Microbialites of Lago Strobel, Santa Cruz Argentina: Environmental and Astrobiological Significance

EMILIANO RIVAROLA; FERNANDO JAVIER GOMEZ; ESTELA CECILIA MLEWSKI

Paris

Workshop; Microbialites: Formation, Evolution and Diagenesis. M-Fed2021; 2021

The Muséum National d?Histoire Naturelle (MNHN)

Microbialites are organosedimentary deposits formed by the interaction between benthicmicrobial communities, detrital sediments and mineral precipitation. Thrombolites aremicrobialites characterized by a clotted mesostructure and lack of lamination. Understandinggeobiological processes involved in their formation helps to inform their significance in modernand ancient environments.The Strobel Lake (SL, GPS S 48° 26’ 52,9’’, W 71° 14’ 46,9’’) is a freshwater closed lake, itspH is 9,3 and its alkalinity range from 450 to 585mL/L CaCo3. It is located on a basaltic plateauin the province of Santa Cruz, Argentina. This lake has an area of 110 km2 approximately anda depth of more than 50 m. Carbonate deposits (microbialites and carbonate crusts) aredistributed in four levels or terraces (L1, L2, L3, L4) that represent the current and previouspositions of the lake level. The altitude difference between the current (L1) and highest level(L4) is about 15-20 m, suggesting significant changes in lake level and volume. Whereas L1and L2 respectively represent the most recently deposits underwater and slightly subaeriallyexposed, L3 and L4 apparently represents older terraces developed over small hillockscomposed of clusters of basaltic blocks.L1 and L2 are represented by dome to columnar shaped, white to light grey colored discreteand compound bioherms that are usually encrusted on a basaltic clast or block and mostlyisolated (not typically laterally linked). These range from 0,15 m to 1m in height and up to 0,8m in diameter. The domes or “active” microbialites from L1 are currently growing underwateror partially exposed (typically 0,10 and 0,50 m water depths). These are generally colonized byball-to pustular- shaped, dark colored (dark green to black) cyanobacterial communities (similarto formed by Nostoc). These form subspherical aggregates that are up to 1 cm in diameter andare an important component in the microbialites conditioning their microtexture. Within theclotted, white colored carbonate framework that characterize the microbialites, a light graycolored, fine grained (micrite) carbonate mud has been observed as well as small gastropodsand very fine grained detrital sediments. In L2, the domes are generally bigger than at L1,reaching 1 m high and 0,8 m in diameter and are totally exposed. At the mesoscale a clottedmesostructure also characterize microbialites in L2. L3 is characterized by thick carbonatecrusts that reach 0,3 m thick, developed over meter-sized basaltic blocks. Although, theseappears as discrete microbialites, the shape, size and distribution is conditioned by the size andshape of the basaltic blocks over which these are nucleated. The crusts are thicker at the top andlaterally thin out to disappear. In L3, the colors, textures and mesostructures of the crusts arevery similar to those recorded in the L1 and L2 microbialites. L4 is represented by thinner (mmto 15 cm scale) carbonate crusts and stains covering the basaltic bedrock. The L4 carbonatecrusts are whiter when compared to other crusts and microbialites.The geological setting of the Strobel lake (arid environment, a basaltic bedrock, a carbonatebelt and associated delta deposits) makes it an excellent analogue for the putative lacustrinedeposits recently discovered in the Jezero Crater of Mars by NASA. Understanding the maincontrols in carbonate precipitation may help to understand what is recorded on Jezero andhighlights the astrobiological significance of the Strobel lake as a planetary analogue.