Sediment supply controls equilibrium channel geometry in gravel rivers.

In many gravel-bedded rivers, floods that fill the channel banks create just enough shear stress to move the median-sized gravel particles on the bed surface (D50). Because this observation is common and is supported by theory, the coincidence of bankfull flow and the incipient motion of D50 has become a commonly used assumption. However, not all natural gravel channels actually conform to this simple relationship; some channels maintain bankfull stresses far in excess of the critical stress required to initiate sediment transport. We use a database of >300 gravel-bedded rivers and >600 10Be-derived erosion rates from across North America to explore the hypothesis that sediment supply drives the magnitude of bankfull shear stress relative to the critical stress required to mobilize the median bed surface grain size ([Formula: see text]). We find that [Formula: see text] is significantly higher in West Coast river reaches (2.35, n = 96) than in river reaches elsewhere on the continent (1.03, n = 245). This pattern parallels patterns in erosion rates (and hence sediment supplies). Supporting our hypothesis, we find a significant correlation between upstream erosion rate and local [Formula: see text] at sites where this comparison is possible. Our analysis reveals a decrease in bed surface armoring with increasing [Formula: see text], suggesting channels accommodate changes in sediment supply through adjustments in bed surface grain size, as also shown through numerical modeling. Our findings demonstrate that sediment supply is encoded in the bankfull hydraulic geometry of gravel bedded channels through its control on bed surface grain size.

Long-term stability of global erosion rates and weathering during late-Cenozoic cooling.

Over geologic timescales, CO(2) is emitted from the Earth's interior and is removed from the atmosphere by silicate rock weathering and organic carbon burial. This balance is thought to have stabilized greenhouse conditions within a range that ensured habitable conditions. Changes in this balance have been attributed to changes in topographic relief, where varying rates of continental rock weathering and erosion are superimposed on fluctuations in organic carbon burial. Geological strata provide an indirect yet imperfectly preserved record of this change through changing rates of sedimentation. Widespread observations of a recent (0-5-Myr) fourfold increase in global sedimentation rates require a global mechanism to explain them. Accelerated uplift and global cooling have been given as possible causes, but because of the links between rates of erosion and the correlated rate of weathering, an increase in the drawdown of CO(2) that is predicted to follow may be the cause of global climate change instead. However, globally, rates of uplift cannot increase everywhere in the way that apparent sedimentation rates do. Moreover, proxy records of past atmospheric CO(2) provide no evidence for this large reduction in recent CO(2) concentrations. Here we question whether this increase in global weathering and erosion actually occurred and whether the apparent increase in the sedimentation rate is due to observational biases in the sedimentary record. As evidence, we recast the ocean dissolved (10)Be/(9)Be isotope system as a weathering proxy spanning the past approximately 12 Myr (ref. 14). This proxy indicates stable weathering fluxes during the late-Cenozoic era. The sum of these observations shows neither clear evidence for increased erosion nor clear evidence for a pulse in weathered material to the ocean. We conclude that processes different from an increase in denudation caused Cenozoic global cooling, and that global cooling had no profound effect on spatially and temporally averaged weathering rates.

Non-native species change the tune of tundra soils: Novel access to soundscapes of the Arctic earthworm invasion.

Over the last decade, an increasing number of studies have used soundscapes to address diverse ecological questions. Sound represents one of the few sources of information capable of providing in situ insights into processes occurring within opaque soil matrices. To date, the use of soundscapes for soil macrofauna monitoring has been experimentally tested only in controlled laboratory environments. Here we assess the validity of laboratory predictions and explore the use of soil soundscape proxies for monitoring soil macrofauna (i.e., earthworm) activities in an outdoor context. In a common garden experiment in northern Sweden, we constructed outdoor mesocosm plots (N = 36) containing two different Arctic vegetation types (meadow and heath) and introduced earthworms to half of these plots. Earthworms substantially altered the ambient soil soundscape under both vegetation types, as measured by both traditional soundscape indices and frequency band power levels, although their acoustic impacts were expressed differently in heath versus meadow soils. While these findings support the as-of-yet untapped promise of using belowground soundscape analyses to monitor soil ecosystem health, direct acoustic emissions from earthworm activities appear to be an unlikely proxy for tracking worm activities at daily timescales. Instead, earthworms indirectly altered the soil soundscape by 're-engineering' the soil matrix: an effect that was dependent on vegetation type. Our findings suggest that long-term (i.e., seasonal) earthworm activities in natural soil settings can likely be monitored indirectly via their impacts on soundscape measures and acoustic indices. Analyzing soil soundscapes may enable larger-scale monitoring of high-latitude soils and is directly applicable to the specific case of earthworm invasions within Arctic soils, which has recently been identified as a potential threat to the resilience of high-latitude ecosystems. Soil soundscapes could also offer a novel means to monitor soils and soil-plant-faunal interactions in situ across diverse pedogenic, agronomic, and ecological systems.

Large shift in source of fine sediment in the upper Mississippi river.

