Direct Fourier transform mass spectral analysis of natural waters with low dissolved organic matter     

Rachel L. Sleighter  Georgina A. McKee  Patrick G. Hatcher 《Organic Geochemistry》2009,40(1):119-125

A major obstacle for characterizing dissolved organic matter (DOM) with ultrahigh resolution mass spectrometry has been its low concentrations in natural waters. Many previous mass spectrometric studies of both terrestrial and marine DOM typically have isolated and concentrated the DOM using solid phase C₁₈ extraction disks, ultrafiltration, or XAD resins, all of which are known to discriminate against many different classes of compounds. We have, for the first time, developed an approach to directly analyze natural water samples with less than 6 mg/l DOC (dissolved organic carbon), using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS). We demonstrate the sensitivity and ability of sequential selective ion accumulation (SSIA) to detect the thousands of components in a single freshwater DOM sample without any significant pretreatment. By utilizing SSIA, the baseline noise decreases while signal to noise ratios of the peaks increase, allowing for approximately 40% more formulas to be assigned to peaks in the mass spectra.  相似文献   

Is iron redox cycling in a high altitude watershed photochemically or thermally driven?     

Christopher J. Borman  B. Patrick Sullivan  Carrick M. Eggleston  Patricia J.S. Colberg 《Chemical Geology》2010,269(1-2):33-39

Many studies have shown that the concentration of aqueous Fe²⁺ increases in surface waters during exposure to sunlight and attribute this phenomenon either to photoreductive dissolution of ferric minerals/colloids or to ligand-to-metal charge transfer within organic complexes of Fe³⁺. In a multi-summer study of iron redox cycling in a relatively high pH stream (Middle Crow Creek, MCC) that drains a mostly-granitic watershed at an altitude of 2400 m, aqueous Fe³⁺ (not Fe²⁺) concentrations were correlated with both sunlight and temperature. A steady state model fails to explain the [Fe²⁺] and [Fe³⁺] data from this stream. However, Fe²⁺ concentrations can be explained using a simple kinetic model in which rate constants for oxidation and reduction were obtained by fitting data from in situ oxidation experiments, including first-order thermal (nonphotochemical) reduction of Fe³⁺. Rate constants obtained from experiments in the dark result in too much Fe²⁺ to match the data from illuminated experiments, requiring a net photooxidation process to explain [Fe³⁺] measured in MCC. The organic content of MCC results in high concentrations of Fe–DOM complexes that not only act as a reservoir contributing to daily changes in [Fe_tot] as measured by our methods, but whose photochemistry may contribute highly oxidizing reactive oxygen species to the stream. In situ studies suggest that photochemical reduction of organically bound Fe³⁺ occurs, followed by thermal release of Fe²⁺ to the water column and subsequent rapid re-oxidation.  相似文献   

Changes to the Editorial Board: meeting the challenges of a changing landscape in international science     

Bernd Lehmann  Patrick J. Williams 《Mineralium Deposita》2010,45(1):1-1

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Hydrodynamics in Holocene Lake Mega-Chad     

Frédéric Bouchette  Mathieu Schuster  Claude Roquin  Patrick Marsaleix 《Quaternary Research》2010,73(2):226-236

Holocene Lake Mega-Chad (LMC) was the largest late Quaternary water-body in Africa. The development of this giant paleo-lake is related to a northward shift of the isohyetes interpreted as evidence for an enhanced Monsoon (African Humid Period). Numerous preserved coastal features have been described all around the LMC shore. Such features reveal the main paleo-hydrodynamical tendencies. In the context of a closed water-body like LMC, hydrodynamics are forced mainly by winds. We use a three-dimensional numerical model (SYMPHONIE) to simulate the mean hydrodynamics in LMC under both Harmattan-like (northeasterly trade winds) and Monsoon-like (southwesterly winds) forcings. The northern part of LMC displays coastal features, such as sand spits, that are consistent with the simulations forced by Harmattan-like winds. Geomorphic features related to Monsoon-driven hydrodynamics are not clearly expressed. They could have developed during the early stage of LMC but subsequently reworked. At the time of sand-spit building, Harmattan-like driven hydrodynamics prevailed and related coastal features were preferentially preserved in the sedimentary record.  相似文献   

Towards Usability Engineering for Online Editors of Volunteered Geographic Information: A Perspective on Learnability     

