High-Altitude Andean Lakes as Natural Laboratories for Planetary Geology and Astrobiology Research: The Laguna Negra case (Argentina)

Introduction

Sedimentary deposits developed in High-Altitude Andean Lakes (HAAL) share some extreme and environmental characteristics that made them excellent analogues for planetary geology and astrobiology research. These conditions favor the development of a diverse and abundant microbial biota that influence mineral precipitation (e.g. carbonates) and the develpoment of microbially influenced sedimentary deposits typically know as stromatolites. To recognize and differentiate stromatolites from similar laminated deposits purelly formed by chemical processes is not straightforward, and Archean stromatolites are a good example. This makes HAAL good environmental analogues to study microbe-mineral proceeses, and the associated biosignatures. The recent findings of putative marginal lacustrine and delta deposits in the Jezero crater on Mars surface highlight the potential of these systems from and astrobiology perspective. The origin and characteristics of these martian carbonates is still unknown so the evaluation potential scenarios in comparable environmental conditions may shed some light into this uncertainties.

The Laguna Negra (a high altitude lake in Catamarca Province, Argentina) is an outstaning example of HAAL where an active microbial mat system and associated carbonate deposits is well developed. These are located in the mixing zone between groundwater spring-fed pools and the main lacustrine system. The Laguna Negra is a unique natural laboratory that fulfills the environmental criteria suggested for early Earth (Archean) and Mars (Noachian) where spectrum of biotic and abiotic process can be studied improving our ability to interpret the sedimentary record on our planet and beyond.

Geological setting

The Laguna Negra is a shallow hypersaline lake where the pH of the main lake and the groundwater springs feeding the lake fluctuates between ~6 and ~8 and salinity between ~320 and ~9 ppt respectively. The mixing zone between the main lake and groundwater is oversaturated with respect to calcite and aragonite. The carbonate belt consists of oncoids, stromatolites, and laminar crusts that are spatially localized in different zones and associated to different microbial mats systems and chemical conditions. Particularly interesting are the laminar crusts, developed in a zone where no significant microbial mats has been observed, but where a diversity of morphologies and microtextures has been recorded. Although interpreted as purelly chemically precipitated, unravelling the different processes that controls this morphological varibality is still challenging.

Oncoids, Stromatolites and Laminar crusts

Oncoids represented by concentrically laminated discs, spheres, and flattened domes (cm to dm in diameter) that can coalesce to form more complex structures and are typically associated with well-stratified diatom-rich microbial mats. The external surface surface can be smooth or can show pillar-like to shrub-shaped millimeter scale protrusions and ornamentations, particularly on the side affected by wind and currents. Oncoids are partially buried and can show lateral protrusions at the sediment–water and the air–water interface. Although oncoids are sub-spherical in shape, they can show asymmetric growth (bigger below the sediment–water interface). Complex lamination is also a result of oncoid rotation, particularly by cryoturbation and bioturbation.

Although water mixing, CO₂ degassing, and evaporation are particularly important to trigger carbonate precipitation the influence of microbial mats is visible in the macromorphologies (differential growth within the anoxic zone related to metabolisms that increase alkalinity) and a diverse set of microtextures some of which are interpreted as microbially influenced.

Stromatolites more localized and represented by centimeter to decimeter-scale laminated structures (up to 25 cm) that typically have a planar or laminar to columnar shape. They are observed associated with dark colored microbial mats and usually are encrusting the upper surface of oncoids. The columnar structures are usually centimeter-sized. Internal lamination is irregular, overlapping, crenulated-micritic to micro-peloidal laminae that preserve abundant organic remains. These features are suggestive of microbially influenced texture.

Laminar crusts show a patchy distribution and represented by millimeter to decimeter carbonate crusts encrusting volcanic rocks, peloidal sediments as well as organic remains. Can also develop dome-shaped morphologies showing concentric growth patterns. These concentric structures can be slightly assymetrical, showing preferential growth towards the upper half (as opposed to oncoids). Oriented and elongated structures are common (by wind-driven currents in the lake). Plates and domes can be rotated and/or coalesce to form more complex structures or more extensive platforms along the lakeshore. The surface can be smooth or show dendritic to pustular patterns or protusions as well as travertine-like microterracetes.

Isopachous regular laminane is the most common building block, as stated showing a concentric pattern but it is worth mentioning that the wind-oriented structures, in cross-section, develop more complex micro-textures (shrub-like to dendritic/micro-stromatolite microfabrics) that resemble microbially influenced structures.

Given the absence of microbial mats, and the macro-morphologies and micro-textures described (e.g., lamina regularity and degree of inheritance, lack of organic remains within the lamina), these structures have been interpreted as predominantly chemically precipitated carbonates, triggered by oversaturation related to water mixing, strong CO2 degassing, and evaporation.

Final considerations

Both, physocochemical and microbial processes can contribute to a diverse range of morphologies and carbonate microtextures and it is not easy to urvanel their relative contributions. Oncoids, stromatolites and laminar crusts show some distinctive features that suggest some of the driving controls, but also some overlapping characteristics that may be difficult to discriminate. As an example, although laminar crusts generally show (in cross section) a strong lamina regularity, a more diverse set of microtextures can be produced by the influence of advective-diffusive processes, localized scarbonate precipitation, rotation due waves, and cryo-bioturbation, thus increasing lamina complexity that can be confused with microbially influenced textures. Possible origins of the carbonates recorded at Jezero crater, for example including carbonate crusts developed over the basaltic substrate, pore-vein-filling carbonate cements, reworked carbonate material, or even stromatolite-like structures. Although chemical biosignatures (trace element distribution and isotope fractionation) are central in the tool box of astrobiologists, to recognize the putative biogenicity of these carbonates it is necessary to combine chemical analysis with the information provided by the external macro-micro morphology and the internal macro and micro microfabric, something that may not be possible when dealing with rover or image based analysis on outcrops or with sample returned to Earth, where part of the context may be lost.