bluuebunny/crossref_metadata_2025 huggingface dataset:
Search for small trans-Neptunian objects by the TAOS project
Authors: Chen W.P. , Alcock C. , Axelrod T. , Bianco F.B. , Byun Y.I. , Chang Y.H. , Cook K.H. , Dave R. , Giammarco J. , Kim D.W. , King S.K. , Lee T. , Lehner M. , Lin C.C. , Lin H.C. , Lissauer J.J. , Marshall S. , Meinshausen N. , Mondal S. , de Pater I. , Porrata R. , Rice J. , Schwamb M.E. , Wang A. , Wang S.Y. , Wen C.Y. , Zhang Z.W.
Published: May 2007

The Taiwan-America Occultation Survey (TAOS) aims to determine the number of small icy bodies in the outer reach of the Solar System by means of stellar occultation. An array of 4 robotic small (D=0.5 m), wide-field (f/1.9) telescopes have been installed at Lulin Observatory in Taiwan to simultaneously monitor some thousand of stars for such rare occultation events. Because a typical occultation event by a TNO a few km across will last for only a fraction of a second, fast photometry is necessary. A special CCD readout scheme has been devised to allow for stellar photometry taken a few times per second. Effective analysis pipelines have been developed to process stellar light curves and to correlate any possible flux changes among all telescopes. A few billion photometric measurements have been collected since the routine survey began in early 2005. Our preliminary result of a very low detection rate suggests a deficit of small TNOs down to a few km size, consistent with the extrapolation of some recent studies of larger (30–100 km) TNOs.
Physical models of asteroids from sparse photometric data
Authors: Ďurech Josef , Scheirich Petr , Kaasalainen Mikko , Grav Tommy , Jedicke Robert , Denneau Larry
Published: May 2007

We present an overview of our work on shape and spin state determination of asteroids from photometric data sparse in time. Our results are based on simulations that were performed using realistic shape and light-scattering models and time sequences that will be provided by Pan-STARRS (Panoramic Survey Telescope and Rapid Response System). We show some typical examples of physical model reconstruction of main belt and near-Earth asteroids and discuss the lightcurve inversion of slow and fast rotators, binary asteroids and tumbling asteroids. We emphasize the scientific potential of sparse photometric data to produce models of a large number of asteroids within the next few years.
X-ray fluorescence modelling for Solar system regoliths: Effects of viewing geometry, particle size, and surface roughness
Authors: Näränen Jyri , Parviainen Hannu , Muinonen Karri
Published: May 2007

Soft X-ray fluorescent emission from the surfaces of asteroids and other atmosphereless solar-system objects is studied using ray-tracing techniques. X-ray observations allow the assessment of the elemental composition and structure of the surface. The model regolith is assumed to consist of close-packed uniformly distributed spherical particles of equal size. The surface is also assumed to be rough according to a fractional-Brownian-motion model. The fluorescent X-ray emission from regolith surfaces is simulated in order to better understand the contribution of viewing-geometry -related phenomena on the signal obtained from, e.g., orbiting platforms. The first results are presented and the applicability of the methods to the interpretation of future asteroid and Mercury mission X-ray data (e.g., BepiColombo) is discussed.
Single and multiple solution algorithms to scan asteroid databases for identifications
Authors: Sansaturio Maria E. , Arratia O.
Published: May 2007

The process of cataloguing the minor planet population of the Solar System has experienced a great advance in the last decades with the start-up of several surveys. The large volume of data generated by them has increased with time and given rise to huge databases of asteroids with uneven qualities.

In fact, a significant fraction of these objects have not been enough observed, thus leading to the computation of very poor quality orbits as to carry out useful predictions of the positions of such asteroids. As a result, some objects can get lost, which is particularly embarrassing for those with Earth crossing orbits.

When this situation persists for a long time, the aforementioned databases end up contaminated in the sense that they contain more than one discovery for the same physical object and some kind of action must be taken. The algorithms for asteroid identifications are thought precisely to mitigate this problem and their design will depend upon the quality of the available data for the objects to be identified.

In this paper we will distinguish two cases: when both objects have a nominal orbit and when one of them lacks it. In addition, when the available data poorly constrain the solution, other orbits in the neighbourhood of the nominal one are also compatible with the observations. Using these alternative orbits allows us to find many identifications that otherwise would be missed. Finally, we will show the efficiency of all these algorithms when applied to the datasets distributed by the Minor Planet Center.
Keplerian consequences of an impact on an asteroid and their relevance for a deflection demonstration mission
Authors: Rathke Andreas , Izzo Dario
Published: May 2007

We investigate upon the change of an asteroid orbit caused by an impact. We find that, given the assumption of two dimensional motion, the asteroid displacement may be described by an analytic and explicit expression that is the vectorial sum of a radial component and a component along the asteroid velocity. The new formulation bridges the gap between the study of short-term effects, using numerical methods and the analytic study of secular changes of the asteroid orbit. The relation of the method to the established formulations is described and the known results are derived as limiting cases.

