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Terminal lakes (without outflow) retain elements and compounds that reach them through fluvial, point source or atmospheric deposition. If the lake sediment is exposed, some of these chemicals could become toxic dust particulates. The Great Salt Lake (GSL) in Utah is a terminal lake that experienced record-low lake elevation in 2021-22, exposing vast areas of playa. Here, we used inductively coupled plasma mass spectrometry to analyze the environmental chemistry of GSL shallow sediment during historic lows in spring, summer, and fall of 2021. Contaminants at the subsurface interface are most able to influence diffusion into the water column and uptake by benthic biota. We focused our analysis on copper, thallium, arsenic, mercury, lead, and zinc, which have been historically deposited in this region and are toxic when at high concentrations. We compared records of regional mining activity to understand the current contamination and assess relevant spatial and temporal gradients. We also used two different extraction methods (EPA 3050b and NH4AcO at pH=7) that can distinguish "environmentally available" vs. tightly associated and less available fractions. We observed consistent concentration of copper across sites indicating a larger relative impact of atmospheric deposition, with some evidence indicating further impacts of point sources. Arsenic, on the other hand, is maintained at high levels in submerged sediments and is likely geologically-and fluvially- derived. Thallium and mercury fluctuate seasonally and correlate with lake elevation. Lead and zinc levels are relatively low in GSL sites compared with freshwater input sites, indicating the deep brine layer may sequester these heavy metals, preventing their release into the water column. Overall, the concentrations of most metals in GSL sediments have declined from historic highs. However, each contaminant has distinct sources, seasonality, mobility and transmission. Complete recovery (if possible) may require many more decades and individual remediation strategies. [ABSTRACT FROM AUTHOR]

Lacustrine carbonates are a powerful archive of paleoenvironmental information but are susceptible to post‐depositional alteration. Microbial metabolisms can drive such alteration by changing carbonate saturation in situ, thereby driving dissolution or precipitation. The net impact these microbial processes have on the primary δ18O, δ13C, and Δ47 values of lacustrine carbonate is not fully known. We studied the evolution of microbial community structure and the porewater and sediment geochemistry in the upper ~30 cm of sediment from two shoreline sites at Green Lake, Fayetteville, NY over 2 years of seasonal sampling. We linked seasonal and depth‐based changes of porewater carbonate chemistry to microbial community composition, in situ carbon cycling (using δ13C values of carbonate, dissolved inorganic carbon (DIC), and organic matter), and dominant allochems and facies. We interpret that microbial processes are a dominant control on carbon cycling within the sediment, affecting porewater DIC, aqueous carbon chemistry, and carbonate carbon and clumped isotope geochemistry. Across all seasons and sites, microbial organic matter remineralization lowers the δ13C of the porewater DIC. Elevated carbonate saturation states in the sediment porewaters (Ω > 3) were attributed to microbes from groups capable of sulfate reduction, which were abundant in the sediment below 5 cm depth. The nearshore carbonate sediments at Green Lake are mainly composed of microbialite intraclasts/oncoids, charophytes, larger calcite crystals, and authigenic micrite—each with a different origin. Authigenic micrite is interpreted to have precipitated in situ from the supersaturated porewaters from microbial metabolism. The stable carbon isotope values (δ13Ccarb) and clumped isotope values (Δ47) of bulk carbonate sediments from the same depth horizons and site varied depending on both the sampling season and the specific location within a site, indicating localized (μm to mm) controls on carbon and clumped isotope values. Our results suggest that biological processes are a dominant control on carbon chemistry within the sedimentary subsurface of the shorelines of Green Lake, from actively forming microbialites to pore space organic matter remineralization and micrite authigenesis. A combination of biological activity, hydrologic balance, and allochem composition of the sediments set the stable carbon, oxygen, and clumped isotope signals preserved by the Green Lake carbonate sediments. [ABSTRACT FROM AUTHOR]

Stromatolites have often served as a diagnostic carbonate facies for deep‐time palaeoclimatic and geobiological studies because they may form under favourable environmental conditions for microbially mediated carbonate production. 'Giant' (<5 m) stromatolites occur in the Laney Member of the Eocene Green River Formation in the Vermillion Creek section of the Sand Wash Basin (north‐west Colorado, USA). Giant stromatolite growth was hypothesized to have been promoted by both availability of large substrates as nucleation sites and physicochemical factors, including increased calcium carbonate mineral saturation states due to the generally warmer Eocene climate and a dynamic period in Lake Gosiute's hydrological balance. Depositional horizons of giant stromatolites were observed at two distinct stratigraphic levels, which demonstrated that the formation of the giant morphotype was not a unique occurrence, and provided an opportunity to examine both onset and cessation of stromatolite development. Coincident increases in carbonate clumped isotope‐derived temperatures, the carbon and oxygen isotopic compositions of lake water (δ18Ow), carbonate mineral saturation states (Ω) estimated by ooid size, and salinity estimated by ostracod assemblages demonstrated that formation of giant stromatolites was facilitated by lake level drawdown. Decreased lake levels: (i) promoted carbonate precipitation; and (ii) positioned benthic microbial communities within the photic zone. Field and petrographic analyses revealed that the giants preserved micritic laminations, 'trapped and bound' grains, and aragonite fans, interpreted as reflecting contributions from both microbially mediated and abiotic carbonate mineral precipitation. Field and microscopic sedimentological and stable isotope data indicated that giant stromatolite growth ceased as a result of subaerial exposure of mounds during lake level lows. Although microbial mediation of carbonate chemistry was seemingly important for initiation of stromatolite growth, this work demonstrated that stromatolite macromorphology was dominantly controlled by availability of large substrates, lake level and resultant solute chemistry, i.e. increased Ω. [ABSTRACT FROM AUTHOR]

