Retreat of the Smith Sound Ice Stream in the Early Holocene     

Anne E. Jennings  John T. Andrews  Brett Oliver  Maureen Walczak  Alan Mix 《Boreas: An International Journal of Quaternary Research》2019,48(4):825-840

Nares Strait, a major connection between the Arctic Ocean and Baffin Bay, was blocked by coalescent Innuitian and Greenland ice sheets during the last glaciation. This paper focuses on the events and processes leading to the opening of the strait and the environmental response to establishment of the Arctic‐Atlantic throughflow. The study is based on sedimentological, mineralogical and foraminiferal analyses of radiocarbon‐dated cores 2001LSSL‐0014PC and TC from northern Baffin Bay. Radiocarbon dates on benthic foraminifera were calibrated with ΔR = 220±20 years. Basal compact pebbly mud is interpreted as a subglacial deposit formed by glacial overriding of unconsolidated marine sediments. It is overlain by ice‐proximal (red/grey laminated, ice‐proximal glaciomarine unit barren of foraminifera and containing >2 mm clasts interpreted as ice‐rafted debris) to ice‐distal (calcareous, grey pebbly mud with foraminifera indicative of a stratified water column with chilled Atlantic Water fauna and species associated with perennial and then seasonal sea ice cover) glacial marine sediment units. The age model indicates ice retreat into Smith Sound as early as c. 11.7 and as late as c. 11.2 cal. ka BP followed by progressively more distal glaciomarine conditions as the ice margin retreated toward the Kennedy Channel. We hypothesize that a distinct IRD layer deposited between 9.3 and 9 (9.4–8.9 1σ) cal. ka BP marks the break‐up of ice in Kennedy Channel resulting in the opening of Nares Strait as an Arctic‐Atlantic throughflow. Overlying foraminiferal assemblages indicate enhanced marine productivity consistent with entry of nutrient‐rich Arctic Surface Water. A pronounced rise in agglutinated foraminifers and sand‐sized diatoms, and loss of detrital calcite characterize the uppermost bioturbated mud, which was deposited after 4.8 (3.67–5.55 1σ) cal. ka BP. The timing of the transition is poorly resolved as it coincides with the slow sedimentation rates that ensued after the ice margins retreated onto land.  相似文献   

Open-source approaches for location cover models: capabilities and efficiency     

Chen  Huanfa  Murray  Alan T.  Jiang  Rui 《Journal of Geographical Systems》2021,23(3):361-380

Journal of Geographical Systems - Location cover models are aimed at siting facilities so as to provide service to demand efficiently. These models are crucial in the management, planning and...  相似文献   

Thermochronological insights into the structural contact between the Tian Shan and Pamirs,Tajikistan          下载免费PDF全文

Gilby Jepson  Stijn Glorie  Dmitry Konopelko  Jack Gillespie  Martin Danišík  Noreen J. Evans  Yunus Mamadjanov  Alan S. Collins 《地学学报》2018,30(2):95-104

Multi‐method thermochronology along the Vakhsh‐Surkhob fault zone reveals the thermotectonic history of the South Tian Shan–Pamirs boundary. Apatite U/Pb analyses yield a consistent age of 251 ± 2 Ma, corresponding to cooling below ~550–350°C, related to the final closure of the Palaeo‐Asian Ocean and contemporaneous magmatism in the South Tian Shan. Zircon (U–Th–Sm)/He ages constrain cooling below ~180°C to the end of the Triassic (~200 Ma), likely related either to deformation induced by the Qiangtang collision or to the closure of the Rushan Ocean. Apatite fission track thermochronology reveals two low‐temperature (<120°C) thermal events at ~25 Ma and ~10 Ma, which may be correlated with tectonic activity at the distant southern Eurasian margin. The late Miocene cooling is confirmed by apatite (U–Th–Sm)/He data and marks the onset of mountain building within the South Tian Shan that is ongoing today.  相似文献   

Field methods for amending marine sediment with activated carbon and assessing treatment effectiveness   总被引：1，自引：0，他引：1  

Cho YM  Smithenry DW  Ghosh U  Kennedy AJ  Millward RN  Bridges TS  Luthy RG 《Marine environmental research》2007,64(5):541-555

