Constraints on the luminosities of gamma-ray bursts

Constraints are found on the gamma-ray burst luminosity function from an analysis of the combined BATSE/PVO intensity distribution. If bursts originate in an extended Galactic halo, then the intrinsic luminosity range is narrow, with bursts spanning only a factor of five or less in luminosity. If bursts originate in a simple Friedmann cosmology with = 1 and = 0, then very few luminosity constraints exist.National Research Council Fellow at NASA/MSFC     

Selection effects and tests of repetition (clustering) in the BATSE 1B catalog

A repeater statistic (derived from the two-point angular correlation function and accounting for location errors) is used to identify constraints on the rate of burst repetition from the BATSE 1B catalog. Because of the anisotropic sky exposure, repeating sources should create additional effects in the large-scale distribution of close burst pairs.National Research Council Fellow at NASA/MSFC     

Wetland-Scale Mapping of Preferential Fresh Groundwater Discharge to the Colorado River

Quantitative evaluation of groundwater/surface water exchange dynamics is universally challenging in large river systems, because existing methodology often does not yield spatially-distributed data and is difficult to apply in deeper water. Here we apply a combined near-surface geophysical and direct groundwater chemical toolkit to refine fresh groundwater discharge estimates to the Colorado River through a 4-km² wetland that borders the town of Moab, Utah, USA. Preliminary characterization of raw electromagnetic imaging (EMI) data, collected by kayak and by walking, was used to guide additional direct-contact electrical measurements and installation of new monitoring wells. Chemical data from the wells strongly supported the EMI spatial characterization of preferential fresh groundwater discharge embedded in natural brine groundwaters and weighted to the southern wetland section. Inversion of the EMI data revealed sub-meter scale detail regarding bulk electrical conductivity zonation across approximately 15.5 km of transects, collected in only 3 days. This electrical detail indicates processes such as salinization of the unsaturated zone and direct discharge through the Colorado River sediments and a tributary creek bed. Overall, the study contributed to a substantial reduction in fresh groundwater discharge estimates previously made using sparse existing well data and a simplified assumption of diffuse fresh groundwater discharge below the entire wetland. EMI will likely become a widely used tool in systems with natural electrical contrast as groundwater/surface water hydrogeologists continue to recognize the prevalence of preferential groundwater discharge processes.     

1DTempPro V2: New Features for Inferring Groundwater/Surface‐Water Exchange

A new version of the computer program 1DTempPro extends the original code to include new capabilities for (1) automated parameter estimation, (2) layer heterogeneity, and (3) time‐varying specific discharge. The code serves as an interface to the U.S. Geological Survey model VS2DH and supports analysis of vertical one‐dimensional temperature profiles under saturated flow conditions to assess groundwater/surface‐water exchange and estimate hydraulic conductivity for cases where hydraulic head is known.     

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.     

Adolf Seilacher's fossil record

Adolf (Dolf) Seilacher, who died earlier this year at the age of 89, was a major figure in palaeontology on both sides of the Atlantic. His interests were diverse, including trace fossils, the processes involved in exceptional preservation, and the evolution of invertebrate morphology. But it is for his views on the nature of the extraordinary fossils of the Ediacaran that he is perhaps best known. His ideas—compelling and provocative—will continue to stimulate research for years to come, not least through the wide use of his terminology (e.g. Lagerstätten, morphodynamics, constructional morphology, vendobiont).     

Batse time-resolved observations of GRBs: the burst ‘topographic’ map

It is a well-known observation that many GRBs have spectra with curvature in the less than 1 MeV regime, but flatten out to approximately a -2 power law at higher energies. The large collecting area of BATSE allows spectral studies with unprecedented time resolution. We are conducting a systematic study of time-resolved spectroscopy of GRBs observed by BATSE. Several aspects of this survey are discussed: correlations among fit parameters and between fit parameters and intensity; screening for spectral features using-residual contour maps; and an introduction to the GRB topographic map and its possible qualitative diagnostic abilities.     

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.