A bimillennial-length tree-ring reconstruction of precipitation for the Tavaputs Plateau, Northeastern Utah     

Troy A. Knight  David M. Meko 《Quaternary Research》2010,73(1):107-159

Despite the extensive network of moisture-sensitive tree-ring chronologies in western North America, relatively few are long enough to document climatic variability before and during the Medieval Climate Anomaly (MCA) ca. AD 800-1300. We developed a 2300-yr tree-ring chronology extending to 323 BC utilizing live and remnant Douglas-fir (Pseudotsuga menziesii) from the Tavaputs Plateau in northeastern Utah. A resulting regression model accounts for 70% of the variance of precipitation for the AD 1918-2005 calibration period. Extreme wet and dry periods without modern analogues were identified in the reconstruction. The MCA is marked by several prolonged droughts, especially prominent in the mid AD 1100s and late 1200s, and a lack of wet or dry single-year extremes. The frequency of extended droughts is not markedly different, however, than before or after the MCA. A drought in the early AD 500s surpasses in magnitude any other drought during the last 1800 yr. A set of four long high-resolution records suggests this drought decreased in severity toward the south in the western United States. The spatial pattern is consistent with the western dipole of moisture anomaly driven by El Niño and is also similar to the spatial footprint of the AD 1930s “Dust Bowl” drought.  相似文献   

Ground-based visible and near-IR observations of Comet 9P/Tempel 1 during the Deep Impact encounter     

Matthew M. Knight  Kevin J. Walsh  Robert A. Swaters  B. Ashley Zauderer  Roberto Vázquez 《Icarus》2007,187(1):199-207

We present the results of our visible and near-IR observations of Comet 9P/Tempel 1 during the Deep Impact encounter. The comet was observed before, during, and after impact from Kitt Peak National Observatory (J, H, K) and Observatorio Astronómico Nacional-San Pedro Mártir, Mexico (B, V, R, I). High time-resolution images in R, J, H, and K the night of impact with a 3^.″5 radius aperture revealed a rapid brightening which had multiple slopes and lasted for approximately 25 min before leveling off. The brightness decreased on subsequent nights and returned to near pre-impact levels by July 8 UT. The R-J, R-H, R-K, J-H, J-K, and H-K colors became bluer the night of impact. The R-J, R-H, and R-K colors remained blue on the night after impact while the J-H, J-K, and H-K colors returned to baseline levels. The observed color changes suggest the bluening was due to an increase in small grains relative to the ambient coma, an increase in ice relative to refractory dust in the coma, or a combination of the two. The ejecta were initially directed towards the southwest but had been driven southeast by solar radiation pressure by the second night after impact. The mean projected ejecta velocity was estimated at 0.20-0.23 km s⁻¹ over the first 24 h after impact.  相似文献   

Impact glass spherules in the Chicxulub K‐Pg event bed at Beloc,Haiti: Alteration patterns          下载免费PDF全文

Xenia Ritter  Alexander Deutsch  Jasper Berndt  Eric Robin 《Meteoritics & planetary science》2015,50(3):418-432

We have investigated six impact glass spherules from the K‐Pg event bed at Beloc, Haiti, using optical and electron microscopy, electron microprobe and in situ laser ablation–mass spectrometry (LA‐ICP‐MS; 37 trace elements, spot size 90–35 μm), in order to understand geochemical changes during alteration. The mm‐sized glass spherules are partly or totally altered to smectite, but original textural features are preserved. The average trace‐element composition of glass matches that one of the upper continental crust. Hints for a “meteoritic component” are lacking (Ni/Cr < 1.3; Pt below detection limit). Compared to this fresh glass, smectites are strongly depleted in trace elements, except for Li, Sc, V, Ni, Ga, Ge, and Ba. The chondrite‐normalized REE distribution patterns are flat with subchondritic abundances, related to their very low degree of crystallinity. We observe a positive Eu and a strong negative Ce anomaly; the latter is explained by formation of an organic Ce⁴⁺‐complex, soluble under reducing conditions. Zr/Hf of glasses and smectites is chondritic to superchondritic (35–40), whereas Nb/Ta in smectite is subchondritic (5–12) compared to Nb/Ta in the glass (~14–18). The low Nb/Ta is due to the low Nb concentrations in the smectite. Using in situ techniques with high spatial resolution, we have documented for the first time the significant changes in diagnostic elemental ratios during alteration of glass spherules. This has to be taken into account in the interpretation of geochemical data of not only impact materials but also volcanic glass, especially if bulk rock methods are used.  相似文献   