Previous laboratory studies have shown reductions in PCB bioavailability for sediments amended with activated carbon (AC). Here we report results on a preliminary pilot-scale study to assess challenges in scaling-up for field deployment and monitoring. The goals of the preliminary pilot-scale study at Hunters Point Shipyard (San Francisco, USA) were to (1) test the capabilities of a large-scale mixing device for incorporating AC into sediment, (2) develop and evaluate our field assessment techniques, and (3) compare reductions in PCB bioavailability found in the laboratory with well-mixed systems to those observed in the field with one-time-mixed systems. In this study we successfully used a large-scale device to mix 500kg of AC into a 34.4m(2) plot to a depth of 1ft, a depth that includes the majority of the biologically active zone. Our results indicate that after 7 months of AC-sediment contact in the field, the 28-day PCB bioaccumulation for the bent-nosed clam, Macoma nasuta, field-deployed to this AC-amended sediment was approximately half of the bioaccumulation resulting from exposure to untreated sediment. Similar PCB bioaccumulation reductions were found in laboratory bioassays conducted on both the bivalve, M. nasuta and the estuarine amphipod, Leptocheirus plumulosus, using sediment collected from the treated and untreated field plots one year after the AC amendment occurred. To further understand the long-term effectiveness of AC as an in situ treatment strategy for PCB-contaminated sediments under field conditions, a 3-year comprehensive study is currently underway at Hunters Point that will compare the effectiveness of two large-scale mixing devices and include both unmixed and mixed-only control plots.  相似文献   

Matrix and whole‐rock fractionations in the Acfer 094 type 3.0 ungrouped carbonaceous chondrite     

John T. WASSON  Alan E. RUBIN 《Meteoritics & planetary science》2010,45(1):73-90

Abstract– We used the electron microprobe to study matrix in the ungrouped type 3.0 carbonaceous chondrite Acfer 094 using 7 × 7‐point, focused‐beam arrays; data points attributable to mineral clasts were discarded. The grid areas show resolvable differences in composition, but differences are less pronounced than we observed in studies of CR2 LaPaz Icefield (LAP) 02342 (Wasson and Rubin [2009]) and CO3.0 Allan Hills A77307 (Brearley [1993]). A key question is why Acfer shows an anomalously uniform composition of matrix compared with these other carbonaceous chondrites. Both whole‐rock and matrix samples of Acfer 094 show enhancements of Ca and K; it appears that these reflect contamination during hot desert weathering. By contrast, the whole‐rock abundance of Na is low. Although weathering effects are responsible for some fractionations, it appears that nebular effects are also resolvable in matrix compositions in Acfer 094. As with LAP 02342, we infer that the observed differences among different areas were inherited from the solar nebula and may have been carried by porous chondrules that experienced low (about 20%) degrees of melting. Acfer 094 has been comminuted by one or more impact events that may also have caused volatile loss. Thus, despite preserving evidence (e.g., an exceptionally high content of presolar SiC) implying a high degree of pristinity, Acfer 094 is far from pristine in other respects. This evidence of comminution and an O‐isotopic composition similar to values measured in metamorphosed CM chondrites suggest that Acfer was hydrated before being outgassed by the inferred impact event. Convection within the plume associated with the impact event probably also contributed to the homogenization of the Acfer 094 matrix.  相似文献   

Decade time-scale modulation of low-mass X-ray binaries     

Martin Durant  Remon Cornelisse  Ron Remillard  Alan Levine 《Monthly notices of the Royal Astronomical Society》2010,401(1):355-361

Regular observations by the All-Sky Monitor aboard the Rossi X-ray Timing Explorer satellite have yielded well-sampled light curves with a time baseline of over 10 years. We find that up to eight of the 16 brightest persistent low-mass X-ray binaries (LMXBs) show significant, possible sinusoidal, variations with periods of the order of 10 years. We speculate on its possible origin and prevalence in the population of LMXBs, and we find the presence of a third object in the system, or long-period variability intrinsic to the donor star, as being attractive origins for the X-ray flux modulation we detect. For some of the objects in which we do not detect a signal, there is substantial short-term variation which may hide modest modulation on long time-scales. Decade time-scale modulations may thus be even more common.  相似文献   

Adaptive meshing in a mixed regime hydrologic simulation model     

Jackie S. Pettway  Joseph H. Schmidt  Alan K. Stagg 《Computational Geosciences》2010,14(4):665-674

Problems in hydrology frequently have moving fronts and dynamic driving mechanisms such as wells. Since the location of important features changes during a simulation, accurate modeling requires uniformly fine resolution or the ability to change resolution during the simulation. We will describe an algorithm for refinement and unrefinement of tetrahedral/triangular meshes that has been implemented in the adaptive hydrology (ADH) code. The codes including the refinement/unrefinement algorithms are implemented in parallel to accommodate problems with large run time and memory requirements. In this paper, we describe the parallel, adaptive grid algorithm used in ADH and show the resulting grids from some example problems.  相似文献   

Reduction of Substituted <Emphasis Type="Italic">p</Emphasis>-Benzoquinones by Fe<Superscript>II</Superscript> Near Neutral pH     

