196,299 research outputs found
Skeletal P/Ca tracks upwelling in Gulf of Panama coral: Evidence for a new seawater phosphate proxy
No full textThe supply of limiting nutrients to the low latitude ocean is controlled by physical processes linked to climate variations, but methods for reconstructing past nutrient concentrations in the surface ocean are few and indirect. Here, we present laser ablation mass spectrometry results that reveal annual cycles of P/Ca in a 4-year record from the scleractinian coral Pavona gigantea (mean P/Ca = 118 mmol mol?1). The P/Ca cycles track variations in past seawater phosphate concentration synchronously with skeletal Sr/Ca-derived temperature variations associated with seasonal upwelling in the Gulf of Panama´. Skeletal P/Ca varies seasonally by 2–3 fold, reflecting the timing and magnitude of dissolved phosphate variations. Solution cleaning experiments on drilled coral powders show that over 60% of skeletal P occurs in intracrystalline organic phases. Coral skeleton P/Ca holds promise as a proxy record of nutrient availability on time scales of decades to millennia
MAGIC method for subnanomolar orthophosphate determination in freshwater
No full textA freshwater magnesium hydroxide co-precipitation method (MAGIC) has been developed to determine accurately and reproducibly low nanomolar to subnanomolar soluble reactive phosphorus (SRP) concentrations in freshwater. The method allows investigation of phosphorus distributions and cycling for systems in which SRP is below the detection limits of conventional methods. In natural waters, both inorganic and organic forms of P are co-precipitated; hence the method is essentially a pre-concentration rather than separation technique. Quantification of SRP on dissolved MAGIC precipitates follows a modified version of the standard molybdenum blue colorimetric method, using a spectrophotometer with ~0.1 milliabsorbance (mAbs) noise. Detection limit is 0.15nM, improving on typical conventional colorimetric methods by a factor of ~50, with precision (RSD of triplicates) of ~10% at the 1nM SRP level, 10% at <0.5nM, and 4-7% at >1nM. Considerable method development was necessary to eliminate or correct for multiple interferences, including a novel finding of potential interference by colored dissolved organic matter, and to optimize recovery, precision, and detection limit. The method was applied to filtered, frozen samples from western Lake Superior, showing that SRP concentrations are characterized by limited seasonal variability, largely uniform vertical distribution, and near-bottom enrichment. Concentrations ranged 0.4-10.9 nM SRP, representing ~10% of the total dissolved phosphorus pool. MAGIC is an easily employed analytical method appropriate for measurement of very low SRP in lakes and rivers
Seawater nutrient and carbonate ion concentrations recorded as P/Ca, Ba/Ca, and U/Ca in the deep-sea coral D. dianthus
No full textAs paleoceanographic archives, deep sea coral skeletons offer the potential for high temporal resolution and precise absolute dating, but have not been fully investigated for geochemical reconstructions of past ocean conditions. Here we assess the utility of skeletal P/Ca, Ba/Ca and U/Ca in the deep sea coral D. dianthus as proxies of dissolved phosphate (remineralized at shallow depths), dissolved barium (trace element with silicate-type distribution) and carbonate ion concentrations, respectively. Measurements of these proxies in globally distributed D. dianthus specimens show clear dependence on corresponding seawater properties. Linear regression fits of mean coral Element/Ca ratios against seawater properties yield the equations: P/Cacoral (lmol/mol) = (0.6 ± 0.1) P/Casw(lmol/mol) – (23 ± 18), R2 = 0.6, n = 16 and Ba/Cacoral(lmol/mol) = (1.4 ± 0.3) Ba/Casw(lmol/mol) + (0 ± 2), R2 = 0.6, n = 17; no significant relationship is observed between the residuals of each regression and seawater temperature, salinity, pressure, pH or carbonate ion concentrations, suggesting that these variables were not significant secondary dependencies of these proxies. Four D. dianthus specimens growing at locations withOarag 6 0.6 displayed markedly depleted P/Ca compared to the regression based on the remaining samples, a behavior attributed to an undersaturation effect. These corals were excluded from the calibration. Coral U/Ca correlates with seawater carbonate ion: U/Cacoral(lmol/mol) = (?0.016 ± 0.003) ½CO2? 3 ? (lmol/kg) + (3.2 ± 0.3), R2 = 0.6, n = 17. The residuals of the U/Ca calibration are not significantly related to temperature, salinity, or pressure. Scatter about the linear calibration lines is attributed to imperfect spatialtemporal matches between the selected globally distributed specimens and available water column chemical data, and potentially to unresolved additional effects. The uncertainties of these initial proxy calibration regressions predict that dissolved phosphate could be reconstructed to ±0.4 lmol/kg (for 1.3–1.9 lmol/kg phosphate), and dissolved Ba to ±19 nmol/kg (for 41–82 nmol/kg Basw). Carbonate ion concentration derived from U/Ca has an uncertainty of ±31lmol/kg (for 60–120 lmol=kg CO2? 3 ). The effect of microskeletal variability on P/Ca, Ba/Ca, and U/Ca was also assessed, with emphasis on centers of calcification, Fe–Mn phases, and external contaminants. Overall, the results show strong potential for reconstructing aspects of water mass mixing and biogeochemical processes in intermediate and deep waters using fossil deep-sea corals
Fabrication of graphene-covered micro-tubes for process intensification
