Marine radiocarbon reservoir corrections (∆R) for Chesapeake Bay and the Middle Atlantic Coast of North America

Radiocarbon dates from known age, pre-bomb eastern oyster (Crassostrea virginica) shells provide local marine reservoir corrections (?R) for Chesapeake Bay and the Middle Atlantic coastal area of eastern North America. These data suggest subregional variability in ?R, ranging from 148 ± 46 ¹⁴C yr on the Potomac River to ? 109 ± 38 ¹⁴C yr at Swan Point, Maryland. The ?R weighted mean for the Chesapeake's Western Shore (129 ± 22 ¹⁴C yr) is substantially higher than the Eastern Shore (? 88 ± 23 ¹⁴C yr), with outer Atlantic Coast samples falling between these values (106 ± 46 and 2 ± 46 ¹⁴C yr). These differences may result from a combination of factors, including ¹⁴C-depleted freshwater that enters the bay from some if its drainages, ¹⁴C-depleted seawater that enters the bay at its mouth, and/or biological carbon recycling. We advocate using different subregional ?R corrections when calibrating ¹⁴C dates on aquatic specimens from the Chesapeake Bay and coastal Middle Atlantic region of North America.     

Assessing lithologic discontinuities and parent material uniformity within the Texas sandy mantle and implications for archaeological burial and preservation potential in upland settings

Alfisols within the Texas Gulf Coast Plain commonly exhibit textural contrasts between sandy, artifact-bearing A–E horizons (i.e., sandy mantle), and artifact-sterile clay-rich Bt (argillic) horizons. This has invoked debate about parent material uniformity and pedogenic versus geomorphic sandy mantle origins, which has implications for the scientific value of buried archaeological sites. To improve our understanding of archaeological burial in upland settings, we evaluated parent material uniformity within five pedons to distinguish pedogenically derived textural changes from geomorphologically created lithologic discontinuities. Depth trends in clay-free particle size classes and stable/immobile Ti and Zr constituents failed to reveal lithologic discontinuities between the sandy mantle and Bt horizons, and the observed textural differences are interpreted to have resulted from pedogenic processes. This interpretation is supported by clay skins, fine clay increases in Bt horizons, and micromorphological observations. Consequently, artifacts buried in upland summits have likely moved down the soil profile due to biomantle processes. Deep sandy mantle sites, non-parallel contacts between the sandy mantle and Bt horizons, and paleogullies incised into Eocene bedrock are better explained by colluvial/soil creep processes adjacent to summits, where archaeological materials may exhibit preservation potential. No single explanation can account for sandy mantle origins, and we advocate a case-by-case approach.     

Predicting the Impact of Coastal Development on Water Quality Using Remote Sensing and GIS‐assisted Hydrologic Modeling Techniques

Abstract

A decline in water quality in the Okatie River, a coastal estuary located in Beaufort County, SC, has resulted in the closure of several shellfish beds. Continuing urban development within the watershed has altered land cover conditions and may be contributing to the recent decline in water quality. Remote sensing and geographic information system (GIS) technology, coupled with a water quality model were used to spatially model stormwater runoff to understand the relationship between recent changes in land cover and watershed runoff characteristics. High spatial resolution imagery acquired in 1994 and 1996 spatially documented pre‐ and post‐development land cover conditions within the watershed. The water quality model Agricultural Nonpoint Source Pollution (AGNPS) evaluated land characteristics such as soil type, topography, and land cover to simulate surface water flow and sediment transport over past and current land cover conditions. Results of the model were used to locate net increases of fresh water discharge and to suggest best management practices (BMP).     

Social Network Analysis of the Academic GIScience Community

There is mounting interest among scientists regarding the use of scientometric social network analysis, or quantitative analysis of the evolution of science as defined by individual researchers and the networks they form. Given that geographers have seldom used this approach compared to researchers in other fields, its implications for research and policy need to be assessed. We applied scientometric social network analysis to geographic information science (GIScience) to understand how the field has evolved over the last sixteen years and to assess the applicability of the standard logistic model of the growth of scientific disciplines. In particular, we examined collaboration in the field at multiple scales, namely, the evolution of the entire research network structure, the nature of subnetworks in defining geographic information science, and the roles individuals play within the community. By delineating how collaborations and research networks have evolved in GIScience, the study addresses the potential of scientometric social network analysis for geography.     

Jobs and the Single Parent: An Analysis of Accessibility to Employment in Toronto

Abstract

In this article, we undertake an analysis of accessibility to jobs from the perspective of single-parent household members. Individuals in this demographic segment are of interest due to the fact they often face the double burden of household and employment responsibilities. A case study of the city of Toronto in Canada, an urban area that has seen an increase in absolute and relative numbers of single-parent households in recent years, is presented. Analysis is based on the application of relative accessibility deprivation indicators (RADI), which are calculated using model-based estimates of distance traveled for various population segments, as well as employment data for the city and its surroundings. The results of the analysis indicate that there are substantial differences in the levels of accessibility to jobs between members of single-parent households, in particular females, and members of other types of households.     