Catherine E. Jones  Patrick Weber 《Transactions in GIS》2012,16(4):523-544

This article presents one of the first systematic usability investigations for a Volunteered Geographic Information (VGI) editor. This research is motivated by the fact that although VGI is now widely consumed, contribution rates are lagging considerably. Compared to traditional GIS interfaces, with complex interfaces resulting in high cognitive loads and barriers to participation, VGI tools and interfaces need to be easy to use and learn to encourage and facilitate contributions. This article develops a case study of OpenStreetMap, one of the most successful VGI projects, and its default editor, Potlatch2. Ten participants with no prior experience of VGI contribution, were instructed to contribute data to OSM in a structured exercise, while being monitored using an eye tracker and audio/video screen capture software. Each participant was asked to Think Aloud, i.e. describe what they were thinking and doing as they completed the tasks. The results highlight significant usability issues impacting learnability, especially from the perspective of a new contributor: hidden functionality, lack of user feedback between interactions and the inefficient and inconsistent placement of functionality and map controls. The facilitation of VGI contributions clearly depends on designing targeted interfaces, optimized to the needs of specific levels of contributors with defined goals and expectations.  相似文献   

Stable isotope (δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N) values of sediment organic matter in subtropical lakes of different trophic status     

Isabela C. Torres  Patrick W. Inglett  Mark Brenner  William F. Kenney  K. Ramesh Reddy 《Journal of Paleolimnology》2012,47(4):693-706

Lake sediments contain archives of past environmental conditions in and around water bodies and stable isotope analyses (δ¹³C and δ¹⁵N) of sediment cores have been used to infer past environmental changes in aquatic ecosystems. In this study, we analyzed organic matter (OM), carbon (C), nitrogen (N), phosphorus (P), and δ¹³C and δ¹⁵N values in sediment cores from three subtropical lakes that span a broad range of trophic state. Our principal objectives were to: (1) evaluate whether nutrient concentrations and stable isotope values in surface deposits reflect modern trophic state conditions in the lakes, and (2) assess whether stratigraphic changes in the measured variables yield information about shifts in trophic status through time, or alternatively, diagenetic changes in sediment OM. Three Florida (USA) lakes of very different trophic status were selected for this study. Results showed that both δ¹³C and δ¹⁵N values in surface sediments of the oligo-mesotrophic lake were relatively low compared to values in surface sediments of the other lakes, and were progressively lower with depth in the sediment core. Sediments of the eutrophic lake had δ¹³C values that declined upcore, whereas δ¹⁵N values increased toward the sediment surface. The eutrophic lake displayed δ¹³C values intermediate between those in the oligo-mesotrophic and hypereutrophic lakes. Sediments of the hypereutrophic lake had relatively higher δ¹³C and δ¹⁵N values. In general, we found greater δ¹³C and δ¹⁵N values with increasing lake trophic state.  相似文献   

The unique frequency spectrum of the Blazhko RRc Star LS Her     

Patrick Wils  Stelios Kleidis  Eric Broens 《Monthly notices of the Royal Astronomical Society》2008,387(2):783-787

The Blazhko effect in RR Lyrae stars is still poorly understood theoretically. Stars with multiple Blazhko periods or in which the Blazhko effect itself varies are particularly challenging. This study investigates the Blazhko effect in the RRc star LS Her. Detailed CCD photometry in the   V , R _C  and I _C band has been performed on 63 nights during six months. LS Her is confirmed to have a Blazhko period of  12.75 ± 0.02  d. However, where normally the side frequencies of the Blazhko triplet are expected, an equidistant group of three frequencies is found on both sides of the main pulsation frequency. As a consequence, the period and amplitude of the Blazhko effect itself vary in a cycle of  109 ± 4  d. LS Her is a unique object turning out to be very important in the verification of the theories for the Blazhko effect.  相似文献   

Narrow associated quasi-stellar object absorbers: clustering, outflows and the line-of-sight proximity effect     

Vivienne Wild  Guinevere Kauffmann  Simon White  Donald York  Matthew Lehnert  Timothy Heckman  Patrick B. Hall  Pushpa Khare  Britt Lundgren  Donald P. Schneider  Daniel Vanden Berk 《Monthly notices of the Royal Astronomical Society》2008,388(1):227-241