The application of the new method for the performance evaluation of an asteroid deflection demonstration mission is illustrated. In such a mission the measurement of the change of the asteroid orbit by an impact will be conducted by radio-ranging to a spacecraft orbiting the deflected asteroid. Hence the measurement will primarily be sensitive to the deflection projected onto the Earth-asteroid line of sight. We discuss how the new formulation of the deflection can conveniently be employed for the estimation of the measurement accuracy and the optimal planning of a deflection demonstration mission.
The cometary impactor flux at the Earth
Authors: Weissman Paul R.
Published: May 2007

Comets account for a small but very significant fraction of impactors on the Earth. Although the total number of Earth-crossing comets is modest as compared with asteroids, the more eccentric and inclined orbits of the comets result in much higher encounter velocities with the planet. Additionally, some Earth-crossing comets are significantly larger than any current near-Earth asteroids (NEAs); comets 1P/Halley and C/1995 O1 Hale-Bopp are good examples of this. Thus, the most energetic impacts on the Earth likely result from comets and not NEAs. The mean impact probability for long-period comets is 2.4× 10−9per comet per perihelion passage, assuming the perihelion distribution of Everhart (1967), with a most probable encounter velocity of 53.5 km sec−1. There are 21 known Earth-crossing Jupiter-family comets with a mean impact probability of 1.6× 10−9per comet per year and a most probable encounter velocity of 17.0 km sec−1. For the 16 known Earth-crossing Halley-type comets the mean impact probability is 1.2× 10−10per year with a most probable encounter velocity of 51.3 km sec−1. The poor knowledge of the size distribution of cometary nuclei makes it difficult to estimate actual impact energies at this time, though that situation is slowly improving, in particular for the Jupiter-family comets.
The IAU Role
Authors: Engvold Oddbjørn
Published: May 2007

One knows currently close to 850 Near Earth Asteroids (NEAs) with diameters 1 km and larger, and one estimates that there may be of the order of 100 000 NEAs with diameters exceeding 140 m. Land and water impacts of NEAs with diameters between 100 m and 500 m will cause major damages.
Near Earth Object impact simulation tool for supporting the NEO mitigation decision making process
Authors: Bailey Nick J. , Swinerd Graham G. , Morley Andrew D. , Lewis Hugh G.
Published: May 2007

This paper describes the development of a computer simulation tool, NEOSim, capable of modelling small NEO impacts and their effect on the global population. The development of the tool draws upon existing models for the atmospheric passage and impact processes. Simulation of the land and ocean impact effects, combined with a population density model, leads to casualty estimation at both a regional and global level. Casualty predictions are based upon the intensity of each impact effect on the local population density, with consideration given to the population inside or outside local infrastructure. Two case studies are presented. The first evaluates the potential threat to the UK, and highlights coastal locations as being at greatest risk. Locations around Cornwall demonstrate an increase in casualties above the local average. The second case study concerns the potential impact of asteroid (99942) Apophis in 2036. Propagation of the possible orbits along the line of variance leads to an extensive path of risk on the Earth. Deflection of the asteroid, by a variety of means, will move the projected impact site along this path. Results generated by NEOSim for the path indicate that South American countries such as Colombia and Venezuela are at a greatest risk with estimated casualty figures in excess of 10 million. Applications of this software to the NEO threat are discussed, along with the next stage of NEO impact simulation development.
Modeling the effect of nonuniform sediment on the dynamics of offshore tidal sandbanks
Authors: Roos Pieter C. , Wemmenhove Rik , Hulscher Suzanne J. M. H. , Hoeijmakers Harry W. M. , Kruyt N. P.
Published: May 2007

Tidal sandbanks are large‐scale bed features present in many shallow shelf seas. Here we investigate the effect of nonuniform sediment on their dynamics, with a particular aim to explain observed surficial grain size variations over tidal sandbanks from a process‐based modeling perspective. To this end, we use a linear stability analysis that describes the positive feedback mechanism between hydrodynamics, sediment, and the seabed responsible for sandbank formation on a horizontal shelf. In this model the sediment transport and bed evolution modules are extended by introducing an active layer and a bimodal sediment mixture. We include a dynamic hiding/exposure description of sediment transport, enhancing the transport of coarse grains and inhibiting the transport of finer grains. The model results show that for symmetrical tidal conditions, coarse grains tend to accumulate at the bank crests. Moreover, the growth rates of the perturbations increase compared to the case of uniform sediment, while the preferred wavelength and bank orientation remain unchanged. For asymmetrical tidal conditions we find a spatial phase shift between topography and the mean grain size fraction, indicating an accumulation of coarse grains on the lee side of the bank. The model results qualitatively agree with observations from banks on the Belgian continental shelf.
Controls on soil methane fluxes: Tests of biophysical mechanisms using stable isotope tracers
Authors: von Fischer Joseph C. , Hedin Lars O.
Published: May 2007

Understanding factors that control methane exchange between soils and the atmosphere remains one of the highest priorities for climate change research. Here we use a novel isotope‐based technique to investigate the relative importance of three mechanisms for explaining landscape‐scale variations in soil methane emissions: (1) consumption of methane by methanotrophic bacteria, (2) quantity of carbon mineralization, or (3) relative amounts of carbon flow through nonmethanogenic versus methanogenic mineralization pathways. Application of a new, nondisruptive, 13CH4 isotope pool dilution technique permitted us to evaluate these mechanisms by distinguishing gross methane fluxes through both productive and consumptive pathways. We quantified each of these pathways in surface soils across broad moisture gradients in tropical montane environments in the Hawaiian Islands and temperate ecosystems in the northeastern United States. We found only limited support for the consumption control hypothesis because consumption was only important in dry soils. We also failed to find support for the carbon supply hypothesis, in that rates of carbon mineralization did not explain the observed variability in net fluxes across landscapes. Rather, dramatic differences in methane production, and thus emission, depended on surprisingly small diversions of soil carbon flow from nonmethanogenic to methanogenic pathways: on average, soils were a net source of methane to the atmosphere if more than 0.04% of total carbon mineralization passed through methanogenic pathways. We infer that fine‐scale heterogeneity of soil redox status is critical for regulating soil methane fluxes.
Regulation of atmospheric CO2 by deep‐sea sediments in an Earth system model
Authors: Ridgwell Andy , Hargreaves J. C.
Published: May 2007