No full textGraphene is known for its high surface-area-to-mass ratio. However, for using graphene in engineering processes such as catalytic reactors or heat exchangers, a high surface-area-to-volume ratio is essential. Currently, graphene is only prepared in sheet form, which limits its surface-area-to-volume ratio to around 200 m2 m−3. Herein, a technique based on chemical vapor deposition (CVD) to realize graphene on a micro-tubular substrate to not only substantially increase its surface-area-to-volume ratio to a value over 2000 m2 m−3 but also to eliminate the maldistribution of flows commonly unavoidable in flat-sheet configurations is proposed and demonstrated. This approach uses a dual-layer micro-tubular substrate fabricated by a phase-inversion technique. In the substrate, a thin copper outer layer is used to enable the CVD growth of graphene, and an inner Cu–Fe layer is adopted to provide a strong mechanical support. This approach is feasible to produce graphene with a very high surface-area-to-volume ratio for possible applications in catalytic reactors or heat exchangers, although problems such as inter-diffusion between the two metal layers and defects in graphene need to be addressed. To the best of our knowledge, this study is the first attempt to prepare graphene with high surface-area-to-volume ratios by a CVD route.</p
Biogeochemical cycling of dissolved zinc in the Western Arctic (Arctic GEOTRACES GN01)
No full textThe biogeochemical cycling of dissolved zinc (dZn) was investigated in the Western Arctic along the U.S. GEOTRACES GN01 section. Vertical profiles of dZn in the Arctic are strikingly different than the classic “nutrient‐type” profile commonly seen in the Atlantic and Pacific Oceans, instead exhibiting higher surface concentrations (~1.1 nmol/kg), a shallow subsurface absolute maximum (~4–6 nmol/kg) at 200 m coincident with a macronutrient maximum, and low deep water concentrations (~1.3 nmol/kg) that are homogeneous (sp.) with depth. In contrast to other ocean basins, typical inputs such as rivers, atmospheric inputs, and especially deep remineralization are insignificant in the Arctic. Instead, we demonstrate that dZn distributions in the Arctic are controlled primarily by (1) shelf fluxes following the sediment remineralization of high Zn:C and Zn:Si cells and the seaward advection of those fluxes and (2) mixing of dZn from source waters such as the Atlantic and Pacific Oceans rather than vertical biological regeneration of dZn. This results in both the unique profile shapes and the largely decoupled relationship between dZn and Si found in the Arctic. We found a weak dZn:Si regression in the full water column (0.077 nmol/μmol, r2 = 0.58) that is higher than the global slope (0.059 nmol/μmol, r2 = 0.94) because of the shelf‐derived halocline dZn enrichments. We hypothesize that the decoupling of Zn:Si in Western Arctic deep waters results primarily from a past ventilation event with unique preformed Zn:Si stoichiometries
A "shallow bathtub ring" of local sedimentary iron input maintains the Palmer Deep biological hotspot on the West Antarctic Peninsula shelf
No full textPalmer Deep (PD) is one of several regional hotspots of biological productivity along the inner shelf of the West Antarctic Peninsula. The proximity of hotspots to shelf-crossing deep troughs has led to the ‘canyon hypothesis’, which proposes that circumpolar deep water flowing shoreward along the canyons is upwelled on the inner shelf, carrying nutrients including iron (Fe) to surface waters, maintaining phytoplankton blooms. We present here full-depth profiles of dissolved and particulate Fe and manganese (Mn) from eight stations around PD, sampled in January and early February of 2015 and 2016, allowing the first detailed evaluation of Fe sources to the area's euphotic zone. We show that upwelling of deep water does not control Fe flux to the surface; instead, shallow sediment-sourced Fe inputs are transported horizontally from surrounding coastlines, creating strong vertical gradients of dissolved Fe within the upper 100 m that supply this limiting nutrient to the local ecosystem. The supply of bioavailable Fe is, therefore, not significantly related to the canyon transport of deep water. Near shore time-series samples reveal that local glacial meltwater appears to be an important Mn source but, surprisingly, is not a large direct Fe input to this biological hotspot
Dr. Duane M. Jackson, Morehouse College, July 2011
No full textThis video is a conversation with Dr. Duane M. Jackson. Dr. Jackson talks about his paper, "Recall and the Serial Position Effect: The Role of Primacy and Recency on Accounting Students' Performance." Jackie Daniel, AUC Woodruff Library, is the interviewer
"Reflections on the subject of Emigration from Europe with a view to Settlement in the United States" By M. Carey.
No full text"Reflections on the subject of Emigration from Europe with a view to Settlement in the United States: containing bried sketches of the moral and political character of those states.
By M. Carey, member of the American philosophical, and of the American Antiquarian Society, and author of The Olive Branch, Cindiciae Hibernicae, essays on banking, on political economy, and on internal improvement.
To which are now added the English editor's comments on the subject; together with Important Advice to Emigrants, and Cautions Against Impositions Practiced in the Outports
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Dr. Glendon Swarthout
No full textHosted by Roger M. Busfield, MSU Assistant Professor of Speech and Theater, Meet the Author is designed to introduce a general audience to a contemporary author and their work through in-depth interviews. This episode features a conversation between Dr. Glendon Swarthout, prolific author and English professor at MSU, and assistant professors Sam S. Baskett and Theodore B. Strandness
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