Minori Uchimiya  Alan T. Stone 《Aquatic Geochemistry》2010,16(1):173-188

The oxidation of dihydroxyaromatics to benzoquinones by Fe^III (hydr)oxides is important in respiratory electron shuttling by microorganisms and has been extensively studied. Prior publications have noted that the Gibbs Free Energy (ΔG) for the forward reaction is sensitive to dihydroxyaromatic structure, pH, and concentrations of reactants and products. Here, we address the back reaction, benzoquinone reduction by Fe^II. Rates markedly increase with increasing pH, in accord with increases in ΔG. Ring substituents that raise the potential of the p-benzoquinone/hydroquinone half reaction raise reaction rates: –OCH₃ < –CH₃ < –C₆H₅ < –H < –Cl. p-Naphthoquinone, with a reduction potential lower than the five substituted p-benzoquinones just listed, yields the lowest reaction rates. The complexity of the reaction is reflected in lag periods and less-pronounced S-shaped time course curves. Benzoquinone reduction by Fe^II may be an important link in networks of electron transport taking place in suboxic and anoxic environments.  相似文献   

Non-nebular origin of dark mantles around chondrules and inclusions in CM chondrites     

Josep M. Trigo-Rodriguez  Alan E. Rubin 《Geochimica et cosmochimica acta》2006,70(5):1271-1290

Our examination of nine CM chondrites that span the aqueous alteration sequence leads us to conclude that compact dark fine mantles surrounding chondrules and inclusions in CM chondrites are not discrete fine-grained rims acquired in the solar nebula as modeled by Metzler et al. [Accretionary dust mantles in CM chondrites: evidence for solar nebula processes. Geochim. Cosmochim. Acta56, 1992, 2873-2897]. Nebular processes that lead to agglomeration produce materials with porosities far higher than those in the dark mantles. We infer that the mantles were produced from porous nebular materials on the CM parent asteroid by impact-compaction (a process that produces the lowest porosity adjacent to chondrules and inclusions). Compaction was followed by aqueous alteration that formed tochilinite, serpentine, Ni-bearing sulfide, and other secondary products in voids in the interchondrule regions. Metzler et al. reported a correlation between mantle thickness and the radius of the enclosed object. In Yamato 791198 we find no correlation when all sizes of central objects and dark lumps are included but a significant correlation (r² = 0.44) if we limit consideration to central objects with radii >35 μm; a moderate correlation is also found in QUE 97990. We suggest that impact-induced shear of a plum-pudding-like precursor produced the observed “mantles”; these were shielded from comminution during impact events by the adjacent stronger chondrules and inclusions. Some mantles in CM chondrites with low degrees of alteration show distinct layers that may largely reflect differences in porosity. Typically, a gray, uniform inner layer is surrounded by an outer layer consisting of darker silicates with BSE-bright speckles. The CM-chondrite objects characterized as “primary accretionary rocks” by Metzler et al. did not form in the nebula, but rather on the parent body. The absence of solar-flare particle tracks and solar-wind-implanted rare gases in these clasts reflect their lithified nature and low surface/volume ratios during the period when they resided in the regolith and were subject to irradiation by solar particles. The clasts are analogous to the light-colored metamorphosed clasts in ordinary-chondrite regolith breccias (which also lack solar-flare particle tracks and solar-wind gas).  相似文献   

Effects on geodetic VLBI measurements due to polarization leakage in S/X receivers     

Alessandra Bertarini  Alan L. Roy  Brian Corey  R. Craig Walker  Walter Alef  Axel Nothnagel 《Journal of Geodesy》2011,85(10):715-721

Geodetic very long baseline interferometry (VLBI) delivers time series of station positions and Earth orientation parameters. These series offer a viable and precise way to study Earth crustal and core dynamics and to support space navigation. Their accuracy is degraded by instrumental errors, of which polarization leakage is considered to be one of the largest that is not yet being addressed. Its effect on the data can be corrected, provided one knows the leakage characteristics of the receivers. For this purpose, we designed a VLBI session to measure the polarization leakage at 15 geodetic and very long baseline array stations over the frequency range from 8,212.99 to 8,932.99 MHz. We describe the polarization leakage measurements and the algorithm that was implemented to correct for its effect on the geodetic delay observables. Subsequently, we applied the correction for polarization leakage to the same data that were used to determine the leakage and checked for the resulting improvement. From the measured leakage terms, one would expect polarization leakage to affect the group delay measurements in 90% of the cases by 1.6 ps or less. This proved to be below the statistical noise in our single VLBI session, and hence, an improvement from the correction could not be detected. Applying this analysis in the context of VLBI2010, we provide a specification for the allowable polarization leakage to achieve the target submillimetre accuracy.  相似文献