We have extended the GENIE‐1 Earth system model to include a representation of sedimentary stratigraphy and the preservation of biogenic carbonates delivered to the ocean floor. This has enabled us to take a novel approach in diagnosing modern marine carbon cycling: assimilating observation of the calcium carbonate (CaCO3) content of deep‐sea sediments with an ensemble Kalman filter. The resulting calibrated model predicts a mean surface sediment content (32.5 wt%) close to the observed value (34.8 wt%), and a global burial rate of CaCO3 in deep sea sediments of 0.121 PgC yr−1, in line with recent budget estimates of 0.10−0.14 PgC yr−1. We employ the GENIE‐1 model in quantifying the multimillennial‐scale fate of fossil fuel CO2 emitted to the atmosphere. In the absence of any interaction between ocean and sediments, an equilibrium partitioning of CO2 is reached within ∼1000 years of emissions ceasing, with 34% (645 ppm) remaining in the atmosphere out of a total fossil fuel burn of 4173 PgC. An additional 12% of CO2 emissions (223 ppm) are sequestered as bicarbonate ions (HCO3−) by reaction with deep‐sea carbonates (“seafloor CaCO3 neutralization”) on a timescale of ∼1.7 ka. Excess of carbonate weathering on land over deep‐sea burial results in a further net transformation of 14% of CO2 emissions (261 ppm) into HCO3− (“terrestrial CaCO3 neutralization”) on a timescale of ∼8.3 ka. We have also assessed the importance of a changing climate in modulating the stabilization of atmospheric CO2 through ocean‐sediment interaction. Increased ocean stratification suppresses particulate organic carbon export, which in turn enhances seafloor CaCO3 preservation. The resulting reduction in the sequestration of fossil fuel CO2 represents a new positive feedback on millennial‐scale climate change.
Earth's anthrobiogeochemical copper cycle
Authors: Rauch J. N. , Graedel T. E.
Published: May 2007

An “anthrobiogeochemical” copper cycle, from Earth's core to the Moon, combining natural biogeochemical and human anthropogenic stocks and flows is derived for the mid‐1990s. Although some aspects of the quantification have moderate to high uncertainty, the anthropogenic mining, manufacturing, and use flows (on the order of 104 Gg Cu/yr) clearly dominate the cycle. In contrast, the natural repositories of Earth's core, mantle, and crust, and of the Moon, hold much higher stocks of copper (>1010 Gg) than do anthropogenic repositories (<106 Gg). The results indicate that the present anthropogenic rate of copper extraction exceeds the natural rate of renewal by ∼106.
Carbon‐cycle feedbacks increase the likelihood of a warmer future
Authors: Matthews H. Damon , Keith David W.
Published: May 2007

Positive carbon‐cycle feedbacks have the potential to reduce natural carbon uptake and accelerate future climate change. In this paper, we introduce a novel approach to incorporating carbon‐cycle feedbacks into probabilistic assessments of future warming. Using a coupled climate‐carbon model, we show that including carbon‐cycle feedbacks leads to large increases in extreme warming probabilities. For example, for a scenario of CO2 stabilization at 550 ppm, the probability of exceeding 2°C warming at 2100 increased by a factor of between 1.7 and 3, while the probability of exceeding 3°C warming increased from a few percent to as much as 22%. CO2 fertilization was found to exert little influence on the amount of future warming, since increased carbon uptake was partially offset by fertilization‐induced surface albedo changes. The effect of positive carbon‐cycle feedbacks on the likelihood of extreme future warming must be incorporated into climate policy‐related decision making.
Ring current at Saturn: Energetic particle pressure in Saturn's equatorial magnetosphere measured with Cassini/MIMI
Authors: Sergis N. , Krimigis S. M. , Mitchell D. G. , Hamilton D. C. , Krupp N. , Mauk B. M. , Roelof E. C. , Dougherty M.
Published: May 2007

The Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft provides measurements of the energetic ion population within the magnetosphere of Saturn. Energetic ion directional intensities, energy spectra and ion composition, are measured by the Charge Energy Mass Spectrometer (CHEMS) over the range ∼3 to 236 keV per charge and by the Low Energy Magnetospheric Measurements System (LEMMS) for ions in the range 0.024 < E < 18 MeV. This work reports preliminary results of partial particle pressure distributions throughout the equatorial magnetosphere and comparison with in situ measurements of the magnetic pressure provided by Cassini's magnetometer. The results cover 11 passes from late 2005 to early 2006, when the spacecraft was particularly close to the nominal magnetic equator in the range 5 < R < 20 RS and can be summarized as follows: (1) the plasma β (particle pressure/magnetic pressure) profile increases radially outward to maximum values of ≥ 1 at L >10 RS; (2) most particle pressure is contained in the range of 10 < E < 150 keV; and (3) in the high beta region 10 < L < 19, where the apparent ring current resides, oxygen generally contributes more than 50% of the total particle pressure. The results demonstrate that typical assumptions of MHD models, whereby particle pressure is presumed to reside with the cold plasma, are not supported by the data.
Observations of turbulent mixing and hydrography in the marginal ice zone of the Barents Sea
Authors: Sundfjord Arild , Fer Ilker , Kasajima Yoshie , Svendsen Harald
Published: May 2007

