Microbialites comprise the mineralized record of early life on Earth and preserve a spectrum of fabrics that reflect complex physical, chemical, and biological interactions. The relatively rarity of microbialites in modern environments, however, challenges our interpretation of ancient structures. Here we report the occurrence of microbial mats, mineral precipitates, and oncoids in the Laguna Negra, a high-altitude hypersaline Andean lake in Catamarca Province, Argentina. Laguna Negra is a Ca-Na-Cl brine where abundant carbonate precipitation takes place. Extreme environmental conditions, including high UV radiation, elevated salinity, and temperature extremes, restrict multicellular life so that mineralization reflects a combination of local hydrologic conditions, lake geochemistry, and microbial activity. The resulting carbonates consist of micritic laminae, botryoidal cement fans, and isopachous cement laminae that are strikingly similar to those observed in Proterozoic stromatolites, providing insight into mechanisms of mineralization. Here, increased saturation with respect to carbonate minerals reflects mixing of spring-fed inlets and lake waters, favoring microbialite formation and preservation. This highlights the importance of hydrological mixing zones in microbialite formation and as taphonomic windows to record microbial activity. Recent discoveries of minerals related to evaporating playa-lake systems on Mars further highlights the potential of Laguna Negra to provide critical insight into biosignature preservation in both terrestrial and extraterrestrial settings.

The post-2.0 Ga Proterozoic C isotope record reveals two distinct, yet interrelated trends: a stepwise increase in average ␦ 13 C from ϳ0‰ (calculated with respect to the Peedee belemnite isotope standard) prior to ca. 1.3 Ga to Ͼ؉5‰ in the Neoproterozoic, and a concomitant increase in the magnitude of isotopic excursions. Steady-state and nonsteady-state models suggest that these fundamental changes are best explained by a combination of evolving burial fluxes and a secular decrease in the size of the marine dissolved inorganic carbon (DIC) reservoir. The DIC reservoir size affects the sensitivity of the isotopic system to biogeochemical perturbation. Major rearrangements of carbon cycling during the Proterozoic, in part related to the evolving marine carbon reservoir, permit elevated ␦ 13 C values to be sustained for geologically long time spans. Recognition of this dependence on DIC reservoir size provides, for the first time, a direct link between changing carbonate precipitation styles and the marine C isotope record and may help constrain estimates of Proterozoic pCO 2 .

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doi: 10.7185/geochemlet.1618Chromium (Cr) isotopes in marine sedimentary rocks can be used as a sensitive proxy for ancient atmospheric oxygen because Cr-isotope fractionation during terrestrial weathering only occurs when pO 2 exceeds a threshold value. This is a useful system when applied to rocks of mid-Proterozoic age, where fundamental questions persist about atmospheric pO 2 and its relationship to biological innovation. Whereas previous studies have focused on temporally limited iron-rich sedimentary rocks, we present new Cr-isotope data from a suite of mid-Proterozoic marine carbonate rocks. Application of the Cr-isotope proxy to carbonate rocks has the potential to greatly enhance the temporal resolution of Proterozoic palaeoredox data. Here we report positive δ 53 Cr values in four carbonate successions, extending the mid-Proterozoic record of Cr-isotope fractionation -and thus pO 2 above threshold values -back to ~1.1 Ga. These data suggest that pO 2 sufficient for the origin of animals was transiently in place well before their Neoproterozoic appearance, although uncertainty in the pO 2 threshold required for Cr-isotope fractionation precludes definitive biological interpretation. This study provides a proof of concept that the Cr-isotopic composition of carbonate rocks can provide important new constraints on the oxygen content of the ancient atmosphere.

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Molar-tooth (MT) is an enigmatic carbonate fabric composed of variously shaped cracks and voids filled with a characteristically uniform, equant microspar. MT is both abundant and widespread in Mesoproterozoic and early Neoproterozoic strata, where void-filling microspar comprises up to 90% of individual beds and 5-25% of preserved carbonate. The temporal restriction of this fabric suggests a potential link between MT formation and the biogeochemical evolution of marine environments. Detailed petrographic relationships among MT crack morphology, distribution of MT microspar, and composition of the surrounding substrate suggest that crack formation and microspar precipitation are intimately linked to the decomposition of sedimentary organic matter in the presence of supersaturated Proterozoic seawater. Laboratory experiments have shown that gas generated within unconsolidated mud can reproduce a variety of MT crack morphologies, yet current gas expansion and migration models do not explicitly consider the role of substrate variability in determining morphologies of MT cracks. A detailed petrographic examination of MT structures from the Mesoproterozoic Belt Supergroup, Montana, permits interpretation of the microscale relationship between crack morphology and lithologic, and potentially rheologic, variability of the surrounding substrate by tracing the distribution of petrographically distinctive MT microspar. Observations of lateral offset of MT cracks at bedding planes or within coarser-grained siltstone or sandstone layers, termination of cracks beneath clay-or organic-rich horizons, grain collapse into underlying MT cracks, and the presence of MT microspar as a pore-filling precipitate suggest that grain size, substrate lithology, and substrate cohesion all play critical roles in the development of MT cracks. By contrast, the presence of a wide range of MT crack morphologies within petrographically homogeneous substrates, and an apparent relationship between crack diameter and sinuosity, suggest that the void-forming process itself also played a role in determining the final morphology of MT cracks. Together, these petrographic observations are used to define a model of microscale gas-sediment interactions that can be used to interpret crack morphology in terms of gas pressure and the strength of sedimentary substrates. The presence of characteristic, void-filling microspar is integral to preservation of MT structures. Cathodoluminescence (CL) identification of this characteristic microspar within MT voids, in pore space of coarse-grained facies, and interstitially within fine-grained facies adjacent to MT voids suggests that MT voids and cement share a common genesis. Because microspar cores are similar in size and morphology to vaterite precipitated experimentally in the presence of a variety of dissolved organic molecules, we suggest that precipitation of MT microspar was intimately linked with gas production during organic decomposition within the host substrate. In this scenario, gas production would resu...

Siberia contains several key reference sections for studies of biological and environmental evolution across the Proterozoic-Phanerozoic transition. The Platonovskaya Formation, exposed in the Turukhansk region of western Siberia, is an uppermost Proterozoic to Cambrian succession whose trace and body fossils place broad limits on the age of deposition, but do not permit detailed correlation with boundary successions elsewhere. In contrast, a striking negative carbon isotopic excursion in the lower part of the Platonovskaya Formation permits precise chemostratigraphic correlation with uppermost Yudomian successions in Siberia, and possibly worldwide. In addition to providing a tool for correlation, the isotopic excursion preserved in the Platonovskaya and contemporaneous successions documents a major biogeochemical event, likely involving the world ocean. The excursion coincides with the palaeontological breakpoint between Ediacaran-and Cambrian-style assemblages, suggesting a role for biogeochemical change in evolutionary events near the Proterozoic-Cambrian boundary.

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