Probing the dark ages with the Square Kilometer Array

The epoch of reionization (EoR) sets a fundamental benchmark in cosmic structure formation, corresponding to the formation of the first luminous objects that act to ionize the neutral intergalactic medium (IGM). Recent observations at near-IR and radio wavelengths imply that we are finally probing into this key epoch of galaxy formation at z 6. The Square Kilometer Array (SKA) will provide critical insight into the EoR, in a number of ways. First, the ability of the SKA to image the neutral IGM in 21-cm emission is a truly unique probe of the process of reionization, and is recognized as the next necessary and fundamental step in our study of the evolution of large scale structure and cosmic reionization. Second, study of HI 21-cm absorption toward the first radio loud objects probes small to intermediate scale structure in the neutral ‘cosmic web’, as well as HI in the first collapsed structures (proto-disks and mini-halos). And third, the incomparable sensitivity of the SKA allows for the study of the molecular gas, dust, and star formation activity in the first galaxies, as well as the radio continuum emission from the first accreting massive black holes. Such objects will be obscured at optical wavelengths due to absorption by the neutral IGM.     

Mass anomalies on Ganymede

Radio Doppler data, generated with NASA's Galileo spacecraft during its second encounter with Jupiter's moon Ganymede, are used to infer the locations and magnitudes of mass anomalies on Ganymede. We construct models for both surface and buried anomalies. With only one flyby and no global coverage, a solution for mass anomalies cannot be uniquely determined. However, we are able to constrain acceptable solutions for mass anomalies to four broad regions—a near polar region and three that are roughly equatorial. If the mass anomalies are constrained to lie at the surface, the centers of the regions are located near the coordinates (77° N, 333° W), (36° N, 0° W), (33° N, 130° W), and (7° N, 194° W). If the mass anomalies are located at the deep ice-rock interface 800 km below the surface, the regions' centers are approximately (65° N, 17° W), (32° N, 30° W), (37° N, 175° W), and (15° N, 211° W). For both models, the regions are up to a few thousand kilometers across. The magnitude of mass anomalies on the surface is on the order of 10¹⁷ kg. Mass anomalies at the ice-rock interface are on average no more than an order of magnitude larger (10¹⁸ kg). There are two positive and two negative mass anomalies in both the surface and ice-rock interface models. One of the positive mass anomalies at the surface is associated with Galileo Regio. The other positive surface mass anomaly is located at high northern latitudes with no obvious geological association. Negative surface mass anomalies lie near Uruk Sulcus and between Perrine Regio and Barnard Regio near Sicyan Sulcus and Phrygia Sulcus. The locations of the ice-rock interface mass anomalies lie approximately radially below the surface anomalies. Positive mass anomalies at the surface could be associated with the silicate-rich ice or accumulated silicate layers of the dark regions. Negative mass anomalies at the surface could be associated with the relatively clean, low-lying ice of sulci. Alternatively, Ganymede's mass anomalies could be associated with the topography or other mass concentrations at the deep ice-rock interface.     

Geostatistical Mapping of Salinity Conditioned on Borehole Logs,Montebello Oil Field,California

We present a geostatistics-based stochastic salinity estimation framework for the Montebello Oil Field that capitalizes on available total dissolved solids (TDS) data from groundwater samples as well as electrical resistivity (ER) data from borehole logging. Data from TDS samples (n = 4924) was coded into an indicator framework based on falling below four selected thresholds (500, 1000, 3000, and 10,000 mg/L). Collocated TDS-ER data from the surrounding groundwater basin were then employed to produce a kernel density estimator to establish conditional probabilities for ER data (n = 8 boreholes) falling below the selected TDS thresholds within the Montebello Oil Field area. Directional variograms were estimated from these indicator coded data, and 500 TDS realizations from conditional indicator simulation were generated for the subsurface region above the Montebello Oil Field reservoir. Simulations were summarized as 3D maps of median TDS, most likely salinity class, and probability for exceeding each of the specified TDS thresholds. Results suggested TDS was below 500 mg/L in most of the study area, with a trend toward higher values (500 to 1000 mg/L) to the southwest; consistent with the average regional groundwater flow direction. Discrete localized zones of TDS greater than 1000 mg/L were observed, with one of these zones in the greater than 10,000 mg/L range; however, these areas were not prevalent. The probabilistic approach used here is adaptable and is readily modified to include additional data and types and can be employed in time-lapse salinity modeling through Bayesian updating.     

