There is a paucity of records on decadal to millennial scales archiving the evolution of the Antarctic subglacial hydrologic system, leaving a gap in our knowledge of the links between meltwater drainage and ice behavior. We employed a suite of sedimentological and geochemical methods to assess the stratigraphic distribution of lithofacies and evaluate sedimentary processes from sediment cores collected from Mercer Subglacial Lake, located beneath the Mercer Ice Stream in West Antarctica. The composite 2.06 m sediment record consisted of massive-to-stratified diamict, massive muds, and laminated muds. Chloride concentrations indicate sediment porewater was primarily derived from glacial melt with sediment deposition in freshwater conditions. Whereas diamicts are associated with basal ice contact during ice stream grounding events (tills), sorted mud beds buried below diamict lack coarse-grained detritus (>2 mm) indicating deposition from suspension settling in slowly flowing or ponded meltwater. Rhythmically laminated sediments capping the sedimentary sequence capture modern subglacial lake conditions and are likely influenced by a continuum of processes controlled by suspended sediment delivered into the lake, water column velocity changes associated with fill-drain cycles, and sediment fallout from basal ice melt. These sedimentary facies characterize a complex subglacial hydrologic system providing evidence that basal conditions alternated from grounded ice to water-filled cavities fed and drained by subglacial meltwater. Our data provide new information on subglacial sediments beneath an Antarctic ice stream that can be used to refine our knowledge of subglacial hydrology, its coupling with ice dynamics, and as an analog for studying ancient glacial deposits.

Ice streams that flow into Ross Ice Shelf are underlain by water-saturated sediments, a dynamic hydrological system, and subglacial lakes that intermittently discharge water downstream across grounding zones of West Antarctic Ice Sheet (WAIS). A 2.06 m composite sediment profile was recently recovered from Mercer Subglacial Lake, a 15 m deep water cavity beneath a 1087 m thick portion of the Mercer Ice Stream. We examined microbial abundances, used 16S rRNA gene amplicon sequencing to assess community structures, and characterized extracellular polymeric substances (EPS) associated with distinct lithologic units in the sediments. Bacterial and archaeal communities in the surficial sediments are more abundant and diverse, with significantly different compositions from those found deeper in the sediment column. The most abundant taxa are related to chemolithoautotrophs capable of oxidizing reduced nitrogen, sulfur, and iron compounds with oxygen, nitrate, or iron. Concentrations of dissolved methane and total organic carbon together with water content in the sediments are the strongest predictors of taxon and community composition. δ¹³C values for EPS (−25 to −30‰) are consistent with the primary source of carbon for biosynthesis originating from legacy marine organic matter. Comparison of communities to those in lake sediments under an adjacent ice stream (Whillans Subglacial Lake) and near its grounding zone provide seminal evidence for a subglacial metacommunity that is biogeochemically and evolutionarily linked through ice sheet dynamics and the transport of microbes, water, and sediments beneath WAIS.

Trace elements sustain biological productivity, yet the significance of trace element mobilization and export in subglacial runoff from ice sheets is poorly constrained at present. Here, we present size-fractionated (0.02, 0.22, and 0.45 µm) concentrations of trace elements in subglacial waters from the Greenland Ice Sheet (GrIS) and the Antarctic Ice Sheet (AIS). Concentrations of immobile trace elements (e.g., Al, Fe, Ti) far exceed global riverine and open ocean mean values and highlight the importance of subglacial aluminosilicate mineral weathering and lack of retention of these species in sediments. Concentrations are higher from the AIS than the GrIS, highlighting the geochemical consequences of prolonged water residence times and hydrological isolation that characterize the former. The enrichment of trace elements (e.g., Co, Fe, Mn, and Zn) in subglacial meltwaters compared with seawater and typical riverine systems, together with the likely sensitivity to future ice sheet melting, suggests that their export in glacial runoff is likely to be important for biological productivity. For example, our dissolved Fe concentration (20,900 nM) and associated flux values (1.4 Gmol y-1) from AIS to the Fe-deplete Southern Ocean exceed most previous estimates by an order of magnitude. The ultimate fate of these micronutrients will depend on the reactivity of the dominant colloidal size fraction (likely controlled by nanoparticulate Al and Fe oxyhydroxide minerals) and estuarine processing. We contend that ice sheets create highly geochemically reactive particulates in subglacial environments, which play a key role in trace elemental cycles, with potentially important consequences for global carbon cycling.

Human-dominated waterways contain thousands of chemicals. Determining which chemical is the most important stressor is important, yet very challenging. The Toxicity Identification Evaluation (TIE) procedure from the US Environmental Protection Agency uses a series of chemical and physical manipulations to fractionate compounds within a matrix and systematically identify potential toxicants through laboratory bioassay testing. Although this may provide useful information, it lacks ecological realism because it is subject to laboratory-related artifacts and is resource intensive. The in situ Toxicity Identification Evaluation (iTIE) technology was developed to improve this approach and has undergone a number of modifications over the past several years. The novel prototype 3 consists of an array of iTIE ambient water fractionation units. Each unit is connected to a peristaltic pumping system with an organism exposure chamber that receives water from a resin chamber to chemically fractionate test site water. Test organisms included freshwater and marine standard toxicity test species. Postfractionation waters are collected for subsequent chemical analyses. Currently, the resins allow for separation of ammonia, metals, and nonpolar organics; the subsequent toxicity responses are compared between treatments and unfractionated, ambient exposures. The iTIE system was deployed to a depth of 3 m and evaluated in streams and marine harbors. Chemical analyses of water and iTIE chemical sorptive resins confirmed chemical groups causing lethal to sublethal responses. The system proved to be as sensitive or more so than the traditional phase 1 TIE test and required almost half of the resources to complete. This iTIE prototype provides a robust technology that improves stressor-causality linkages and thereby supports strong evidence for ecological risk weight-of-evidence assessments. Environ Toxicol Chem 2020;39:1746-1754. © 2020 SETAC.