Measurements of hydrography, currents, microstructure shear, and temperature were made at ice drift stations in the marginal ice zone (MIZ) of the northern Barents Sea. Highly variable mixing regimes were observed within and below the pycnocline. Elevated turbulent dissipation (5−15 × 10−7 W kg−1) was associated with strong vertical shear between the surface layer and the subsurface currents, as well as strong tidal flow over shallow topography. Dissipation in the pycnocline was enhanced at stations with strong wind forcing. During drifts under relatively calm wind and away from strong fronts and abrupt topography, station‐mean dissipation values were up to a factor 50 lower and double diffusion contributed significantly to the vertical heat flux where hydrography favored diffusive layering. Independent measures of turbulent length scale from density overturns compared well with those inferred from the dissipation measurements. The variability of dissipation was better captured using a scaling by shear suggested for shelves rather than shear variance models appropriate to the deep open ocean. Sufficiently resolved patches of enhanced temperature microstructure used in combination with dissipation measurements suggest mixing efficiency of Rf ∼ 0.2 for patches stable to double diffusion, comparable to the conventional upper bound of Rf ∼ 0.17. Mixing efficiency for double‐diffusive convection favorable cases is found to be significantly larger, Rf ∼ 0.36. Water mass modification and fluxes of nutrients and dissolved carbon were found to have large local variability in accordance with the observed variability of vertical mixing in the MIZ.
Uncertainties in the mean ocean dynamic topography before the launch of the Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE)
Authors: Vossepoel Femke C.
Published: May 2007

In anticipation of the future observations of the gravity mission Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE), the present‐day accuracy of mean dynamic topography (MDT) is estimated from both observations and models. A comparison of five observational estimates illustrates that RMS differences in MDT vary from 4.2 to 10.5 cm after low‐pass filtering the fields with a Hamming window with wavenumberN= 120 (corresponding to an effective resolution of 167 km). RMS differences in observational MDT reduce to 2.4–8.3 cm forN= 15 (1334 km). Differences in data sources (geoid model, in situ data) are mostly visible in the small‐scale oceanic features, while differences in processing (filtering, inverse modeling techniques) are reflected at larger scales. A comparison of seven different numerical ocean models demonstrates that model estimates differ mostly in western boundary currents and in the Antarctic Circumpolar Current. RMS differences between modeled and observed MDT are at best 8.8 cm forN= 120, and reduce to 6.4 cm forN= 15. For models with data assimilation, a minimal RMS difference of 6.6 cm (N= 120) to 3.4 cm (N= 15) is obtained with respect to the observational MDTs. The reduction of differences between MDTs with increasing filtering scales is smaller than expected. While it is expected that GOCE will improve MDT estimates at small spatial scales, improvement of mean sea surface estimates from satellite altimetry may be needed to improve MDT estimates at larger scales.
Statistical inference of OH concentrations and air mass dilution rates from successive observations of nonmethane hydrocarbons in single air masses
Authors: Arnold S. R. , Methven J. , Evans M. J. , Chipperfield M. P. , Lewis A. C. , Hopkins J. R. , McQuaid J. B. , Watson N. , Purvis R. M. , Lee J. D. , Atlas E. L. , Blake D. R. , Rappenglück B.
Published: May 2007

Bayesian inference has been used to determine rigorous estimates of hydroxyl radical concentrations () and air mass dilution rates (K) averaged following air masses between linked observations of nonmethane hydrocarbons (NMHCs) spanning the North Atlantic during the Intercontinental Transport and Chemical Transformation (ITCT)‐Lagrangian‐2K4 experiment. The Bayesian technique obtains a refined (posterior) distribution of a parameter given data related to the parameter through a model and prior beliefs about the parameter distribution. Here, the model describes hydrocarbon loss through OH reaction and mixing with a background concentration at rate K. The Lagrangian experiment provides direct observations of hydrocarbons at two time points, removing assumptions regarding composition or sources upstream of a single observation. The estimates are sharpened by using many hydrocarbons with different reactivities and accounting for their variability and measurement uncertainty. A novel technique is used to construct prior background distributions of many species, described by variation of a single parameter α. This exploits the high correlation of species, related by the first principal component of many NMHC samples. The Bayesian method obtains posterior estimates of , K and α following each air mass. Median values are typically between 0.5 and 2.0 × 106 molecules cm−3, but are elevated to between 2.5 and 3.5 × 106 molecules cm−3, in low‐level pollution. A comparison of estimates from absolute NMHC concentrations and NMHC ratios assuming zero background (the “photochemical clock” method) shows similar distributions but reveals systematic high bias in the estimates from ratios. Estimates of K are ∼0.1 day−1 but show more sensitivity to the prior distribution assumed.
Estimates of Cl atom concentrations and hydrocarbon kinetic reactivity in surface air at Appledore Island, Maine (USA), during International Consortium for Atmospheric Research on Transport and Transformation/Chemistry of Halogens at the Isles of Shoals
Authors: Pszenny Alexander A. P. , Fischer Emily V. , Russo Rachel S. , Sive Barkley C. , Varner Ruth K.
Published: May 2007

