35 research outputs found
Origin and Provenance of Spherules and Magnetic Grains at the Younger Dryas Boundary
One or more bolide impacts are hypothesized to have triggered the Younger Dryas cooling at âŒ12.9 ka. In support of this hypothesis, varying peak abundances of magnetic grains with iridium and magnetic microspherules have been reported at the Younger Dryas boundary (YDB). We show that bulk sediment and/or magnetic grains/microspherules collected from the YDB sites in Arizona, Michigan, New Mexico, New Jersey, and Ohio have (187)Os/(188)Os ratios â„1.0, similar to average upper continental crust (= 1.3), indicating a terrestrial origin of osmium (Os) in these samples. In contrast, bulk sediments from YDB sites in Belgium and Pennsylvania exhibit (187)Os/(188)Os ratios \u3c\u3c1.0 and at face value suggest mixing with extraterrestrial Os with (187)Os/(188)Os of âŒ0.13. However, the Os concentration in bulk sample and magnetic grains from Belgium is 2.8 pg/g and 15 pg/g, respectively, much lower than that in average upper continental crust (=31 pg/g), indicating no meteoritic contribution. The YDB site in Pennsylvania is remarkable in yielding 2- to 5-mm diameter spherules containing minerals such as suessite (Fe-Ni silicide) that form at temperatures in excess of 2000 °C. Gross texture, mineralogy, and age of the spherules appear consistent with their formation as ejecta from an impact 12.9 ka ago. The (187)Os/(188)Os ratios of the spherules and their leachates are often low, but Os in these objects is likely terrestrially derived. The rare earth element patterns and Sr and Nd isotopes of the spherules indicate that their source lies in 1.5-Ga Quebecia terrain in the Grenville Province of northeastern North America
Surficial Redistribution of Fallout 131iodine in a Small Temperate Catchment
Isotopes of iodine play significant environmental roles, including a limiting micronutrient (127I), an acute radiotoxin (131I), and a geochemical tracer (129I). But the cycling of iodine through terrestrial ecosystems is poorly understood, due to its complex environmental chemistry and low natural abundance. To better understand iodine transport and fate in a terrestrial ecosystem, we traced fallout 131iodine throughout a small temperate catchment following contamination by the 11 March 2011 failure of the Fukushima Daiichi nuclear power facility. We find that radioiodine fallout is actively and efficiently scavenged by the soil system, where it is continuously focused to surface soils over a period of weeks following deposition. Mobilization of historic (pre-Fukushima) 137cesium observed concurrently in these soils suggests that the focusing of iodine to surface soils may be biologically mediated. Atmospherically deposited iodine is subsequently redistributed from the soil system via fluvial processes in a manner analogous to that of the particle-reactive tracer 7beryllium, a consequence of the radionuclidesâ shared sorption affinity for fine, particulate organic matter. These processes of surficial redistribution create iodine hotspots in the terrestrial environment where fine, particulate organic matter accumulates, and in this manner regulate the delivery of iodine nutrients and toxins alike from small catchments to larger river systems, lakes and estuaries
Contrasting sensitivity of lake sediment n-alkanoic acids and n-alkanes to basin-scale vegetation and regional-scale precipitation ÎŽ2H in the Adirondack Mountains, NY (USA)
The hydrogen isotope values of plant waxes (ÎŽ2Hwax) primarily reflect plant source water. ÎŽ2Hwax preserved in lake sediments has therefore been widely used to investigate past hydroclimate. The processes by which plant waxes are integrated at regional and catchment scales are poorly understood and may affect the ÎŽ2Hwax values recorded in sediments. Here, we assess the variability of sedimentary ÎŽ2Hwax for two plant wax compound classes (n-alkanes and n-alkanoic acids) across 12 lakes in the Adirondack Mountains that receive similar regional precipitation ÎŽ2H but vary at the catchment-scale in terms of vegetation structure and basin morphology. Total long-chain (n-C27 to n-C35) alkane concentrations were similar across all sites (191âŻÂ±âŻ53âŻÂ”g/g TOC) while total long-chain (n-C28 and n-C30) alkanoic acid concentrations were more variable (117âŻÂ±âŻ116âŻÂ”g/g TOC) and may reflect shoreline vegetation composition. Lakes with shorelines dominated by evergreen gymnosperm plants had significantly higher concentrations of long-chain n-alkanoic acids relative to n-alkanes, consistent with our observations that deciduous angiosperms