Aerobic methane production in aquatic ecosystems impacts the global atmospheric budget of methane, but the extent, mechanism, and taxa responsible for producing this greenhouse gas are not fully understood. Lake Bonney (LB), a perennially ice‐covered Antarctic lake, has cold hypersaline waters underlying an oxygenated freshwater layer. We present temporal methane concentration profiles in LB indicating methane production in the oxygenated (> 200% air saturation) water. Experiments amended with methylphosphonate (MPn) yielded methane generation, suggesting in situ methanogenesis via the carbon‐phosphorus (C‐P) lyase pathway. Enrichment cultures from the lake were used to isolate five bacterial strains capable of generating methane when supplied with MPn as the sole P source. Based on 16S rRNA gene sequencing, the isolates belong to the Proteobacteria (closely related to Marinomonas, Hoeflea, and Marinobacter genera) and Bacteroidetes (Algoriphagus genus). 16S rRNA metagenomic sequencing confirms the presence of these taxa in LB. None of the isolated species were reported to be capable to produce methane. In addition, orthologs of the phosphoenolpyruvate mutase gene (PepM) and methylphosphonate synthase (MPnS), enzymes involved in phosphonate and MPn biosynthesis, were widely spread in the LB shotgun metagenomic libraries; genes related to C‐P lyase pathways (phn gene clusters) were also abundant. 16S rRNA and mcrA genes of anaerobic methanogens were absent in both 16S rRNA and metagenomics libraries. These data reveal that in situ aerobic biological methane production is likely a significant source of methane in LB.

It is difficult to assess the toxicity of a single stressor and establish a strong stressor-causality link when multiple stressors coexist. Toxicity identification evaluation (TIE) methodology uses a series of chemical and physical manipulations to fractionate compounds within a matrix and systematically identify potential toxicants. The current US Environmental Protection Agency application of TIE can provide valuable information but often lacks ecological realism and is subject to laboratory-related artifacts. An in situ TIE device (iTIED) was designed to assess the sources of toxicity in aquatic ecosystems. For this laboratory validation, each unit was equipped with a sorbent resin chamber, an organism exposure chamber, a water collection container, and a peristaltic pump. Chemical analyses of water processed by each iTIED unit were compared with both lethal and sublethal molecular responses of the organisms. The compound removal effectiveness of different sorbent resins was also compared. In addition to successfully fractionating diverse chemical mixtures, the iTIED demonstrated a potential for early detection of molecular biomarkers, which could identify chronic toxicity that may go unnoticed in traditional TIE assays. Utilizing this novel in situ system will reduce the uncertainty associated with laboratory-based simulations and aid management efforts in targeting compounds that pose the greatest threat. (C) 2016 SETAC

Mineral dust is transported in the atmosphere and deposited in oceans, ice sheets and the terrestrial biosphere. Temporal changes in locations of dust source areas and transport pathways have implications for global climate and biogeochemical cycles. The chemical and physical characterization of the dust record preserved in ice cores is useful for identifying of dust source regions, dust transport, dominant wind direction and storm trajectories. Here, we present a 50,000-year geochemical characterization of mineral dust entrapped in a horizontal ice core from the Taylor Glacier in East Antarctica. Strontium (Sr) and neodymium (Nd) isotopes, grain size distribution, trace and rare earth element (REE) concentrations, and inorganic ion (Cl- and Na+) concentrations were measured in 38 samples, corresponding to a time interval from similar to 46 kyr before present (BP) to present. The Sr and Nd isotope compositions of insoluble dust in the Taylor Glacier ice shows distinct changes between the Last Glacial Period (LGP in this study ranging from similar to 46.7-15.3 kyr BP) the early Holocene (in this study ranging from 14.5-8.7 kyr BP), and zero-age samples. The 87Sr/86Sr isotopic composition of dust in the Taylor Glacier ice ranged from 0.708 to 0.711 during the LGP, while the variability during the early Holocene is higher ranging from 0.707 to 0.714. The eNd composition ranges from 0.1 to 3.9 during the LGP, and is more variable from 1.9 to -8.2 during the early Holocene. The increased isotopic variability during the early Holocene suggests a shift in dust provenance coinciding with the major climate transition from the LGP to the Holocene. The isotopic composition and multiple physical and chemical constraints support previous work attributing Southern South America (SSA) as the main dust source to East Antarctica during the LGP, and a combination of both local Ross Sea Sector dust sources and SSA after the transition into the Holocene. This study provides the first high time resolution data showing variations in dust provenance to East Antarctic ice during a major climate regime shift, and we provide evidence of changes in the atmospheric transport pathways of dust following the last deglaciation. (C) 2017 Elsevier Ltd. All rights reserved.