Average hydroxyl radical (OH) to chlorine atom (Cl·) ratios ranging from 45 to 119 were determined from variability‐lifetime relationships for selected nonmethane hydrocarbons (NMHC) in surface air from six different transport sectors arriving at Appledore Island, Maine, during July 2004. Multiplying these ratios by an assumed average OH concentration of 2.5 × 106 cm−3 yielded estimates of Cl· concentrations of 2.2 to 5.6 × 104 cm−3. Summed reaction rates of methane and more than 30 abundant NMHCs with OH and Cl· suggest that Cl· reactions increased the kinetic reactivity of hydrocarbons by 16% to 30% over that due to OH alone in air associated with the various transport sectors. Isoprene and other abundant biogenic alkenes were the most important hydrocarbon contributors after methane to overall kinetic reactivity.
Thermal emission spectra of terrestrial alkaline volcanic rocks: Applications to Martian remote sensing
Authors: Dunn Tasha L. , McSween Harry Y. , Christensen Philip R.
Published: May 2007

The long‐standing assessment that Martian igneous compositions are strictly subalkaline has recently been brought into question by the detection of alkaline rocks at Gusev crater. In this study, we assess the utility of thermal emission spectra (5−25 μm at 10 cm−1 spectral sampling) in identifying and classifying terrestrial alkalic volcanic rocks, and then apply our results to MGS/TES data. Using spectral linear deconvolution, we determined modal mineralogies of a suite of alkaline rocks (basalts, trachyandesites, trachytes, rhyolites). We then compared modeled data to modes measured using an electron microprobe mapping technique. Most phases modeled well (within 10 vol%). Modeled plagioclase abundances, however, were too high, reflecting the presence of devitrified glass and the absence of appropriate glass end‐members in the spectral library. Systematic inaccuracies in bulk‐rock chemistries, derived by combining modeled mineral end‐member compositions in proportion to their abundances, reflect the overdetermination of plagioclase and resulting mass balance problems. Nevertheless, modeled mineralogies and derived bulk‐rock chemistries clearly reveal the alkaline character of these rocks, and many rocks are correctly classified petrologically. To determine if alkaline rocks can be detected in global and regional TES spectra, we deconvolved ST1, ST2, and four compositionally distinct regional spectral classes using our end‐member library. Our results suggest that alkaline rocks are not readily detected in most regionally‐ and globally‐derived TES spectra. We do not rule out the possibility that areas of regional‐scale alkalic volcanism exist, although they are more likely to be identified by in situ surface investigations.
Methane thermodynamics in nanoporous ice: A new methane reservoir on Titan
Authors: Voss L. F. , Henson B. F. , Robinson J. M.
Published: May 2007

A porous, icy regolith has been proposed to house a methane/ethane/nitrogen liquid reservoir on Titan. The thermodynamics of such a mesoporous reservoir with a pore diameter fixed at 2 mm are equivalent to the previously proposed surface ocean. This work shows that if such a porous hydrocarbon/nitrogen reservoir contains a significant volume within pores of nanometer dimension, then the modification of methane thermodynamics through the Kelvin effect significantly changes the reservoir‐atmosphere interaction on Titan. We measure the capillary uptake of pure methane liquid into well‐characterized ice grown at temperatures and partial pressures simulating Titan at present. From these results, a model of a nanoporous reservoir housing the methane/ethane/nitrogen solution is constructed. Modifying a previously published analytical climate model for Titan to incorporate such a reservoir, we report calculations of the coupled reservoir‐atmosphere system. This modification leads to a very sensitive dependence of the state of the atmosphere on the total reservoir volume. In particular, assuming a nanoporous volume distribution reported here, calculations show that such a reservoir must comprise at least 1.5% of a 10 km deep ice regolith in order to yield a climate consistent with that currently observed on Titan. In contrast to the mesoporous model, it reduces the sensitivity of the system to temperature, mitigating the runaway greenhouse effect observed in the previous model. Furthermore, the model indicates that in a past epoch a surface ocean subsided into the reservoir as the total surface liquid volume reduced with time.
Channel width response to differential uplift
Authors: Amos Colin B. , Burbank Douglas W.
Published: May 2007

The role of channel width and slope adjustments to differential uplift in rivers within actively deforming terrains remains contentious. Here high‐resolution topographic surveying of formerly antecedent outwash channels demonstrates marked changes in channel width as a primary response to differential uplift. For five Late Quaternary alluvial paleochannels crossing small folds along the active Ostler fault zone of southern New Zealand, nearly continuous measurements of paleochannel width and concomitant incision reveal abrupt narrowing of widths toward minimum values at channel positions coincident with the initial uplift. When the magnitude of differential uplift is sufficiently small, narrowing alone permits these channels to remain antecedent. In the context of a unit stream power model for fluvial erosion, observed limits on the magnitude of channel narrowing suggest that above some threshold amount of differential uplift, continued incision requires concomitant changes in channel gradient. Thus when crossing small growing folds, alluvial rivers simply narrow their channels, whereas larger folds that demand greater incision prompt an initial narrowing followed by channel steepening.
Role of nitrification and denitrification on the nitrous oxide cycle in the eastern tropical North Pacific and Gulf of California
Authors: Yamagishi Hiroaki , Westley Marian B. , Popp Brian N. , Toyoda Sakae , Yoshida Naohiro , Watanabe Shuichi , Koba Keisuke , Yamanaka Yasuhiro
Published: May 2007

