Residence time control on hot moments of net nitrate production and uptake in the hyporheic zone

The retention capacity for biologically available nitrogen within streams can be influenced by dynamic hyporheic zone exchange, a process that may act as either a net source or net sink of dissolved nitrogen. Over 5 weeks, nine vertical profiles of streambed chemistry (NO₃^? and NH₄⁺) were collected above two beaver dams along with continuous high‐resolution vertical hyporheic flux data. The results indicate a non‐linear relation of net NO₃^? production followed by net uptake in the hyporheic zone as a function of residence time. This Lagrangian‐based relation is consistent through time and across varied morphology (bars, pools, glides) above the dams, even though biogeochemical and environmental factors varied. The empirical continuum between net NO₃^? production and uptake and residence time is useful for identifying two crucial residence time thresholds: the transition to anaerobic respiration, which corresponds to the time of peak net nitrate production, and the net sink threshold, which is defined by a net uptake in NO₃^? relative to streamwater. Short‐term hyporheic residence time variability at specific locations creates hot moments of net production and uptake, enhancing NO₃^? production as residence times approach the anaerobic threshold, and changing zones of net NO₃^? production to uptake as residence times increase past the net sink threshold. The anaerobic and net sink thresholds for beaver‐influenced streambed morphology occur at much shorter residence times (1.3 h and 2.3 h, respectively) compared to other documented hyporheic systems, and the net sink threshold compares favorably to the lower boundary of the anaerobic threshold determined for this system with the new oxygen Damkohler number. The consistency of the residence time threshold values of NO₃^? cycling in this study, despite environmental variability and disparate morphology, indicates that NO₃^? hot moment dynamics are primarily driven by changes in physical hydrology and associated residence times. Copyright © 2013 John Wiley & Sons, Ltd.     

Microbial community distribution in sediments from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

We studied the microbial communities collected from hydrate-bearing sediments on the North Slope of Alaska to determine how abiotic variables (e.g., grain size, hydrate presence, formation fluid gases) may correspond to the type and distribution of microbes in the sediments. The cores were acquired from sub-permafrost, Eocene (46-55 million year old) sediments in the BPXA-DOE-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well within which hydrates are believed to have formed 1.5 mya. Forty samples, eight of which originally contained hydrates, were acquired from depths of ca. 606-666 m below land surface. Five drilling fluid samples acquired from the same depth range were included in the analysis as a control for possible contamination by drilling fluid microbes during the drilling and handling of cores. DNA was extracted from 15 samples (typically <1 ng DNA/g sediment was recovered) and then amplified using polymerase chain reaction with primers specific for bacterial and archaeal 16S rDNA genes, which indicates the likelihood that microbes were present in all analyzed samples. Only bacterial DNA amplicons were detected. Terminal-restriction fragment length polymorphism (T-RFLP) was used to measure bacterial diversity in the respective samples. Non-metric multidimensional scaling (NMS) was used to determine the abiotic variables that may have influenced bacterial diversity. NMS analysis revealed that the microbial taxa present in the sediment were distinct from the taxa present in the drilling fluids suggesting that the sediments were not contaminated by the drilling fluids. Multi-response permutation procedures (MRPP) found no significant difference between three sample groups identified a priori as being from within a hydrate zone, outside of hydrate zone, or on the edge of a hydrate/non-hydrate zone according to downhole nuclear magnetic resonance spectroscopy logs. However, among the several other abiotic parameters that were evaluated mud gas methane concentration and variables related to hydrate presence (e.g., chloride concentration, salinity, and resistivity) appeared to define the arrangement of microbial community signatures in plots resulting from NMS analysis. Communities from hydrate and non-hydrate layers each contained unique taxa as determined by the T-RFLP assay.     

High-resolution observations of aggregate flux during a sub-polar North Atlantic spring bloom

An aggregate flux event was observed by ship and by four underwater gliders during the 2008 sub-polar North Atlantic spring bloom experiment (NAB08). At the height of the diatom bloom, aggregates were observed as spikes in measurements of both particulate backscattering coefficient (b_bp) and chlorophyll a fluorescence. Optical sensors on the ship and gliders were cross-calibrated through a series of simultaneous profiles, and b_bp was converted to particulate organic carbon. The aggregates sank as a discrete pulse, with an average sinking rate of ∼75 m d⁻¹; 65% of aggregate backscattering and 90% of chlorophyll fluorescence content was lost between 100 m and 900 m. Mean aggregate organic carbon flux at 100 m in mid-May was estimated at 514 mg C m⁻² d⁻¹, consistent with independent flux estimates. The use of optical spikes observed from gliders provides unprecedented coupled vertical and temporal resolution measurements of an aggregate flux event.     