Organic and inorganic stable isotopes of lacustrine carbonate sediments are commonly used in reconstructions of ancient terrestrial ecosystems and environments. Microbial activity and local hydrological inputs can alter porewater chemistry (e.g., pH, alkalinity) and isotopic composition (e.g., δ18Owater, δ13CDIC), which in turn has the potential to impact the stable isotopic compositions recorded and preserved in lithified carbonate. The fingerprint these syngenetic processes have on lacustrine carbonate facies is yet unknown, however, and thus, reconstructions based on stable isotopes may misinterpret diagenetic records as broader climate signals. Here, we characterize geochemical and stable isotopic variability of carbonate minerals, organic matter, and water within one modern lake that has known microbial influences (e.g., microbial mats and microbialite carbonate) and combine these data with the context provided by 16S rRNA amplicon sequencing community profiles. Specifically, we measure oxygen, carbon, and clumped isotopic compositions of carbonate sediments (δ18Ocarb, δ13Ccarb, ∆47), as well as carbon isotopic compositions of bulk organic matter (δ13Corg) and dissolved inorganic carbon (DIC; δ13CDIC) of lake and porewater in Great Salt Lake, Utah from five sites and three seasons. We find that facies equivalent to ooid grainstones provide time‐averaged records of lake chemistry that reflect minimal alteration by microbial activity, whereas microbialite, intraclasts, and carbonate mud show greater alteration by local microbial influence and hydrology. Further, we find at least one occurrence of ∆47 isotopic disequilibrium likely driven by local microbial metabolism during authigenic carbonate precipitation. The remainder of the carbonate materials (primarily ooids, grain coatings, mud, and intraclasts) yield clumped isotope temperatures (T(∆47)), δ18Ocarb, and calculated δ18Owater in isotopic equilibrium with ambient water and temperature at the time and site of carbonate precipitation. Our findings suggest that it is possible and necessary to leverage diverse carbonate facies across one sedimentary horizon to reconstruct regional hydroclimate and evaporation–precipitation balance, as well as identify microbially mediated carbonate formation. [ABSTRACT FROM AUTHOR]

Field investigations of the properties of heavily melted "rotten" Arctic sea ice were carried out on shorefast and drifting ice off the coast of Utqiaġvik (formerly Barrow), Alaska, during the melt season. While no formal criteria exist to qualify when ice becomes rotten, the objective of this study was to sample melting ice at the point at which its structural and optical properties are sufficiently advanced beyond the peak of the summer season. Baseline data on the physical (temperature, salinity, density, microstructure) and optical (light scattering) properties of shorefast ice were recorded in May and June 2015. In July of both 2015 and 2017, small boats were used to access drifting rotten ice within ∼32 km of Utqiaġvik. Measurements showed that pore space increased as ice temperature increased (-8 to 0 ∘ C), ice salinity decreased (10 to 0 ppt), and bulk density decreased (0.9 to 0.6 g cm -3). Changes in pore space were characterized with thin-section microphotography and X-ray micro-computed tomography in the laboratory. These analyses yielded changes in average brine inclusion number density (which decreased from 32 to 0.01 mm -3), mean pore size (which increased from 80 µ m to 3 mm), and total porosity (increased from 0 % to > 45 %) and structural anisotropy (variable, with values of generally less than 0.7). Additionally, light-scattering coefficients of the ice increased from approximately 0.06 to > 0.35 cm -1 as the ice melt progressed. Together, these findings indicate that the properties of Arctic sea ice at the end of melt season are significantly distinct from those of often-studied summertime ice. If such rotten ice were to become more prevalent in a warmer Arctic with longer melt seasons, this could have implications for the exchange of fluid and heat at the ocean surface. [ABSTRACT FROM AUTHOR]

The article discusses a study which describes the giant stromatolites that occur in the Green River Formation deposited in alkaline lakes that covered much of Wyoming, Colorado and Utah for several million years in the Eocene. Topics covered include a description of stromatolites and other microbialites from a paleontologic and astrobiologic perspective, the different properties of stromatolites that are critical to oil recovery, and stromatolites as paleoenvironmental records.