology 2cm to 3cm in relief and internal clotting without any sign of laminations. Outward from this zone, thrombolitic construction thins to merge with a white calcified crust below which a soft substratum of dark organic material 4cm to 6cm in thickness is concealed. The substratum is laminated and heavily mucilaginous, as observed along the edges of extensive shrinkage cracks in the overlying crust. The thrombolitic crust is anchored to the shore, while the thinner crust and associated stromatolitic mats float on the surface of the lagoons. Laboratory cultures of the dark organic material yielded the solitary cyanobacterium Chroococcidiopsis as the predominant taxon interspersed with filamentous forms. In decreasing order of abundance, other morphotypes present include Phormidium, Oscillatoria, Geitlerinema, Chroococus, and probably Spirulina. The larger of the two island lagoons follows an east–west azimuth and covers 0.225km2, while the smaller lagoon has a roughly north–south axis and covers only 0.023km2. The salinity of water in the smaller lagoon was measured as148ppt. Pliocene strata along the edge of the smaller modern lagoon include siltstone bearing calcified platelets suggestive of a microbial origin. Dry lagoons abandoned during the later Quaternary occur inland at higher elevations on the island, but retain no fossils except for sporadic white crusts cemented on cobbles around distinct margins. Raised Quaternary lagoons parallel to the big lagoon on Isla Angel de la Guarda are partly obscured by flood damage, but still easily mapped from aerial photos. These features suggest that Isla Angel de la Guarda experienced Quaternary uplift similar in scale to many other gulf islands on which marine terraces are preserved. Closed lagoons around the Gulf of California represent a stable oligotrophic ecosystem affected by extreme aridity and hypersalinity, punctuated episodically by the injection of floodwater from tropical storms. The taxonomic and geographic ranges of microbial communities throughout the lar