Ice lines, planetesimal composition and solid surface density in the solar nebula

To date, there is no core accretion simulation that can successfully account for the formation of Uranus or Neptune within the observed 2–3 Myr lifetimes of protoplanetary disks. Since solid accretion rate is directly proportional to the available planetesimal surface density, one way to speed up planet formation is to take a full accounting of all the planetesimal-forming solids present in the solar nebula. By combining a viscously evolving protostellar disk with a kinetic model of ice formation, which includes not just water but methane, ammonia, CO and 54 minor ices, we calculate the solid surface density of a possible giant planet-forming solar nebula as a function of heliocentric distance and time. Our results can be used to provide the starting planetesimal surface density and evolving solar nebula conditions for core accretion simulations, or to predict the composition of planetesimals as a function of radius. We find three effects that favor giant planet formation by the core accretion mechanism: (1) a decretion flow that brings mass from the inner solar nebula to the giant planet-forming region, (2) the fact that the ammonia and water ice lines should coincide, according to recent lab results from Collings et al. [Collings, M.P., Anderson, M.A., Chen, R., Dever, J.W., Viti, S., Williams, D.A., McCoustra, M.R.S., 2004. Mon. Not. R. Astron. Soc. 354, 1133–1140], and (3) the presence of a substantial amount of methane ice in the trans-saturnian region. Our results show higher solid surface densities than assumed in the core accretion models of Pollack et al. [Pollack, J.B., Hubickyj, O., Bodenheimer, P., Lissauer, J.J., Podolak, M., Greenzweig, Y., 1996. Icarus 124, 62–85] by a factor of 3–4 throughout the trans-saturnian region. We also discuss the location of ice lines and their movement through the solar nebula, and provide new constraints on the possible initial disk configurations from gravitational stability arguments.     

The Virtual Solar Observatory—A Resource for International Heliophysics Research

The Virtual Solar Observatory (VSO) has been developed to allow researchers, educators, and the general public to access data and images from the major sources of on-line solar data. The VSO substantially reduces the effort required to locate disparate data sets, and removes the need for the user to locate the data and learn multiple interfaces. The VSO provides a single interface to about 60 geographically distributed data sets including space- and ground-based sources. These data sets incorporate several physical variables including magnetic field, intensity, Doppler velocity, etc., and all wavelengths from X-ray to radio. All layers of the sun, from the interior to the corona, are included. In this paper we describe the system and present the interface that the user will encounter. We also discuss future enhancements planned for the system.     

Towards integrated governance for water, health and social–ecological systems: The watershed governance prism

This article proposes a shift toward the integrated governance of watersheds as a basis for fostering health, sustainability and social–ecological resilience. The authors suggest that integrated watershed governance is more likely when different perspectives, including health and well-being, are explicitly understood, communicated, and sought as co-benefits of watershed management. A new conceptual device – the watershed governance prism – is introduced in relation to the multiple facets of governance that characterize contemporary water resources management and examined as an integrative framework to link social and environmental concerns with the determinants of health in the watershed context. The authors assess the diagnostic and communicative potential of such a framework, discussing its utility as a concise depiction of multiple, interacting policy priorities and as a guide to integrate different research and policy domains into the governance of water, health and social–ecological systems.     

Temporal evolution of parent volatiles and dust in Comet 9P/Tempel 1 resulting from the Deep Impact experiment

The Deep Impact encounter with the Jupiter family Comet 9P/Tempel 1 on UT 2005 July 4 was observed at high spectral resolving power (λ/δλ∼25,000) using the cross-dispersed near-infrared echelle spectrometer (NIRSPEC) at Keck-2. We report the temporal evolution of parent volatiles and dust (simultaneously measured) resulting from the event. Column abundances are presented for H₂O and C₂H₆ beginning 30 min prior to impact (T−30) and ending 50 min following impact (T+50), and for H₂O and HCN from T+50 until T+96, in time steps of approximately 6 min post-impact. The ejecta composition was revealed by an abrupt increase in H₂O and C₂H₆ near T+25. This showed C₂H₆/H₂O to be higher than its pre-impact value by a factor 2.4±0.5, while HCN/H₂O was unchanged within the uncertainty of the measurements. The mixing ratios for C₂H₆ and HCN in the ejecta agree with those found in the majority of Oort cloud comets, perhaps indicating a common region of formation. The expanding dust plume was tracked by continuum measurements, both through the 3.5-μm spectral continuum and through 2-μm images acquired with the SCAM slit-viewing camera, and each showed a monotonic increase in continuum intensity following impact. A Monte Carlo model that included dust opacity was applied to the dust coma, and its parameters were constrained by observations; the simulated continuum intensities reproduced both spectral and SCAM data. The relatively sudden appearance of the volatile ejecta signature is attributed to heating of icy grains (perhaps to a threshold temperature) that are decreasingly shadowed by intervening (sunward) dust particles in an optically thick ejecta plume, perhaps coupled with an accelerated decrease in dust optical depth near T+25.     

