<div> <div> <div> <p>Appeared more than 3.5 billion years ago, microbialites represent one of the oldest ecosystems on Earth. These architects of oxygenic photosynthesis dominate the fossil record for nearly 80% of Earth's history, having influenced the evolution of the planet notably by changing the properties of the atmosphere. Despite a dramatic decline in their abundance from the start of the Phanerozoic, they still develop today in a wide spectrum of depositional environments (The Bahamas, Australia, Brazil, etc.). The spatio-temporal distribution of microbialites therefore make them a valuable archive of both life and Earth evolution. However, after nearly 100 years of research, their origin as well as their environmental significance is still a matter of debate. Little is known about microbialite formation, in particular the relative roles of microbial versus environmental factors ruling their growth. Laguna de Los Cisnes located at 53 &#176; 25' S and 70 &#176; 40' W in Chilean Tierra del Fuego, Patagonia, provides us with a unique site to fill this gap. This basin was formed during the retreat of the ice following the last glaciation about 10,000 years ago. Subsequently, the lake was densely colonized by microbial mats that developed the presently living and fossil carbonate microbialites. We have explored the relative contribution of environmental versus biological factors controlling microbialite morphogenesis across various scales.</p> <p>Macroscopically, these organo-sedimentary deposits have an extension of almost 8 km<sup>2</sup> encompassing several morphologies exceptionally large with maximum heights and widths of 1.5 m and 5.0 m respectively. Crater-like shapes are dominant, displaying a spherical to elongated character most frequently unfilled. Both spatial distribution and temporal succession of morphotypes indicate that the dominant physico-chemical character of the water is critical in the localization as well as in the style of the microbial carbonate factory, which in turn is reflected in the morphological character of the subsequent deposit. The microbialite meso-structure reveals a pattern of three lithological distinctive stacked layers. This fabric reflects a multiphase history of formation, linked with the ecological succession of specific bacterial communities throughout time that are still strongly influenced by the prevailing environmental conditions. Interestingly, the simultaneous occurrence of various living bacterial mats provides insights regarding the microscale interactions between the different compounds of the bacterial ecosystem (cyanobacteria, sulfate-reducing bacteria, green algae and diatoms) and their relative roles in the calcification processes.</p> </div> </div> Finally, the presence of extraordinary well-preserved fossil outcrops along with living microbialites gives a temporal dimension to this study, laying the foundation for the development of a new formation model. By applying the latter to other microbialites outcropping at different geographical and temporal scales, the microbial carbonates of Laguna de Los Cisnes can provide critical information to better reconstruct the dominant environmental conditions during the early evolution of life on Earth.</div>
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<p>Saline lakes are known to be sensitive to changes in environmental conditions on a broad temporal scale. Therefore, variations in the mineralogical, geochemical, and sedimentological characteristics of these settings have often been interpreted to reflect oscillations in climatic conditions. However, recent work has shown that microbial communities can also influence the formation of carbonate and evaporite minerals in saline lake environments, especially in the salars of South America. Here, both abiotic and organomineralization pathways can be found to exist within the same salar environments, indicating a high degree of spatial heterogeneity of mineralization processes in such settings. Thus, the drivers of the resulting mineral assemblage can be complicated to disentangle through space and time. A process-level understanding of first-order controls on mineral assemblages can provide new insights into sedimentological dynamics of salar environments.</p> <p>Babel (2004) published a conceptual model based on marine-fed systems that established links between salinity and the style of gypsum mineral deposition. Based on field and laboratory analyses conducted on sediments in the Salar de Llamara, we adapted this model for a continental saline lake setting (Reid et al., 2021). In the present study, we aimed to test whether our salar-scale conceptual model was applicable more generally to continental saline lake environments. To accomplish this goal, we investigated a 15-year time series of electrical conductivity, a proxy for salinity, collected in five saline lake/wetland systems situated along the margin of the Salar de Atacama. Based on this dataset, we predicted the style and mineralogy of mineral deposition in each setting using our salar-scale conceptual model. Next, we compared our predictions with published field descriptions of the occurrences of biofilms, microbial mats, microbialites, and evaporite deposits in these lakes. Through a principal component analysis, we evaluated environmental characteristics such as electrical conductivity, pH, and dissolved oxygen as controls on mineral morphology and mineralogy.