A Parkes radio telescope study of giant pulses from PSR J1823–3021A     

H. S. Knight 《Monthly notices of the Royal Astronomical Society》2007,378(2):723-729

We report on 685-MHz observations of PSR J1823–3021A using the Parkes radio telescope. A total of 120 giant pulses (GPs) were found by searching for spiky emission at 16-μs time resolution. The energies of these pulses follow a power law that has a very steep exponent of −3.1. This means that the emission mechanism that gives rise to the GPs almost always produces pulses that only have moderate energies. The profile formed by adding all the GPs has components that are narrower and more widely separated than the profile formed from all pulses. Aberration and retardation of emission from a corotating volume mean that components emitted at high altitude will have asymmetric phases compared to those emitted at low altitude. By assuming that the components of the pulse profile form conal pairs, we use this effect to limit the GPs to be emitted no higher than 4 km above ordinary emission. The arrival times of the GPs are well modelled by Poisson statistics at time-scales around 100 s. We report a GP with spikes of emission at the phases of both components. The probability of two independent GPs occurring within a single pulse period is     , so an interpretation can be conjectured that the two pulses are not independent. This may mean that the magnetosphere can remain in a state that is susceptible to discrete 'giant' emission events for as long as 2 ms.  相似文献   

Spatial processing of signals received by platform mounted sonar     

Solomon  I.S.D. Knight  A.J. 《Oceanic Engineering, IEEE Journal of》2002,27(1):57-65

Acoustic signals received by platform mounted sonar arrays can be spatially processed to enhance the detection of targets in the presence of both ambient and platform generated (self) noise. Ambient noise in the ocean, such as that due to distant shipping or biological choruses, are known to be spatially correlated. The platform generated noise will be of near-field origin and may not be received by all elements in the array. In this paper we investigate the performance of the minimum variance distortionless response (MVDR) beamformer and the recently introduced Fourier integral method (FIM) and compare their performances with the conventional beamformer. Real passive sonar data, obtained from a platform mounted sparse linear array of hydrophones, is used to study the performance of the beamformers in a typical sonar environment. It is shown that in the absence of self noise, when the array is accurately calibrated the MVDR beamformer will perform very well, but when sensor gain or phase errors are present the performance of the MVDR beamformer is degraded. Further, the MVDR beamformer is unable to reject the self noise which is not "seen" by the entire array. FIM however seems to perform well and a modified version of FIM, which we call weighted FIM (WFIM), is shown to perform better and is at worst comparable to a well calibrated MVDR beamformer  相似文献   