Effects of Groundwater Development on Uranium: Central Valley,California, USA

Uranium (U) concentrations in groundwater in several parts of the eastern San Joaquin Valley, California, have exceeded federal and state drinking water standards during the last 20 years. The San Joaquin Valley is located within the Central Valley of California and is one of the most productive agricultural areas in the world. Increased irrigation and pumping associated with agricultural and urban development during the last 100 years have changed the chemistry and magnitude of groundwater recharge, and increased the rate of downward groundwater movement. Strong correlations between U and bicarbonate suggest that U is leached from shallow sediments by high bicarbonate water, consistent with findings of previous work in Modesto, California. Summer irrigation of crops in agricultural areas and, to lesser extent, of landscape plants and grasses in urban areas, has increased Pco₂ concentrations in the soil zone and caused higher temperature and salinity of groundwater recharge. Coupled with groundwater pumping, this process, as evidenced by increasing bicarbonate concentrations in groundwater over the last 100 years, has caused shallow, young groundwater with high U concentrations to migrate to deeper parts of the groundwater system that are tapped by public-supply wells. Continued downward migration of U-affected groundwater and expansion of urban centers into agricultural areas will likely be associated with increased U concentrations in public-supply wells. The results from this study illustrate the potential long-term effects of groundwater development and irrigation-supported agriculture on water quality in arid and semiarid regions around the world.     

In-situ hafnium and lead isotope analyses of detrital zircons from the Devonian sedimentary basin of NE Greenland: a record of repeated crustal reworking

An intramontane collapse basin developed within the hanging wall above the large-scale extensional Fjord Regional Detachment of NE Greenland in middle to late Devonian times. The continental clastic sediments within the basin are derived locally from Laurentian source rocks, which makes them well suited for a study of the crustal evolution of the source terrain. This is the first integrated in-situ Pb and Hf isotope study to be presented, and zircon data on a selected sandstone from the basin are combined with Sm-Nd whole-rock data on sand/siltstones. Nd whole-rock ages of two samples of sandstones and a siltstone are 2.0-2.1 Ga. Peak frequencies of zircon ²⁰⁷Pb/²⁰⁶Pb ages at 1,764-1,912 Ma, and ¹⁷⁶Hf/¹⁷⁷Hf values at 0.28142-0.28163 (t_DM=2.47 to 2.06) for the sandstone suggest the generation of a considerable volume of juvenile continental crust in the ultimate zircon provenance at 1.9-2.0 Ga. The Hf isotopic compositions of Archaean zircons in the sandstone are distinct from those of the source materials of Proterozoic protocrust at 1.9-2.0 Ga, but zircons with elevated Hf-t_DM ages of up to 2.47 Ga can be related to a component of Archaean crust or reworked Archaean material in the ultimate zircon source area. Zircon ²⁰⁷Pb/²⁰⁶Pb ages are also recorded at 1,480-1,572, 1,318 and 1,014 Ma (Grenvillian). The Hf isotope compositions of these zircons are consistent with reworking of the Proterozoic protocrust at these times, with little or no juvenile input. The Proterozoic zircons form two distinct groups defined by ¹⁷⁶Yb/¹⁷⁷Hf>0.05055 and ¹⁷⁶Yb/¹⁷⁷Hf<0.03301, and the latter group overlaps with Yb-Hf isotope data on the Archaean zircons. The two groups may represent zircons derived from evolved granites and intermediate to mildly felsic rocks, respectively. The repeated reworking of the continental crust also comprised erosion and deposition of sediments in the Proterozoic (the Krummedal sequence and the Eleonore Bay Supergroup, EBS) and intrusion of Caledonian anatectic granites in the EBS, which both represent provenance components to the Devonian sediments. No discrete Caledonian Pb-Pb zircon ages are recorded, but Caledonian magmatism may be represented by strongly discordant zircons which form arrays with a lower intercept age at ca. 400 Ma and an upper intercept at 1,600-2,000 Ma. One undated zircon records a ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.282218, higher than that of the Proterozoic protocrust in Caledonian/late-Caledonian times (380-450 Ma) which may represent a Caledonian mantle contribution.     