Spatiotemporal index for analyzing controls on snow climatology: application in the Colorado Front Range

Mountain snowpacks are important water supplies that are susceptible to climate change, yet snow measurements are sparse relative to snowpack heterogeneity. We used remote sensing to derive a spatiotemporal index of snow climatology that reveals patterns in snow accumulation, persistence, and ablation. Then we examined how this index relates to climate, terrain, and vegetation. Analyses were based on Moderate Resolution Imaging Spectroradiometer eight-day snow cover from 2000 to 2010 for a mountain watershed in the Colorado Front Range, USA. The Snow Cover Index (SCI) was calculated as the fraction of years that were snow covered for each pixel. The proportion of SCI variability explained by independent variables was evaluated using regression analysis. Independent variables included elevation, northing, easting, slope, aspect, northness, solar radiation, precipitation, temperature, and vegetation cover. Elevation was the dominant control on SCI patterns, due to its influence on both temperature and precipitation. Grouping SCI values by elevation, we identified three distinct snow zones in the basin: persistent, transitional, and intermittent. The transitional snow zone represents an area that is sensitive to losing winter snowpack. The SCI can be applied to other basins or regions to identify dominant controls on snow cover patterns and areas sensitive to snow loss.     

A novel repeat-coring approach to reconstruct recent sediment, phosphorus, and mercury loading from the upper Mississippi River to Lake Pepin, USA

It can be advantageous to revisit coring locations in lakes years after an initial paleolimnological study is completed, to assess environmental changes in the intervening time interval. We revisited sediment core sites in Lake Pepin (Minnesota, Wisconsin) more than a decade after an original set of 10 cores was collected, dated radiometrically, and studied in 1996. Prominent magnetic susceptibility features were used to align the new core set with the older set, such that traditional radiometric dating was not necessary to obtain a chronology for the new cores. The procedure used to align the two core sets accounted for compaction of former surface sediments by burial with new sediment. The amount of new sediment, mercury, and phosphorus accumulated at each core site was determined and extrapolated to the depositional area of the lake to estimate recent (1996–2008) whole-basin loads. Recent sediment accumulation in Lake Pepin compared well (within 3%) with monitored inflow data from a gauging station on the upper Mississippi River just before it enters the lake. Bulk sediment accumulation rate remained very high (772,000 t/year) for the recent period (1996–2008), down slightly from the peak in 1990–1996 (876,000 t/year), and almost an order of magnitude above pre-settlement rates. Total phosphorus deposition remained constant since a peak in the 1960s, but was also well above pre-settlement rates. Mercury continued its precipitous decline since peaking in the 1960s.     

Controls on fluvial sedimentary architecture and sediment‐fill state in salt‐walled mini‐basins: Triassic Moenkopi Formation,Salt Anticline Region,SE Utah,USA

The Triassic Moenkopi Formation in the Salt Anticline Region, SE Utah, represents the preserved record of a low‐relief ephemeral fluvial system that accumulated in a series of actively subsiding salt‐walled mini‐basins. Development and evolution of the fluvial system and its resultant preserved architecture was controlled by the following: (1) the inherited state of the basin geometry at the time of commencement of sedimentation; (2) the rate of sediment delivery to the developing basins; (3) the orientation of fluvial pathways relative to the salt walls that bounded the basins; (4) spatially and temporally variable rates and styles of mini‐basin subsidence and associated salt‐wall uplift; and (5) temporal changes in regional climate. Detailed outcrop‐based tectono‐stratigraphic analyses demonstrate how three coevally developing mini‐basins and their intervening salt walls evolved in response to progressive sediment loading of a succession of Pennsylvanian salt (the Paradox Formation) by the younger Moenkopi Formation, deposits of which record a dryland fluvial system in which flow was primarily directed parallel to a series of elongate salt walls. In some mini‐basins, fluvial channel elements are stacked vertically within and along the central basin axes, in response to preferential salt withdrawal and resulting subsidence. In other basins, rim synclines have developed adjacent to bounding salt walls and these served as loci for accumulation of stacked fluvial channel complexes. Neighbouring mini‐basins exhibit different styles of infill at equivalent stratigraphic levels: sand‐poor basins dominated by fine‐grained, sheet‐like sandstone fluvial elements, which are representative of nonchannelised flow processes, apparently developed synchronously with neighbouring sand‐prone basins dominated by major fluvial channel‐belts, demonstrating effective partitioning of sediment route‐ways by surface topography generated by uplifting salt walls. Reworked gypsum clasts present in parts of the stratigraphy demonstrate the subaerial exposure of some salt walls, and their partial erosion and reworking into the fill of adjoining mini‐basins during accumulation of the Moenkopi Formation. Complex spatial changes in preserved stratigraphic thickness of four members in the Moenkopi Formation, both within and between mini‐basins, demonstrates a complex relationship between the location and timing of subsidence and the infill of the generated accommodation by fluvial processes.