Inertial currents in the northern North sea     

P. J. Knight  M. J. Howarth  T. P. Rippeth 《Journal of Sea Research》2002,47(3-4)

Current profiles were measured in the northern North Sea during the autumnal breakdown of stratification (September and October) in 1998. The site was in 110 m of water and the depth-averaged M₂ tidal current amplitudes were about 0.15 m s⁻¹. The surface and bed mixed layers were initially well separated. The measurements were made principally with Acoustic Doppler Current Profilers (ADCP) which gave good coverage of the majority of the water column.During a two-month period several episodes of inertial currents were observed, exhibiting a range of responses some of which corresponded very closely to that predicted by theory. The structure of the inertial currents was primarily first mode baroclinic, with no inertial energy in the depth-averaged current. This implies that the currents in the lower layer are strongly linked to those in the surface layer and also that dissipation could be generated by bed friction, but the nature of the link is unclear. The level of least motion coincided with the thermocline. Since the currents in the upper and lower layers are 180° out of phase, large shears can occur across the thermocline; occasionally the bulk Richardson number determined with a four-metre vertical resolution was less than one.Turbulence measurements suggest that when large inertial current shears are present across the thermocline, which exceed the buoyancy frequencies, then mixing within and across the thermocline is significant. Future experiments should concentrate on enhanced dissipation measurements around the thermocline and higher spatial resolution time series measurements of current and density.  相似文献   

Sulfide oxidation as a process for the formation of copper-rich magmatic sulfides   总被引：1，自引：0，他引：1  

Cora C. Wohlgemuth-Ueberwasser  Raúl O. C. Fonseca  Chris Ballhaus  Jasper Berndt 《Mineralium Deposita》2013,48(1):115-127

Typical magmatic sulfides are dominated by pyrrhotite and pentlandite with minor chalcopyrite, and the bulk atomic Cu/Fe ratio of these sulfides is typically less than unity. However, there are rare magmatic sulfide occurrences that are dominated by Cu-rich sulfides (e.g., bornite, digenite, and chalcopyrite, sometimes coexisting with metallic Cu) with atomic Cu/Fe as high as 5. Typically, these types of sulfide assemblages occur in the upper parts of moderately to highly fractionated layered mafic–ultramafic intrusions, a well-known example being the Pd/Au reef in the Upper Middle Zone of the Skaergaard intrusion. Processes proposed to explain why these sulfides are so unusually rich in Cu include fractional crystallization of Fe/(Ni) monosulfide and infiltration of postmagmatic Cu-rich fluids. In this contribution, we explore and experimentally evaluate a third possibility: that Cu-rich magmatic sulfides may be the result of magmatic oxidation. FeS-dominated Ni/Cu-bearing sulfides were equilibrated at variable oxygen fugacities in both open and closed system. Our results show that the Cu/Fe ratio of the sulfide melt increases as a function of oxygen fugacity due to the preferential conversion of FeS into FeO and FeO_1.5, and the resistance of Cu₂S to being converted into an oxide component even at oxygen fugacities characteristic of the sulfide/sulfate transition (above FMQ?+?1). This phenomenon will lead to an increase in the metal/S ratio of a sulfide liquid and will also depress its liquidus temperature. As such, any modeling of the sulfide liquid line of descent in magmatic sulfide complexes needs to address this issue.  相似文献   

Mg and Si isotopic fractionation patterns in types B1 and B2 CAIs: Implications for formation under different nebular conditions     

Emma S. Bullock  Kim B. Knight  Frank M. Richter  Noriko T. Kita  Takayuki Ushikubo  Glenn J. MacPherson  Andrew M. Davis  Ruslan A. Mendybaev 《Meteoritics & planetary science》2013,48(8):1440-1458

Magnesium and silicon isotopic profiles across melilite grains in two type B1 and two type B2 calcium‐aluminum‐rich inclusions (CAIs) reveal differing but constant enrichments in heavy isotopes everywhere except ≤1000 μm from the CAI margins. There is no close correlation in the B1s or the B2s between isotopic composition and åkermanite content of the melilite, a measure of progressive igneous crystallization, yet such a correlation might be expected in a type B2: without a melilite mantle (as in B1s) to seal the interior off and prevent further evaporation, the melt would have maintained communication with the external gas. These observations indicate a model in which B1s and B2s solidified under differing conditions. The B2s solidified under lower hydrogen pressures ( ≤ 10^?4 – 10^?5 bars) than did B1s ( > 10^?4 bars), so surface volatilization was slower in the B2s and internal chemical and isotopic equilibrium was maintained over the interval of melilite crystallization. The outermost zones of the CAIs (≤1000 μm from the edge) are not consistently enriched in heavy isotopes relative to the interiors, as might be expected from diffusion‐limited surface evaporation of the melt. In all cases, the magnesium in the CAI margins is lighter than in the interiors. In one case, silicon in the margin also is lighter, but locally in some CAIs, it is isotopically heavier near the surface. If melt evaporation played a role in the formation of these outer zones, a later event in many cases caused isotopic re‐equilibration with an external and isotopically near‐normal reservoir.  相似文献   