Although sediment is a natural constituent of rivers, excess loading to rivers and streams is a leading cause of impairment and biodiversity loss. Remedial actions require identification of the sources and mechanisms of sediment supply. This task is complicated by the scale and complexity of large watersheds as well as changes in climate and land use that alter the drivers of sediment supply. Previous studies in Lake Pepin, a natural lake on the Mississippi River, indicate that sediment supply to the lake has increased 10-fold over the past 150 years. Herein we combine geochemical fingerprinting and a suite of geomorphic change detection techniques with a sediment mass balance for a tributary watershed to demonstrate that, although the sediment loading remains very large, the dominant source of sediment has shifted from agricultural soil erosion to accelerated erosion of stream banks and bluffs, driven by increased river discharge. Such hydrologic amplification of natural erosion processes calls for a new approach to watershed sediment modeling that explicitly accounts for channel and floodplain dynamics that amplify or dampen landscape processes. Further, this finding illustrates a new challenge in remediating nonpoint sediment pollution and indicates that management efforts must expand from soil erosion to factors contributing to increased water runoff.

Mid-Miocene cooling and the extinction of tundra in continental Antarctica.

A major obstacle in understanding the evolution of Cenozoic climate has been the lack of well dated terrestrial evidence from high-latitude, glaciated regions. Here, we report the discovery of exceptionally well preserved fossils of lacustrine and terrestrial organisms from the McMurdo Dry Valleys sector of the Transantarctic Mountains for which we have established a precise radiometric chronology. The fossils, which include diatoms, palynomorphs, mosses, ostracodes, and insects, represent the last vestige of a tundra community that inhabited the mountains before stepped cooling that first brought a full polar climate to Antarctica. Paleoecological analyses, (40)Ar/(39)Ar analyses of associated ash fall, and climate inferences from glaciological modeling together suggest that mean summer temperatures in the region cooled by at least 8 degrees C between 14.07 +/- 0.05 Ma and 13.85 +/- 0.03 Ma. These results provide novel constraints for the timing and amplitude of middle-Miocene cooling in Antarctica and reveal the ecological legacy of this global climate transition.

Chemical and physical drivers of beryllium retention in two soil endmembers.

Meteoric 10Be and 7Be produced in the atmosphere from high-energy spallation reactions are deposited onto the Earth's surface through wet and dry deposition and are sorbed onto the surfaces of particles. On land, the sorbed concentrations scale with the residence time of sediments in a landscape-offset by slow (10Be) and fast (7Be) radioactive decay. Additionally, the amount of native 9Be, leached from minerals, correlates with the chemical weathering of soils. However, previous work has shown that chemical and physical properties of soils and river sediments affects sorption of beryllium. Therefore, the magnitude of sorbed beryllium concentrations may be more representative of the sorption capacity of the system rather than its erosional or weathering history. Although previous work has examined the physical and chemical properties of soil that influence beryllium sorption, these studies either lack consensus or exclude potentially important variables. In this work, we provide a thorough examination of variables previously reported to have influence on beryllium chemistry as well as new variables such as nitrogen, phosphorus and sulfur concentrations in order to determine which factors best predict beryllium sorption. We selected two soil endmembers with differing compositions, separated them into different size fractions, and characterized the surface area, cation exchange capacity (CEC), mineralogy, sulfur, carbon, nitrogen and phosphorus concentrations. We determined that the inverse percent abundance of quartz and the CEC best predict beryllium sorption potential in these soils. By deriving a model that relates these two variables to the percent sorbed beryllium, we were able to predict the sorption capacity of our system and reduced the error in sorbed beryllium amounts due to differences in soil properties by about 42%. From these results, we provide insight as to why there is inconsistency in the literature with regards to the physio-chemical controls on the environmental behavior of beryllium.

Siderophore-mediated iron removal from chrysotile: Implications for asbestos toxicity reduction and bioremediation.

Asbestos fibers are highly toxic (Group 1 carcinogen) due to their high aspect ratio, durability, and the presence of iron. In nature, plants, fungi, and microorganisms release exudates, which can alter the physical and chemical properties of soil minerals including asbestos minerals. We examined whether exudates from bacteria and fungi at environmentally relevant concentrations can alter chrysotile, the most widely used asbestos mineral, and lower its toxicity. We monitored the release of iron from chrysotile in the presence of organic acid ligands and iron-specific siderophores derived from bacteria and fungi and measured any change in fiber toxicity toward peritoneal macrophages harvested from mice. Both fungal and bacterial siderophores increased the removal of iron from asbestos fibers. In contrast, organic acid ligands at environmentally relevant concentrations neither released iron from fibers nor helped in siderophore-mediated iron removal. Removal of plant-available or exchangeable iron did not diminish iron dissolution by both types of siderophores, which indicates that siderophores can effectively remove structural iron from chrysotile fibers. Removal of iron by siderophore lowered the fiber toxicity; fungal siderophore appears to be more effective than bacterial siderophore in lowering the toxicity. These results indicate that prolonged exposure to siderophores, not organic acids, in the soil environment decreases asbestos fiber toxicity and possibly lowers the health risks. Thus, bioremediation should be explored as a viable strategy to manage asbestos-contaminated sites such as Brownfield sites, which are currently left untreated despite dangers to surrounding communities.