Use of a simplified Mahalanobis distance approach to constrain the dispersion and provenance of Cr-pyrope populations at the Chidliak kimberlite province,Nunavut, Canada

Exploration for diamond-bearing kimberlites in the Chidliak project area by Peregrine Diamonds has generated a grid-like till sampling pattern across four discrete areas of interest totalling 402 km² that is densely populated with research-grade compositional data for 10,743 mantle-derived Cr-pyrope garnets. The available dataset is well suited to statistical analysis, in part due to the relatively unbiased spatial coverage. Previous workers showed empirically that the TiO₂ and Mn thermometry (Ti-TMn) attributes of Cr-pyrope populations at the Chidliak project may serve as source-specific “fingerprints”. In this work, we employ a simplified version of the multivariate Mahalanobis distance technique to formally examine the variability of, and differences between, Ti-TMn attributes of Cr-pyrope subpopulations recovered from a Laurentide-age glaciated terrain that also contains 30 known kimberlites within the four areas of interest. We show the simplified Mahalanobis distance approach enables accurate discrimination of Cr-pyrope subpopulations with subtly to distinctly different Ti-TMn attributes, and permits proper demarcation of their respective kimberlite source(s), specifically in areas with straightforward glacial histories. Redistribution and blending of Cr-pyrope subpopulations from known kimberlite sources is also observed, and typifies areas at Chidliak with complex late-glacial histories. Our results support <1 km horizontal scale subtle to obvious variability in the proportions of TiO₂-rich and high-temperature (> 1100 °C) Cr-pyropes between closely spaced kimberlite source(s) and also between physically adjacent magma batches within single kimberlite pipes. The local scale variability is attributed to protokimberlite fluid or melt interacting with, and metasomatizing discrete conduits within, the ambient diamond-facies peridotitic mantle at times closely preceding eruption of kimberlite magma batches at Chidliak.

     

How Population Decline Can Impact Genetic Diversity: a Case Study of Eelgrass (<Emphasis Type="Italic">Zostera marina</Emphasis>) in Morro Bay,California

Seagrass populations are in decline worldwide. Eelgrass (Zostera marina L.), one of California’s native seagrasses, is no exception to this trend. In the last 8 years, the estuary in Morro Bay, California, has lost 95% of its eelgrass. Population bottlenecks like this one often result in severe reductions in genetic diversity; however, this is not always the case. The decline of eelgrass in Morro Bay provides an opportunity to better understand the effects of population decline on population genetics. Furthermore, the failure of recent restoration efforts necessitates a better understanding of the genetic underpinnings of the population. Previous research on eelgrass in California has demonstrated a link between population genetic diversity and eelgrass bed health, ecosystem functioning, and resilience to disturbance and extreme climatic events. The genetic diversity and population structure of Morro Bay eelgrass have not been assessed until this study. We also compare Morro Bay eelgrass to Bodega Bay eelgrass in Northern California. We conducted fragment length analysis of nine microsatellite loci on 133 Morro Bay samples, and 20 Bodega Bay samples. We found no population differentiation between the remaining beds in Morro Bay and no difference among samples growing at different tidal depths. Comparisons with Bodega Bay revealed that Morro Bay eelgrass contains three first-generation migrants from the north, but Morro Bay remains considerably genetically differentiated from Bodega Bay. Despite the precipitous loss of eelgrass in Morro Bay between 2008 and 2017, genetic diversity remains relatively high and comparable to other populations on the west coast.     

Panoptic geographies: an examination of all U.S. geographic dissertations

The study of geography as a discipline has often employed a top‐down approach that examines the output of the field's most prominent members. In this paper, we provide a bottom‐up approach by analyzing geography dissertations. We start by assembling a database that includes the title, author, date, and university for every geographic doctoral dissertation written in the United States over the last 120 years. The database allows us to track the rise and fall of regions of interest and specific topics. It gives a broad overview of the different schools in geography and their influence within separate historical periods. And it permits a more empirically based delineation of geographical “eras” based on the commonality of words in dissertation titles. This work is at the convergence of the quantitative and qualitative; we quantify and visualize textual data to provide a more robust history of geography. The result is a new, more comprehensive way to look at the changing discipline of geography.     

Managing Groundwater to Ensure Ecosystem Function

Groundwater is a critical resource not only for human communities but also for many terrestrial, riparian, and aquatic ecosystems and species. Yet groundwater planning and management decisions frequently ignore or inadequately address the needs of these natural systems. As a consequence, ecosystems dependent on groundwater have been threatened, degraded, or eliminated, especially in arid regions. There is growing acknowledgment that governmental protections for these ecological resources are necessary, but current legal, regulatory and voluntary provisions are often inadequate. Groundwater management premised on “safe yield,” which aims to balance human withdrawals with natural recharge rates, typically provides little to no consideration for water needed by ecosystems. Alternatively, the “sustainable yield” concept aims to integrate social, economic and environmental needs for groundwater, but the complexity of groundwater systems creates substantial uncertainty about the impact that current or future groundwater withdrawals will have on ecosystems. Regardless of the legal or regulatory framework, guidance is needed to help ensure environmental water needs will be met, especially in the face of pressure to increase human uses of groundwater resources. In this paper, we describe minimum provisions for planning, managing, and monitoring groundwater that collectively can lower the risk of harm to groundwater-dependent ecosystems and species, with a special emphasis on arid systems, where ecosystems and species may be especially reliant upon and sensitive to groundwater dynamics.