Nitrous oxide (N2O) is an important atmospheric greenhouse gas and is involved in stratospheric ozone depletion. Analysis of the isotopomer ratios of N2O (i.e., the intramolecular distribution of 15N within the linear NNO molecule and the conventional N and O isotope ratios) can elucidate the mechanisms of N2O production and destruction. We analyzed the isotopomer ratios of dissolved N2O at a site in the eastern tropical North Pacific (ETNP) and a site in the Gulf of California (GOC). At these sites, the flux of N2O to the atmosphere is extremely high but denitrification activity in the oxygen minimum zone (OMZ) also reduces N2O to N2. We estimated the isotopomeric enrichment factors for N2O reduction by denitrification. The factor was −11.6 ± 1.0‰ for the bulk (average) N, −19.8 ± 2.3‰ for the center N (α‐site nitrogen), −3.4 ± 0.3‰ for the end N (β‐site nitrogen), and −30.5 ± 3.2‰ for the 18O of N2O. Isotopomer analysis of N2O suggests that nitrifiers should contribute to N2O production more than denitrifiers at the oxycline above the OMZs in the ETNP (50–80 m) and in the GOC (80–300 m). In contrast, denitrifiers should largely contribute to the N2O production and consumption in the OMZs both in the ETNP (120–130 m) and in the GOC (600–800 m). The N2O isotopomer analysis will be a useful tool for resolving the distribution of water masses that carry a signal of N loss by denitrification.
Storm event exports of dissolved organic nitrogen (DON) across multiple catchments in a glaciated forested watershed
Authors: Inamdar Shreeram P. , Mitchell Myron J.
Published: May 2007

Storm event patterns of dissolved organic nitrogen (DON) were studied for multiple events across four catchments (1.6–696 ha) in a forested, glaciated watershed in western New York State. Highest concentrations of DON in the watershed were recorded for litter leachate followed by throughfall. Storm event concentrations of DON consistently peaked at or before peak discharge while dissolved organic carbon (DOC) concentrations peaked on the hydrograph recession limbs. Concentrations of DON in stream water were derived from throughfall and litter layer while the DOC expression was attributed to throughfall, litter, and the flushing of the mineral soil by a rising water table. Temporal patterns of ammonium (NH4+) concentrations during events consistently matched those of DON indicating similar sources and flow paths. A previously validated end‐member mixing analysis (EMMA) for NO3− failed to predict the DON concentrations observed in streamflow. DON concentrations and DON as % of total dissolved nitrogen (TDN) differed considerably between baseflow (% DON: 6 to 19%) and storm events (% DON: 6 to 64%). DON concentrations and % DON of TDN increased with catchment size and amount of saturated/wetland areas. A wetland catchment that consistently yielded high storm‐event DOC concentrations produced variable amounts of DON, indicating a decoupling of DOC and DON dynamics in the wetland. Our study suggests that storm events and watershed characteristics, especially the proportion of saturated and wetland areas, may have a greater influence on DON exports than atmospheric N deposition.
Sea surface temperatures of the western Arabian Sea during the last deglaciation
Authors: Saher M. H. , Jung S. J. A. , Elderfield H. , Greaves M. J. , Kroon D.
Published: May 2007

In this study we present a sea surface temperature (SST) record from the western Arabian Sea for the last 20,000 years. We produced centennial‐scale δ18O and Mg/Ca SST time series of core NIOP929 with focus on the glacial‐interglacial transition. The western Arabian Sea is influenced by the seasonal NE and SW monsoon wind systems. Lowest SSTs occur during the SW monsoon season because of upwelling of cold water, and highest SSTs can be found in the low‐productivity intermonsoon season. The Mg/Ca‐based temperature record reflects the integrated SST of the SW and NE monsoon seasons. The results show a glacial‐interglacial SST difference of ∼2°C, which is corroborated by findings from other Arabian Sea cores. At 19 ka B.P. a yet undescribed warm event of several hundred years duration is found, which is also reflected in the δ18O record. A second centennial‐scale high SST/low δ18O event is observed at 17 ka B.P. This event forms the onset of the stepwise yet persistent trend toward Holocene temperatures. Highest Mg/Ca‐derived SSTs in the NIOP929 record occurred between 13 and 10 ka B.P. Interglacial SST is ∼24°C, indicating influence of upwelling. The onset of Arabian Sea warming occurs when the North Atlantic is experiencing minimum temperatures. The rapid temperature variations at 19, 17, and 13 ka B.P. are difficult to explain with monsoon changes alone and are most likely also linked to regional hydrographic changes, such as trade wind induced variations in warm water advection.
Development of three‐dimensional basement structure in Taiwan deduced from past plate motion: Consistency with the present seismicity
Authors: Takada Youichiro , Fukahata Yukitoshi , Hashima Akinori , Terakawa Toshiko , Fukui Kenji , Yanagisawa Takatoshi , Ikeda Yasutaka , Kimura Gaku , Matsu'ura Mitsuhiro
Published: May 2007