Influence of late Holocene hillslope processes and landforms on modern channel dynamics in upland watersheds of central Nevada

Stratigraphic, geomorphic, and paleoecological data were collected from upland watersheds in the Great Basin of central Nevada to assess the relationships between late Holocene climate change, hillslope processes and landforms, and modern channel dynamics. These data indicate that a shift to drier, warmer climatic conditions from approximately 2500 to 1300 YPB led to a complex set of geomorphic responses. The initial response was massive hillslope erosion and the simultaneous aggradation of both side-valley alluvial fans and the axial valley system. The final response was fan stabilization and axial channel incision as fine-grained sediments were winnowed from the hillslope sediment reservoirs, and sediment yield and runoff processes were altered. The primary geomorphic response to disturbance for approximately the past 1900 years has been channel entrenchment, suggesting that the evolutionary history of hillslopes has produced watersheds that are prone to incision. The magnitude of the most recent phase of channel entrenchment varies along the valley floor as a function of geomorphic position relative to side-valley alluvial fans. Radial fan profiles suggest that during fan building, fan deposits temporarily blocked the flow of sediment down the main stem of the valley, commonly creating a stepped longitudinal valley profile. Stream reaches located immediately upvalley of these fans are characterized by low gradients and alternating episodes of erosion and deposition. In contrast, reaches coincident with or immediately downstream of the fans exhibit higher gradients and limited valley floor deposition. Thus, modern channel dynamics and associated riparian ecosystems are strongly influenced by landforms created by depositional events that occurred approximately 2000 years ago.     

Lowermost mantle anisotropy beneath the Pacific: Imaging the source of the Hawaiian plume

We utilized recordings of seismic shear phases provided by several North American broadband seismometer arrays to provide unique constraints on shear wave anisotropy beneath the northern and central Pacific Ocean. Using a new analysis method that reduces measurement errors and enables the analysis of a larger number of available waveforms, we examined relative travel times of teleseismic S and Sdiff that sample a large area of lowermost mantle structure. The results of this study provide evidence for small-scale lateral and depth variations in shear wave anisotropy for a broad region of the lowermost mantle beneath the Pacific Ocean. In particular, we image a localized zone of anomalously strong anisotropy whose strength increases toward the top of D″ beneath Hawaii. Our results, combined with a previous study of V_P/V_SH ratios, indicate that ancient subducted slab material may be responsible for observations beneath the northern Pacific, while lenses or layers of core–mantle boundary reaction products or partial melt, oriented by horizontal inflow of mantle material to the Hawaiian plume source, can explain observations beneath the central Pacific.     

Indication of a geomagnetic low-inclination excursion in supposed Middle Weichselian interstadial marine clay at Rubjerg,Denmark

Palaeomagnetic measurements have been made on specimens from Late Pleistocene sediments of a piston boring at Rubjerg in Vendsyssel, northern Jutland. The stratigraphy of the deposits is based on content of foraminifera. A total of 70 relatively oriented specimens were investigated palaeomagnetically. Normal steep inclinations close to that of the axial dipole field were found in the Upper Saxicava Sand and in the Younger Yoldia Clay (radiocarbon dated at 14,650 ± 190?12,650 ± 180 B.P.), and a secular variation with an amplitude of 10–12° in the inclincation and a “period” roughly estimated at about 350–400 years was found in the Younger Yoldia Clay.Seventeen relatively oriented specimens from undisturbed older marine deposits revealed a stable low inclination of 11° with α₉₅ = 3°. The age of this apparent geomagnetic excursion falls somewhere between 23,000 and 40,000 B.P. (Older Yoldia Clay). Among other known geomagnetic excursions and events within this interval are Laschamp in France, Mono Lake in California and Lake Mungo in Australia. Until more definite ages have been obtained, the excursion is provisionally named the “Rubjerg Excursion”.     

Benthic macrofaunal communities in partially impounded salt marshes in Delaware: Comparisons with natural marshes and responses to sediment exposure

This two-part study examined the benthic macrofaunal community in Delaware salt marsh impoundments having partial tidal restriction. The first part compared abundance, diversity, and taxonomic composition in three habitat types in impoundments—creeks, vegetated creek banks, and ponds—to those found in natural marshes. Impoundment effects were present but were habitat-specific. Abundances were higher in natural marsh creeks than in impoundment creeks, and diversities were higher in impoundment ponds than in natural marsh ponds. Vegetated bank communities in impoundments were about 50% insects and arachnids and 50% oligochaetes, while natural bank communities were primarily oligochaetes and the polychaeteManayunkia aestuarina. This is likely due to the decrease in flooding of the vegetated high marsh caused by partial impoundment. Pond and creek community composition also showed impoundment effects: in comparison with natural marshes, impoundments had higher proportions of the burrowing anemoneNematostella vectensis, nemerteans, andTubificoides sp. oligochaetes and lower proportions of the oligochaeteClitellio arenarius. The second part of the study compared benthic macrofauna in an impoundment before, during, and after the water level was lowered so that some bottom sediments were exposed and some covered with just a few centimeters of water for several weeks. During this event, macrofaunal abundances were reduced and the community shifted from being dominated by annelids, anemones, and nemerteans toward one dominated by annelids and insects. About 6 wk after reflooding, persistent effects of this disturbance were still suggested by greatly increased abundances and 96% dominance by one species of oligochaete,Paranais litoralis. Impoundment management plans calling for periods of sediment exposure or very low water may want to consider the potential for strong and persistent effects on the macrofaunal community.