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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.
Conodont apatite δ18OV-SMOW values from Middle though Upper Pennsylvanian (Desmoinesian–Missourian) laminated, marine black shale units within cyclic deposits of intercalated terrestrial and marine strata (cyclothems) from the Illinois Basin (United States) were measured in order to evaluate their utility as a proxy for changes in the oxygen isotopic composition of the epicontinental Late Pennsylvanian Midcontinent Sea (LPMS). The average δ18OV-SMOW values of well-preserved monogeneric (Idiognathodus) separates of conodont apatite from 12 lithologic units representing nine cyclothems range from 17.0‰ to 20.1‰ and average 19.0‰ ± 0.4‰ (1σ). Within the limits of analytical uncertainty of stable isotope measurements, the stratigraphic distribution of conodont apatite δ18O values is nontrending; particularly, there is no significant shift in δ18O values across the Desmoinesian–Missourian boundary, a period that has been interpreted to preserve a shift toward a warmer climate, increased seasonality, and shorter periods of wet conditions in the terrestrial record. Conodont apatite δ18O values from stratigraphically equivalent black shale members across the Illinois Basin vary up to 2.6‰, which is nearly equivalent to the observed stratigraphic range of conodont apatite δ18O values, and suggests differences in local (basin-scale) seawater δ18O values affected the conodont apatite δ18O values. Within analytical uncertainty, conodont apatite δ18O values from the Illinois Basin and Midcontinent Basin (United States) are indistinguishable, suggesting a component of overarching broader regional to global controls on seawater δ18O values. Nevertheless, if the large variability observed in stratigraphically equivalent black shale members in the Illinois Basin is attributed to regional factors, these results indicate caution should be used when attempting to interpret temporal shifts from single aliquots of conodonts in epicontinental settings.
Changes in the stratigraphic distribution of manganese (Mn)-bearing columnar structures interpreted as rhizocretions indicate a shift in paleohydrological conditions in the Bighorn Basin during the Paleocene–Eocene Thermal Maximum (PETM). While most studies agree that significant warming occurred during the PETM, interpretations differ as to the effects of warming on the paleohydrologic regime. The columns consist of sand and silt cemented by calcite that also contains black, Mn-bearing nodules. The rhizocretions are typically vertically oriented and range from a few cm to over 50 cm in diameter and up to 75 cm tall. They are interpreted to have formed in the rhizospheres of relatively large root systems. The rhizocretions are restricted to intervals, showing only weak paleopedogenic development, that are interpreted as crevasse-splay or avulsion deposits. While such deposits are present throughout the study section at Polecat Bench in the northern Bighorn Basin, Wyoming, the Mn-bearing rhizocretions are found only below and above the PETM interval and within the initial carbon isotope excursion (CIE) marking the onset of the PETM. The abundance of black, Mn-rich nodules (< 1 cm in diameter) in the rhizocretions and presence of other pedogenic features indicate that the soils in which the plants were growing underwent seasonal flooding and repeated redox alternations. Their distribution in similar host deposits throughout the study interval, except for the main body of the PETM, supports climate as the driving factor in preservation of the Mn-bearing rhizocretions. The Mn-bearing rhizocretions support recent studies that suggest transient drying in northern Wyoming during the PETM.
Previous studies of the sequence stratigraphic distribution of fossils have focused on the record of relatively abundant marine invertebrates. Only a handful of studies have examined how sequence stratigraphic architecture influences the occurrence of vertebrates, particularly large and rare tetrapods. The Jurassic Sundance Formation of the Bighorn Basin, Wyoming, USA, contains a rich suite of invertebrate and vertebrate fossils, including large and rare marine reptiles, and this allows the sequence stratigraphic controls on the distribution of these groups to be compared. The Sundance Formation consists of four depositional sequences, with the lower two being carbonate dominated and the upper two siliciclastic dominated. Two incised valley fills are also present. The presence of multiple depositional sequences and strongly erosional sequence boundaries is the likely cause of the complicated lithostratigraphic nomenclature of the Sundance. Invertebrates (mollusks and echinoderms) in the Sundance conform to well-established patterns of occurrences, including strong facies control and fossil concentrations at maximum flooding surfaces, in the upper portion of parasequences, and within lags overlying sequence boundaries. As expected from their rarity, marine reptiles (ichthyosaurs, plesiosaurs, and pliosaurs) show a weaker connection to sequence stratigraphic architecture. Nonetheless, they do display facies control and are found primarily in offshore mudstone, rather than shoreface and estuarine sandstone. They are also more common at hiatal surfaces, including a zone of concretions at the maximum flooding surface and in lag deposits overlying sequence boundaries. These associations suggest that sequence stratigraphic architecture may be a useful approach for discovery of marine vertebrates and that sequence stratigraphic context should be considered when making paleobiological interpretations of marine vertebrates as well as invertebrates.
Small shelly fossils are preserved as apatite steinkerns in the Cambrian Series 2–3 Thorntonia Limestone, Australia. Petrological observations indicate that phosphorus delivered to Thorntonia sediment was remobilized before precipitating in microenvironments defined by the matrix-filled interiors of small, mostly conical skeletons. A previous geochemical study concluded that both organic matter and iron oxides sourced phosphorus to Thorntonia sediments, and that anoxia governed phosphorus remobilization within the sediment column. This contribution asks: What factors allowed for the selective preservation of skeleton interiors, and what biases result from this preservation? We find that small shells physically trapped phosphorus-laden pore waters, creating local conditions where kinetic barriers to apatite precipitation could be overcome. Only a subset of Thorntonia Limestone skeletons is preserved by apatite, showing evidence of selectivity with respect to shell size, shape, and orientation. Both the biological and physical factors that govern phosphorus remineralization and precipitation have changed through time, accounting for the opening and closing of the Ediacaran-Cambrian phosphatization taphonomic window. The opening of this window may have required a global increase in phosphate delivery to the oceans.
Burgess Shale–type deposits represent exceptional preservational windows for examining the biodiversity and ecological structure of some of the earliest metazoan communities that evolved during the Cambrian Explosion. While much attention has been paid to the original Burgess Shale locality, the Walcott Quarry on Fossil Ridge, temporal and regional variations of the depositional environment of the Burgess Shale biota as a whole are still poorly understood. Here we present the first comprehensive taphonomic and sedimentological study of the Tulip Beds on Mount Stephen (Campsite Cliff Shale Member, Burgess Shale Formation), based on a time-averaged assemblage of nearly 10,000 specimens. The taphonomic characteristics—size sorting, resistance to decay, and potential flow alignment—and mode of deposition of this assemblage are compared specifically to those of the nearby and stratigraphically younger Walcott Quarry assemblage. Like other Burgess Shale–type deposits, the Tulip Beds consist of millimeter-laminated, event-derived claystone, but lack the thicker claystone layers and prominent carbonate interbeds that occur in the Walcott Quarry. These differences suggest a depositional environment lower in energy and possibly more distal to the Cathedral Escarpment. Overall, taphonomic analyses suggest no significant decay biases, transport, or sorting of the assemblage, and most specimens, benthic taxa in particular, appear to have been buried close to their living environments. Single bedding planes with large accumulations dominated by a single taxon, e.g., isolated claws of Anomalocaris, suggest short time-averaged assemblages with limited background sedimentation. Overall the Tulip Beds locality is environmentally and taphonomically comparable to the Walcott Quarry and biotic variations between the two sites are likely to be primary in nature, thus paving the way for more detailed paleoecological investigations in the future.