Modern Microbialites in High-Altitude Andean Lakes of Catamarca Province (Argentina): An Analogue to Precambrian Aedimentary Environments and Carbonate Microfabrics

GOMEZ, F.J., KAH, L.C., BARTLEY, J.K., AND ASTINI, R.A.,

Mendoza

Congreso; International Sedimentological Congress; 2010

International Association of Sedimentologists

Stromatolites are usually considered laminated organo-sedimentary structures, common in the ancient rock record and evidence of primitive microbial activity although stromatolite-like structures can also be the product of abiotic processes. Because stromatolites are the final product of interacting physical, chemical and biological processes they usually show a spectrum from biotic to abiotic signatures. Also, the common overprint by secondary alteration processes and the lack of modern analogues for Precambrian stromatolites makes the study of fossil microbial life challenging in geological ancient materials. Here we report modern microbial mats, mineral precipitates and stromatolites in Laguna Negra, a high-altitude (~4000 m.a.s.l.) Andean lake of Catamarca, Argentina, that represents an excellent analogue to Precambrian stromatolite microfabrics. Laguna Negra is located at the southeast end of the Laguna Verde Complex (LVC), in the Puna region of the Catamarca Province, in northwest Argentina, close to the border with Chile. The Puna region is a high-altitude plateau and is dominated by andesitic to basaltic volcanic rocks with minor rhyolite, dacite, and ignimbrites. The volcano-sedimentary setting as well as the unusual extreme environmental conditions of this lake, including high UV-radiation influx, extremes of temperature, salinity and water activity, can be compared with those supposed to have existed on early Earth as well as Mars, which highlights its astrobiological potential. These extreme conditions restrict eukaryotic life so that microfabrics are primarily controlled by microbial and environmental processes. The Laguna Negra is a slightly acidic (pH=5.5-6) calcium-chlorine rich brine (19 g/L and 198 g/L respectively), salinity is 285.6 ppt with and shows low sulfate concentration (~100 mg/). It has moderate alkalinity (690-930 mg/L) and a concentration of CO2 = 2601-3520 mg/L. Detritus cover most of the present lake area and is represented by silty to sandy immature siliciclastic sediment due to incipient weathering of volcanic bedrock at low water-rock ratios. Mineral precipitation in Laguna Negra consists of both carbonate (typically calcite) and evaporate (mostly halite and poly-hydrated salts) deposition. Carbonate deposits occur largely along the south and southwestern margin of the lake, and this is represented by stromatolites/oncolites and mineral crusts in an area with extensive microbial mats development. This Stromatolite Belt (SB) has an area of 298236 m2 and is located on the southern side of the lake in the mixing zone between groundwater springs and lake water. At a macro-scale, the stromatolites/oncolites and the associated mineral crusts show a broad diversity of both macroscopic and microscopic morphologies. These consist of millimeter to decimeter-scale laminar crusts, discs, flattened domes, columns and mounds which can contain both vertical columnar growth as well as lateral extensions (mineral precipitation along the water-line of partially exposed structures and/or at the water-sediment interface). There is a bimodal size distribution with bigger structures currently subaerially exposed and smaller one subacueous. At the micro-scale, mounds, discs and flattened domes show a generally concentric pattern of laterally continuous irregular, wrinkled lamina of 50-100 μm thickness. In polished surfaces it is mostly defined by an alternation of differentially colored lamina, generally whitish to dark grayish or greenish to reddish. Under the plain polarized microscope these are represented by dark micritic to microsparitic carbonate interlaminated with irregular to regular translucent lamina represented by either individual or stacked micro-laminated botryoids or isopachous fibrous crusts. They form both convex-upward micro-domes as well as mini-columnar convex-upward structures with isopachous and non-isopachous laminae. Fluorescence microscopy shows organic matter, putative microbial remains, usually present within irregular micritic laminae, suggesting mineral precipitation associated with microbial mat activity. Microbial filaments are locally preserved as well as coccoid-like partially degraded organic matter. Diatoms attached to micritic substrates or preserved within fibrous and botryoidal carbonates were noted. Clearly, lamina accretion is controlled by mineral precipitation and trapping and binding of detrital sediments is minor. Scarce siliciclastic material within lamina is primarily composed of silt to sand sized immature weathering products of volcanic rocks.   Extremes in temperature and salinity strongly limit the extent of non-microbial life in Laguna Negra. The standing biological community consists of a variety of well stratified microbial mats, yielding the structure commonly recorded in hypersaline environments, with different functional groups as photosynthetic cyanobacteria atop, followed by anoxigenic phototrophic purple- and green-sulfur bacteria and sulfate reducing bacteria and/or methanogens in the anoxic zone. Centric, pennate and filamentous diatoms are common in the upper surface. The pervasive red to pink color of microbial mats suggests production of pigments (e.g., carotenoids, and probably scytonemin) that may act to aid in protecting against high UV radiation influx and salinity stress. Green, olive and brownish, gelatinous, pustular and pinnacle microbial mats are also recorded. Biofilms atop most mineral precipitates are also common. The Laguna Negra stromatolites show the same basic building blocks as the Precambrian examples, developed under similar environmental conditions but where later alteration processes are absent. We consider that the mineralizing microbial system in Laguna Negra is a unique natural laboratory that fulfills most of the environmental features suggested for early Earth and Mars and where a spectrum of ongoing biotic and abiotic process and potential biosignatures can be studied and tested. These unique environmental conditions and the absence of secondary alteration make this place an excellent target to study microbe-mineral interaction in currently mineralizing microbial mats, improving our ability to interpret the sedimentary record on our planet and beyond.   Thin and thick mineral crusts show isopachous laminites as the main component. These range from planar laminites to more complex shapes including radial fibrous micro-botryoids, columnar, chevron-like and convex upward micro-domes. Lamination is more regular when compared with mounds, discs and flattened domes. A high degree of inheritance during lamina accretion and significant changes where small perturbations are propagated upward is observed. This feature is typically recorded in Precambrian stromatolites interpreted as abiotic.  The laminated micro-textures found in Laguna Negra stromatolites/oncolites and mineral crusts we described notably resemble those commonly recorded in well-preserved Precambrian sedimentary rocks. This suggest that here similar processes during precipitation and lamina accretion are maybe taking place. The Laguna Negra stromatolites show micro-laminated botryoidal crystals nucleated on degraded organics, and include organics during later crystal growth which are remarkably similar to those recorded in the Precambrian. The organic degradation by microbial heterotrophic activity on incipient biofilms possibly triggered carbonate precipitation by increasing alkalinity and ionic activity or providing nucleation sites. Microbes were passively entombed and better preserved during later fast crystal growth. This could explain why well preserved microfossils in the geological record are occasionally encased in similar micro-laminated crystals. The more irregular micritic fabrics that usually alternate could indicate better developed biofilms where biological influence on mineral precipitation has been more important and where nucleation of small (micrite) crystals predominate.