Using colored clay, we examined geometrical evolution of the three‐dimensional basement structure in Taiwan due to relative plate motion under the following assumptions: the motion of the Philippine Sea plate relative to the Eurasian plate has been constant during the last 15 Myr, the Ryukyu and Manila trenches had been connected by a transform fault before the collision between the Luzon arc and Eurasian continental margin, and the collision started at 5 Ma. The basement structure obtained from the clay model was fully consistent with observed deep seismicity, which led us to the following conclusions. There are no oceanic slabs beneath central Taiwan. Instead, the Eurasian continental margin is underthrusting beneath it. The Philippine Sea slab extends northwestward from the Ryukyu trench, while the South China Sea slab extends southeastward from the Manila trench. The downdip length of the South China Sea slab becomes shorter from south to north.
Plio‐Quaternary exhumation history of the central Nepalese Himalaya: 1. Apatite and zircon fission track and apatite [U‐Th]/He analyses
Authors: Blythe A. E. , Burbank D. W. , Carter A. , Schmidt K. , Putkonen J.
Published: May 2007

New apatite and zircon fission track and (U‐Th)/He analyses serve to document the bedrock cooling history of the central Nepalese Himalaya near the Annapurna Range. We have obtained 82 apatite fission track (AFT), 7 zircon fission track (ZFT), and 7 apatite (U‐Th)/He (AHe) ages from samples collected along the Marsyandi drainage, including eight vertical relief profiles from ridges on either side of the river averaging more than 2 km in elevation range. In addition, three profiles were sampled along ridge crests that also lie ∼2 km above the adjacent valleys, and a transect of >20 valley bottom samples spans from the Lesser Himalaya across the Greater Himalaya and into the Tethyan strata. As a consequence, these data provide one of the more comprehensive low‐temperature thermochronologic studies within the Himalaya. Conversely, the youthfulness of this orogen is pushing the limits of these dating techniques. AFT ages range from >3.8 to 0 Ma, ZFT ages from 1.9 to 0.8 Ma, and AHe ages from 0.9 to 0.3 Ma. Most ridges have maximum ages of 1.3–0.8 Ma at 2 km above the valley bottom. Only one ridge crest (in the south central zone of the field area) yielded significantly older ZFT and AFT ages of ∼2 Ma; we infer that a splay of the Main Central Thrust separates this ridge from the rest of the Greater Himalaya. ZFT and AFT ages from a vertical transect along this ridge indicate exhumation rates of ∼1.5 km Myr−1(r2> 0.7) from ∼2 to 0.6–0.8 Ma, whereas AHe ages indicate a faster exhumation rate of ∼2.6 km Myr−1(r2= 0.9) over the last 0.8 Myr. Exhumation rates calculated for six of the remaining seven vertical profiles ranged from 1.5 to 12 km Myr−1(all with low r2values of <0.6) for the time period from ∼1.2 to 0.3 Ma, with no discernible patterns in south to north exhumation rates evident. The absence of a trend in exhumation rates, despite a strong spatial gradient in rainfall, argues against a correlation of long‐term exhumation rates with modern patterns of rainfall. AFT ages in the Tethyan strata are, on average, older than in the Greater Himalaya and may be a response to a drier climate, slip on the South Tibetan Detachment, or a gentler dip of the underlying thrust ramp. These data are further evaluated with thermokinematic modeling in the companion paper by Whipp et al.
Plio‐Quaternary exhumation history of the central Nepalese Himalaya: 2. Thermokinematic and thermochronometer age prediction model
Authors: Whipp David M. , Ehlers Todd A. , Blythe Ann E. , Huntington Katharine W. , Hodges Kip V. , Burbank Douglas W.
Published: May 2007

In the Himalaya and other active convergent orogens, linear relationships between thermochronometer sample age and elevation are often used to estimate long‐term exhumation rates. In these regions, high‐relief topography and nonvertical exhumation pathways may invalidate such one‐dimensional (1‐D) interpretations and lead to significant errors. To quantify these errors, we integrate apatite fission track (AFT) ages from the central Himalaya with a 3‐D coupled thermokinematic model, from which sample cooling ages are predicted using a cooling‐rate‐dependent algorithm. By changing the slip partitioning between faults near the Main Central thrust and the Main Frontal thrust system at the Himalayan range front, we are able to explore the significance of different tectonic scenarios on predicted thermochronometer ages. We find that the predicted AFT cooling ages are not sensitive to the different slip partitioning scenarios but depend strongly on erosion rate: Predicted ages are most consistent with kinematic models that produce erosion rates of 1.8–5.0 mm/yr. This range is considerably smaller than that derived from regression lines through the data (−2.6–12.2 mm/yr). The pattern of observed AFT ages shows more complexity than our models predict. None of the kinematic scenarios are able to fit >80% of all of the AFT data, most likely because erosion is spatially variable. Such complexities notwithstanding, we conclude that the use of thermokinematic modeling and thermochronologic data sets to explore detailed fault kinematics in rapidly eroding active orogens has great promise but requires integration of higher‐temperature (>∼350°C) data sets to maximize effectiveness.
A colloid‐facilitated transport model with variable colloid transport properties
Authors: Robinson Bruce A. , Wolfsberg Andrew V. , Viswanathan Hari S. , Reimus Paul W.
Published: May 2007