produced more long-chain n-alkanes than evergreen gymnosperms (471 and 33âŻÂ”g/g TOC, respectively). In sediments, the most abundant chain lengths in each compound class were n-C29 alkane and n-C28 alkanoic acid, which had mean ÎŽ2H values of â188âŻÂ±âŻ6â° and â164âŻÂ±âŻ9â°, respectively. Across sites, the range in sedimentary n-C29 alkane (22â°) and n-C28 alkanoic acid ÎŽ2H (35â°) was larger than expected based on the total range in modeled mean annual precipitation ÎŽ2H (4â°). We observed larger mean Δapp (based on absolute values) for n-alkanes (â123â°) than for n-alkanoic acids (â97â°). Across sites, the ÎŽ2H offset between n-C29 alkane and the biosynthetic precursor n-C30 alkanoic acid (ΔC29-C30) ranged from â8 to â58â°, which was more variable than expected based on observations in temperate trees (â20 to â30â°). Sediments with greater aquatic organic matter contributions (lower C/N ratios) had significantly larger (absolute) ΔC29-C30 values, which may reflect long-chain n-alkanoic acids from aquatic sources. Concentration and ÎŽ2Hwax data in Adirondack lakes suggest that long-chain n-alkanes are more sensitive to regional-scale precipitation signals, while n-alkanoic acids are more sensitive to basin-scale differences in catchment vegetation and wax sourcing
Lessons learned from organizing and teaching virtual phylogenetics workshops
No abstract available
Quantitative Retention of Atmospherically Deposited Elements by Native Vegetation Is Traced by the Fallout Radionuclides <sup>7</sup>Be and <sup>210</sup>Pb
Atmospheric
deposition is the primary mechanism by which remote
environments are impacted by anthropogenic contaminants. Vegetation
plays a critical role in intercepting atmospheric aerosols, thereby
regulating the timing and magnitude of both contaminant and nutrient
delivery to underlying soils. However, quantitative models describing
the fate of atmospherically derived elements on vegetation are limited
by a lack of long-term measurements of both atmospheric flux and foliar
concentrations. We addressed this gap in understanding by quantifying
weekly atmospheric deposition of the naturally occurring radionuclide
tracers <sup>7</sup>Be and <sup>210</sup>Pb, as well as their activities
in leaves of colocated trees, for three years in New Hampshire, U.S.
The accumulation of both <sup>7</sup>Be and <sup>210</sup>Pb in deciduous
and coniferous vegetation is predicted by a model that is based solely
on measured atmospheric fluxes, duration of leaf exposure, and radioactive
decay. Any âwash offâ processes that remove <sup>7</sup>Be and <sup>210</sup>Pb from foliage operate with a maximum half-time
of greater than 370 days (<i>P</i> > 99%), which is an
order
of magnitude longer than previously assumed. The retention of both <sup>7</sup>Be and <sup>210</sup>Pb on leaves is thus quantitative and
permanent, coupling the fate of <sup>7</sup>Be, <sup>210</sup>Pb and
similar atmospheric species to that of the leaf matter itself. These
findings demonstrate that the long-standing paradigm of a short âenvironmental
half-lifeâ for atmospheric contaminants deposited on natural
surfaces must be re-evaluated
Tungsten Speciation and Solubility in Munitions-Impacted Soils
Considerable
questions persist regarding tungsten geochemistry
in natural systems, including which forms of tungsten are found in
soils and how adsorption regulates dissolved tungsten concentrations.
In this study, we examine tungsten speciation and solubility in a
series of soils at firing ranges in which tungsten rounds were used.
The metallic, mineral, and adsorbed forms of tungsten were characterized
using X-ray absorption spectroscopy and X-ray microprobe, and desorption
isotherms for tungsten in these soils were used to characterize its
solidâsolution partitioning behavior. Data revealed the complete
and rapid oxidation of tungsten metal to hexavalent tungstenÂ(VI) and
the prevalence of adsorbed polymeric tungstates in the soils rather
than discrete mineral phases. These polymeric complexes were only
weakly retained in the soils, and porewaters in equilibrium with contaminated
soils had 850 mg L<sup>â1</sup> tungsten, considerably in excess
of predicted solubility. We attribute the high solubility and limited
adsorption of tungsten to the formation of polyoxometalates such as
W<sub>12</sub>SiO<sub>40</sub><sup>4â</sup>, an α-Keggin
cluster, in soil solutions. Although more research is needed to confirm
which of such polyoxometalates are present in soils, their formation
may not only increase the solubility of tungsten but also facilitate
its transport and influence its toxicity