Using data from the Sloan Digital Sky Survey data release 3 (SDSS DR3), we investigate how narrow (<700 km s⁻¹) C  iv and Mg  ii quasar absorption-line systems are distributed around quasars. The C  iv absorbers lie in the redshift range 1.6 < z < 4 and the Mg  ii absorbers in the range 0.4 < z < 2.2. By correlating absorbers with quasars on different but neighbouring lines of sight, we measure the clustering of absorbers around quasars on comoving scales between 4 and 30 Mpc. The observed comoving correlation lengths are   r _o∼ 5 h ⁻¹Mpc  , similar to those observed for bright galaxies at these redshifts. Comparing correlations between absorbers and the quasars, in whose spectra they are identified, then implies: (i) that quasars destroy absorbers to comoving distances of ∼300 kpc (C  iv ) and ∼800 kpc (Mg  ii ) along their lines of sight; (ii) that ≳40 per cent of C  iv absorbers within 3000 km s⁻¹ of the quasi-stellar object are not a result of large-scale clustering but rather are directly associated with the quasar itself; (iii) that this intrinsic absorber population extends to outflow velocities of the order of 12 000 km s⁻¹; (iv) that this outflow component is present in both radio-loud and radio-quiet quasars and (v) that a small high-velocity outflow component is also observed in the Mg  ii population. We also find an indication that absorption systems within 3000 km s⁻¹ are more abundant for radio-loud quasars than for radio-quiet quasars. This suggests either that radio-loud objects live in more massive haloes, or that their radio activity generates an additional low-velocity outflow, or both.  相似文献   

Secondary and sesquinary craters on Europa     

Kevin Zahnle  Jose L. Alvarellos  Patrick Hamill 《Icarus》2008,194(2):660-674

We address impact cratering on Io and Europa, with the emphasis on the origin of small craters on Europa as secondary to the primary impacts of comets on Io, Europa, and Ganymede. In passing we also address the origin of secondary craters generated by Zunil, a well-studied impact crater on Mars that is a plausible analog to impact craters on Io. At nominal impact rates, and taking volcanic resurfacing into account, we find that there should be 1.3 impact craters on Io, equally likely to be of any diameter between 100 m and 20 km. The corresponding model age of Europa's surface is between 60 and 100 Ma. This range of ages does not include a factor three uncertainty stemming from the uncertain sizes and numbers of comets. The mass of basaltic impact ejecta from Io to reach Europa is found to meet or exceed the micrometeoroid flux as a source of rock-forming elements to Europa's ice crust. To describe impact ejecta in more detail we adapt models for impact-generated spalls and Grady-Kipp fragments originally developed by Melosh. Our model successfully reproduces the observed size-number distributions of small craters on both Mars and Europa. However, the model predicts that a significant fraction of the 200-500 m diameter craters on Europa are not traditional secondary craters but are instead sesquinary craters caused by impact ejecta from Io that had gone into orbit about Jupiter. This prediction is not supported by observation, which implies that high speed spalls usually break up into smaller fragments that make smaller sesquinary craters. Iogenic basalts are also interesting because they provide stratigraphic horizons on Europa that in principle could be used to track historic motions of the ice, and they provide materials suitable to radiometric dating of Europa's surface.  相似文献   

Dust devil speeds, directions of motion and general characteristics observed by the Mars Express High Resolution Stereo Camera     

Christina Stanzel  Martin Pätzold  Patrick L. Whelley  Ronald Greeley  the HRSC Co-Investigator Team 《Icarus》2008,197(1):39-51

A total of 205 dust devils were detected in 23 High Resolution Stereo Camera (HRSC) images taken between January 2004 and July 2006 with the ESA Mars Express orbiter, in which average dust devil heights were ∼660 m and average diameters were ∼230 m. For the first time, dust devil velocities were directly measured from orbit, and range from 1 to 59 m/s. The observed dust devil directions of motion are consistent with data derived from a General Circulation Model (GCM). In some respects HRSC dust devil properties agree favorably with data from the NASA Mars Exploration Rover Spirit dust devil analyses. The spatial distribution of the active dust devils detected by HRSC supports the conjecture that the ascending branch of the Hadley circulation is responsible for the increase in dust devil activity, especially observed during southern summer between 50° and 60° S latitude. Combining the dust-lifting rate of 19 kg/km²/sol derived from the Spirit observations with the fewer in number but larger in size dust devils from various other locations observed by HRSC, we suggest that dust devils make a significant contribution to the dust entrainment into the atmosphere and to the martian dust cycle.  相似文献