</p> <p>Results indicate that salinity is a first-order control on sedimentological expression in the lakes of the Salar de Atacama, although the transition between organomineralization pathways and physicochemical precipitation may occur at different salinity values than observed in other saline lake settings. Broadly in agreement with our model from the Salar de Llamara, granular precipitates of carbonate minerals formed within microbial mats were associated with environments characterized by low salinity, while microbial mats with laminated precipitates were found in settings with moderate salinity in the Salar de Atacama. High salinity environments contained crystalline bottom types characterized by selenitic morphology. Because some South American salars have been cited as living laboratories analogous to the ancient conditions that fostered the evolution of terrestrial and Martian life, these insights into mineralization are important. Improved constraints on the controls of carbonate and evaporite mineral deposition in saline lake environments will elucidate the definition of habitable environments, and provide a testing ground for the production and preservation of chemical and morphological biosignatures through time.</p>
<p>Lake levels in hydrologically closed-basins are very sensitive to climatically and/or anthropogenically triggered environmental changes. Their record through time can provide valuable information to forecast changes that can have substantial economical and societal impact.</p><p>Increasing precipitation in eastern Patagonia (Argentina) have been documented following years with strong El Ni&#241;o (cold) events using historical and meteorological data. Quantifying changes in modern lake levels allow determining the impact of rainfall variations while contributing to anticipate the evolution of lacustrine systems over the next decades with expected fluctuations in ENSO frequencies. Laguna Carrilaufquen Grande is located in the intermontane Maquinchao Basin, Argentina. Its dimension fluctuates greatly, from 20 to 55 km<sup>2</sup> water surface area and an average water depth of 3 m. Several well-preserved gravelly beach ridges witness rainfall variations that can be compared to meteorological data and satellite images covering the last ~50 years. Our results show that in 2016 lake level was the lowest of the past 44 years whereas the maximum lake level was recorded in 1985 (+11.8 m above the current lake level) in a position 1.6 km to the east of the present shoreline. A five-years moving average rainfall record of the area was calculated smoothing the extreme annual events and correlated to the determined lake level fluctuations. The annual variation of lake levels was up to 1.2 m (e.g. 2014) whereas decadal variations related to humid-arid periods for the interval 2002 to 2016 were up to 9.4 m. These data are consistent with those from other monitored lakes and, thus, our approach opens up new perspectives to understand the historical water level fluctuations of lakes with non-available monitoring data.</p><p>&#160;</p><p>Laguna de los Cisnes in the Chilean section of the island of Tierra del Fuego, is a closed-lake presently divided into two sections of 2.2 and 11.9 km<sup>2</sup>, respectively. These two water bodies were united in the past forming a single larger lake. The lake level was&#160; ca. 4 m higher than today as shown by clear shorelines and the outcropping of large Ca-rich microbialites. Historical data, aerial photographs and satellite images indicate that the most recent changes in lake level are the result of a massive decrease of water input during the last half of the 20<sup>th</sup> century triggered by an indiscriminate use of the incoming water for agricultural purposes. The spectacular outcropping of living and fossil microbialites is not only interesting from a scientific point of view but has also initiated the development of the site as a local touristic attraction. However, if the use of the incoming water for agriculture in the catchment remains unregulated the lake water level might drop dangerously and eventually the lake might fully desiccate.</p><p>These two examples illustrate how recent changes in lake level can be used to anticipate the near future of lakes. They show that ongoing climate changes along with the growing demand of natural resources have already started to impact lacustrine systems and this is likely to increase in the decades to come.</p>
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