Anomalous contaminant transport velocities in groundwater for species generally considered to be immobile are often attributed to the mechanism of colloid‐facilitated transport. In some of the field observations attributed to colloid facilitation, an extremely small fraction of the total contaminant mass introduced to the groundwater is detected downstream. In this study, a new model of colloid‐facilitated contaminant transport is proposed that explains this phenomenon as the variability of mobility of individual colloids in the population. The process of retardation via attachment and detachment of colloids on immobile surfaces is often modeled with time and space invariant parameters; here it is modeled assuming a diverse population of transport properties that account for the inherent variability of colloid size, surface charge and chemical properties, mineralogy, and the concomitant impact on colloid mobility. When the contaminant is assumed to irreversibly attach to or form colloids, the migration of the contaminant plume exhibits extremely non‐Fickian behavior. The plume's center of mass travels with a velocity governed by the groundwater velocity divided by the mean colloid retardation factor. However, small quantities of contaminant attached to a few highly mobile colloids travel at velocities up to the groundwater velocity, far exceeding the velocity of the centroid of the plume. This paper introduces the colloid diversity model, presents some sensitivity calculations for an idealized case, and discusses the implications of such a model on data needs, simulation of field observations, and model scaling.
Effects of acute exercise and training on insulin action and sensitivity: focus on molecular mechanisms in muscle
Authors: Wojtaszewski Jørgen F.P. , Richter Erik A.
Published: May 2007

A single bout of exercise increases insulin sensitivity for several hours and the effect is mainly restricted to the muscles recruited during exercise. When exercise is repeated over time, adaptations to physical training occur that include more long-lasting increases in insulin sensitivity. The present review explores the molecular mechanisms involved in both the acute and chronic effects of exercise on insulin sensitivity in skeletal muscle.
Muscle metabolism and control of capillary blood flow: insulin and exercise
Authors: Rattigan Stephen , Bradley Eloise A. , Richards Stephen M. , Clark Michael G.
Published: May 2007

The evidence that muscle metabolism is determined by available capillary surface area is examined. From newly developed methods it is clear that exercise and insulin mediate capillary recruitment as part of their actions in vivo. In all insulin-resistant states examined thus far, insulin-mediated capillary recruitment is impaired with little or no change to the exercise response. Control mechanisms for capillary recruitment for exercise and insulin are considered, and the failure of the microvasculature to respond to insulin is examined for possible mechanisms that might account for impaired vascular responses to insulin in insulin resistance.
Integration of the metabolic and cardiovascular effects of exercise
Authors: Wagenmakers Anton J.M. , van Riel Natal A.W. , Frenneaux Michael P. , Stewart Paul M.
Published: May 2007

Most of the essays in this volume have adopted a reductionist approach and have focused on the biochemistry either in skeletal muscle or in the vascular wall. There is however a complex interaction between the biochemistry in the endothelium of the microvascular wall, the vascular smooth muscle and the skeletal muscle fibres involving signalling pathways in the three tissues and an intense exchange of signal molecules between them. In the present essay an integrative overview is given of this complex metabolic interaction and the impairments in it that lead to type 2 diabetes and cardiovascular disease.A reduced nitric oxide production by the (micro)vascular endothelium is identified as the key event and is reversible by regular exercise and a reduced calorie intake. The chapter also contains a description of the complex metabolic network controlled by the inducible transcription factor nuclear factor-κB, that is activated in more advanced stages of the chronic diseases, and either leads to repair of the microvascular wall or to irreversible damage and the severe complications of end stage cardiovascular disease and type 2 diabetes.
Regulation and enzymatic basis of bone resorption by human osteoclasts
Authors: Fuller Karen , Kirstein Barrie , Chambers Timothy J.
Published: May 2007

Although much has been learned recently of the mechanisms that regulate osteoclastic differentiation, much less is known of the means through which their resorptive activity is controlled. This is especially so for human osteoclasts. We have recently developed an assay that allows us to measure resorptive activity while minimizing confounding effects on differentiation by optimizing osteoclastogenesis, so that measurable resorption occurs over a short period, and by relating resorption in each culture during the test period to the resorption that had occurred in the same culture in a prior control period. In the present study, we found that RANKL (receptor activator of nuclear factor κB ligand) strongly stimulated the release of CTX-I (C-terminal telopeptide degradation product of type I collagen) by osteoclasts over a similar range to that over which it induces osteoclastic differentiation, consistent with a distinct action on osteoclastic function. CT (calcitonin) dose-dependently inhibited bone resorption, whereas PTH (parathyroid hormone), IL (interleukin)-1, TNF-α (tumour necrosis factor-α), IL-6, IL-8, VEGF (vascular endothelial growth factor), MCP-1 (monocyte chemoattractant protein-1), MIP-1γ (macrophage inflammatory protein-1γ), IFN (interferon)-γ and dibutyryl cGMP had no significant effect. Ca2+, cyclosporin A, IFN-β and dibutyryl cAMP all strongly suppressed resorption. Bone resorption was also strongly suppressed by alendronate, the cysteine protease inhibitor E64 and the cathepsin K inhibitor MV061194. Inhibitors of MMPs (matrix metalloproteinases) had no effect on CTX-I release. Moreover, the release of the MMP-derived collagen fragment ICTP (C-terminal cross-linked telopeptide of type I collagen) represented less that 0.01% of the quantity of CTX-I released in our cultures. This suggests that MMPs make, at most, a very small contribution to the bone-resorptive activity of osteoclasts.