Differential elemental uptake in three pseudo-metallophyte C4 grasses in situ in the eastern USA.

BACKGROUND AND AIMS: Elemental uptake in serpentine floras in eastern North America is largely unknown. The objective of this study was to determine major and trace element concentrations in soil and leaves of three native pseudo-metallophyte C4 grasses in situ at five sites with three very different soil types, including three serpentine sites, in eastern USA. METHODS: Pseudo-total and extractible concentrations of 15 elements were measured and correlated from the soils and leaves of three species at the five sites. RESULTS: Element concentrations in soils of pseudo-metallophytes varied up to five orders of magnitude. Soils from metalliferous sites exhibited higher concentrations of their characteristic elements than non-metalliferous. In metallicolous populations, elemental concentrations depended on the element. Concentrations of major elements (Ca, Mg, K) in leaves were lower than typical toxicity thresholds, whereas concentrations of Zn were higher. CONCLUSIONS: In grasses, species can maintain relatively low metal concentrations in their leaves even when soil concentrations are richer. However, in highly Zn-contaminated soil, we found evidence of a threshold concentration above which Zn uptake increases drastically. Finally, absence of main characteristics of serpentine soil at one site indicated the importance of soil survey and restoration to maintain serpentinophytes communities and avoid soil encroachment.

Framework for assessment and phytoremediation of asbestos-contaminated sites.

We examine the feasibility of phytoremediation as an alternative strategy to limit the exposure of asbestos in site with asbestos-containing materials. We collected soils from four locations from two sites-one with naturally occurring asbestos, and another, a superfund site, where asbestos-containing materials were disposed over decades-and performed ecotoxicology tests. We also performed two experiments with crop cultivar and two grasses from serpentine ecotype and cultivar to determined best choice for phytoremediation. Asbestos concentrations in different size fractions of soils varied by orders of magnitude. However, different asbestos concentrations had little effect on germination and root growth. Presence of co-contaminants such as heavy metals and lack of nutrients affected plant growth to different extents, indicating that several of these limiting factors should be considered instead of the primary contaminant of concern. Crop cultivar survived on asbestos-contaminated soil. Grasses from serpentine ecotype did not show higher biomass than the cultivar. Overall, these results showed that soil conditions play a critical role in screening different crop species for phytoremediation and that asbestos concentration has limited to no effect on plant growth. Our study provided a framework for phytoremediation of asbestos-contaminated sites to limit long-term asbestos exposure.

Asbestos Fiber Preparation Methods Affect Fiber Toxicity.

To measure the toxic potential of asbestos fibers-a known cause of asbestosis, lung cancer, and malignant mesothelioma-asbestos minerals are generally first ground down to small fibers, but it is unknown whether the grinding condition itself changes the fiber toxicity. To evaluate this, we ground chrysotile ore with or without water for 5-30 min and quantified asbestos-induced reactive oxygen species generation in elicited murine peritoneal macrophages as an indicator of fiber toxicity. The toxicity of dry-ground fibers was higher than the toxicity of wet-ground fibers. Grinding with or without water did not materially alter the mineralogical properties. However, dry-ground fibers contained at least 7 times more iron than wet-ground fibers. These results indicate that grinding methods significantly affect the surface concentration of iron, resulting in changes in fiber-induced reactive oxygen species generation or toxicity. Therefore, fiber preparation conditions should be accounted for when comparing the toxicity of asbestos fibers between reported studies.

Extreme decay of meteoric beryllium-10 as a proxy for persistent aridity.

The modern Antarctic Dry Valleys are locked in a hyper-arid, polar climate that enables the East Antarctic Ice Sheet (EAIS) to remain stable, frozen to underlying bedrock. The duration of these dry, cold conditions is a critical prerequisite when modeling the long-term mass balance of the EAIS during past warm climates and is best examined using terrestrial paleoclimatic proxies. Unfortunately, deposits containing such proxies are extremely rare and often difficult to date. Here, we apply a unique dating approach to tundra deposits using concentrations of meteoric beryllium-10 ((10)Be) adhered to paleolake sediments from the Friis Hills, central Dry Valleys. We show that lake sediments were emplaced between 14-17.5 My and have remained untouched by meteoric waters since that time. Our results support the notion that the onset of Dry Valleys aridification occurred ~14 My, precluding the possibility of EAIS collapse during Pliocene warming events. Lake fossils indicate that >14 My ago the Dry Valleys hosted a moist tundra that flourished in elevated atmospheric CO2 (>400 ppm). Thus, Dry Valleys tundra deposits record regional climatic transitions that affect EAIS mass balance, and, in a global paleoclimatic context, these deposits demonstrate how warming induced by 400 ppm CO2 manifests at high latitudes.