The role of renewable energy in climate stabilization: results from the EMF27 scenarios     

Gunnar Luderer  Volker Krey  Katherine Calvin  James Merrick  Silvana Mima  Robert Pietzcker  Jasper Van Vliet  Kenichi Wada 《Climatic change》2014,123(3-4):427-441

This paper uses the EMF27 scenarios to explore the role of renewable energy (RE) in climate change mitigation. Currently RE supplies almost 20 % of global electricity demand. Almost all EMF27 mitigation scenarios show a strong increase in renewable power production, with a substantial ramp-up of wind and solar power deployment. In many scenarios, renewables are the most important long-term mitigation option for power supply. Wind energy is competitive even without climate policy, whereas the prospects of solar photovoltaics (PV) are highly contingent on the ambitiousness of climate policy. Bioenergy is an important and versatile energy carrier; however—with the exception of low temperature heat—there is less scope for renewables other than biomass for non-electric energy supply. Despite the important role of wind and solar power in climate change mitigation scenarios with full technology availability, limiting their deployment has a relatively small effect on mitigation costs, if nuclear and carbon capture and storage (CCS)—which can serve as substitutes in low-carbon power supply—are available. Limited bioenergy availability in combination with limited wind and solar power by contrast, results in a more substantial increase in mitigation costs. While a number of robust insights emerge, the results on renewable energy deployment levels vary considerably across the models. An in-depth analysis of a subset of EMF27 reveals substantial differences in modeling approaches and parameter assumptions. To a certain degree, differences in model results can be attributed to different assumptions about technology costs, resource potentials and systems integration.  相似文献   

Land-use transition for bioenergy and climate stabilization: model comparison of drivers,impacts and interactions with other land use based mitigation options   总被引：1，自引：0，他引：1  

Alexander Popp  Steven K. Rose  Katherine Calvin  Detlef P. Van Vuuren  Jan Phillip Dietrich  Marshall Wise  Elke Stehfest  Florian Humpenöder  Page Kyle  Jasper Van Vliet  Nico Bauer  Hermann Lotze-Campen  David Klein  Elmar Kriegler 《Climatic change》2014,123(3-4):495-509

In this article, we evaluate and compare results from three integrated assessment models (GCAM, IMAGE, and ReMIND/MAgPIE) regarding the drivers and impacts of bioenergy production on the global land system. The considered model frameworks employ linked energy, economy, climate and land use modules. By the help of these linkages the direct competition of bioenergy with other energy technology options for greenhouse gas (GHG) mitigation, based on economic costs and GHG emissions from bioenergy production, has been taken into account. Our results indicate that dedicated bioenergy crops and biomass residues form a potentially important and cost-effective input into the energy system. At the same time, however, the results differ strongly in terms of deployment rates, feedstock composition and land-use and greenhouse gas implications. The current paper adds to earlier work by specific looking into model differences with respect to the land-use component that could contribute to the noted differences in results, including land cover allocation, land use constraints, energy crop yields, and non-bioenergy land mitigation options modeled. In scenarios without climate change mitigation, bioenergy cropland represents 10–18 % of total cropland by 2100 across the different models, and boosts cropland expansion at the expense of carbon richer ecosystems. Therefore, associated emissions from land-use change and agricultural intensification as a result of bio-energy use range from 14 and 113 Gt CO2-eq cumulatively through 2100. Under climate policy, bioenergy cropland increases to 24–36 % of total cropland by 2100.  相似文献