The largest Au deposits in the St Ives Goldfield (Yilgarn Craton, Western Australia) may be located in a major Neoarchean volcano-sedimentary depo-centre

The largest Neoarchean gold deposits in the world-class St Ives Goldfield, Western Australia, occur in an area known as the Argo–Junction region (e.g. Junction, Argo and Athena). Why this region is so well endowed with large deposits compared with other parts of the St Ives Goldfield is currently unclear, because gold deposits at St Ives are hosted by a variety of lithologic units and were formed during at least three different deformational events. This paper presents an investigation into the stratigraphic architecture and evolution of the Argo–Junction region to assess its implications for gold metallogenesis. The results show that the region’s stratigraphy may be subdivided into five regionally correlatable packages: mafic lavas of the Paringa Basalt; contemporaneously resedimented feldspar-rich pyroclastic debris of the Early Black Flag Group; coarse polymictic volcanic debris of the Late Black Flag Group; thick piles of mafic lavas and sub-volcanic sills of the Athena Basalt and Condenser Dolerite; and the voluminous quartz-rich sedimentary successions of the Early Merougil Group. In the Argo–Junction region, these units have an interpreted maximum thickness of at least 7,130 m, and thus represent an unusually thick accumulation of the Neoarchean volcano-sedimentary successions. It is postulated that major basin-forming structures that were active during deposition and emplacement of the voluminous successions later acted as important conduits during mineralisation. Therefore, a correlation exists between the location of the largest gold deposits in the St Ives Goldfield and the thickest parts of the stratigraphy. Recognition of this association has important implications for camp-scale exploration.     

Quantitative guidance for efficient vertical flow measurements at the sediment–water interface using temperature–depth profiles

Upward discharge to surface water bodies can be quantified using analytical models based on temperature–depth (T-z) profiles. The use of sediment T-z profiles is attractive as discharge estimates can be obtained using point-in-time data that are collected inexpensively and rapidly. Previous studies have identified that T-z methods can only be applied at times of the year when there is significant difference between the streambed–water interface and deeper sediment temperatures (e.g., winter and summer). However, surface water temperatures also vary diurnally, and the influence of these variations on discharge estimates from T-z methods is poorly understood. For this study, synthetic T-z profiles were generated numerically using measured streambed interface temperature data to assess the influence of diurnal temperature variations on discharge estimation and provide insight into the suitable application of T-z methods. Results show that the time of day of data collection can have a substantial influence on vertical flux estimates using T-z methods. For low groundwater discharge fluxes (e.g., 0.1 m d⁻¹), daily transience in streambed temperatures led to relatively large errors in estimated flow magnitude and direction. For higher discharge fluxes (1.5 m d⁻¹), the influence of transient streambed temperatures on discharge estimates was strongly reduced. Discharge estimates from point-in-time T-z profiles were most accurate when the uppermost point in the T-z profile was near the bed interface daily mean (two time periods daily). Where temperature time series data are available, daily averaged T-z profiles can produce accurate discharge estimates across a wide range of discharge rates. Seasonality in shallow groundwater temperature generally had a negligible influence on vertical flow estimates. These findings can be used to plan field campaigns and provide guidance on the optimal application of T-z methods to quantify vertical groundwater discharge to surface water bodies.     

Are galactic GRB models still an option?

The traditional paradigm of a Galactic origin of gamma-ray bursts (GRBs) suffered a major reduction in popularity when BATSE revealed an isotropic but radially inhomogeneous distribution of GRBs. The lack of pronounced galactic anisotropies places severe constraints on models including significant source contributions from the usual disk, bulge, or halo components of the Milky Way. Although models can be designed to preserve the local disk origin, a perhaps more promising approach invokes a very extended Galactic halo. Populating such halos with neutron stars requires very high velocities. Injection of such neutron stars might be restricted to a phase of the early galaxy, or it may continue to the present. We discuss several observational constraints that address the question presented in the title.     

Structural adjustment and the contemporary sub-Saharan African city

Although it has been suggested that structural adjustment policies have slowed Third World urban growth and have stimulated a spatial deconcentration of economic activity, this paper argues that African cities continue to grow and mainly through peri-urban development. This investment comes mainly from domestic sources and migrants' remittances, and tends to be in consumption rather than production. Reasons include cultural factors, lack of confidence in the national economy and in the state's long-term economic objectives, an increasing demand for housing, improvements in intraurban transport, and a desire to spread investment risk among a range of alternatives including housing.