Surface sediment gases as indicators of subsurface hydrocarbons – examining the record in laboratory and field studies

Near-surface sediment gases are commonly examined in surface geochemistry surveys to evaluate subsurface hydrocarbon generation and entrapment. Sediment gases reside in interstitial spaces, bound to mineral or organic surfaces, and/or entrapped in carbonate inclusions. Characterizing migrated sediment gases with minimal compositional and isotopic fractionation is challenging, and different methods are currently in use to extract and analyze surface sediment gases. These methods have not been compared in controlled laboratory experiments. The key to calibration experiments is to know the charge gas composition and thus have a basis to evaluate the sediment gas extraction procedure.     

An Experimental Apparatus for Laboratory and Field-Based Perfusion of Sediment Porewater with Dissolved Tracers

The water–sediment interface is a dynamic zone where the benthic and pelagic environments are linked through exchange and recycling of organic matter and nutrients. However, it is often difficult to measure rate processes in this zone. To that end, we designed an experimental apparatus for continuous and homogeneous perfusion of sediment porewater with dissolved conservative (SF₆, Rhodamine WT dye) and isotopic (H¹³CO₃⁻ and ¹⁵NH₄⁺) tracers to study nitrogen and carbon cycling by the sediment microbial community of shallow illuminated sediments. The perfusionator consists of a 60-cm ID × 60-cm high cylinder that includes a reservoir for porewater at the base of the sediment column. Porewater amended with conservative and stable isotopic tracers was pumped through a mixing reservoir and upward through the overlying sediments. We tested the perfusionator in a laboratory setting, as part of an outdoor mesocosm array, and buried in coastal sediments. Conservative and isotopic tracers demonstrated that the porewater tracers were distributed homogeneously through the sediment column in all settings. The perfusionator was designed to introduce dissolved stable isotope tracers but is capable of delivering any dissolved ionic, organic, or gaseous constituent. We see a potentially wide application of this technique in the aquatic and marine sciences in laboratory and field settings.     

Dayglow emissions of the O2 Herzberg bands and the Rayleigh backscattered spectrum of the earth

Numerous fluorescent emissions from the Herzberg bands of molecular oxygen lie in the spectral region 242–300 nm. This coincides with the wavelength range used by orbiting spectrometers which observe the Rayleigh backscattered spectrum of the earth for the purpose of monitoring the vertical distribution of stratospheric ozone. Model calculations indicate that Herzberg band emissions in the dayglow could provide significant contamination of the ozone measurements if the quenching rate of O₂(A³Σ) is sufficiently small. This is especially true near 255 nm, where the most intense fluorescent emissions relative to the Rayleigh scattered signal are located and where past satellite measurements show a persistent excess radiance above that expected for a pure ozone absorbing and molecular scattering atmosphere. However, very small quenching rates are adequate to reduce the dayglow emission to negligible levels. Available laboratory data have not definitely established the quenching on the rate of O₂(A³Σ) as a function of vibrational level, and such information is required before the Herzberg band contributions can be evaluated with confidence.     

Using GIS in Areawide Pest Management: A Case Study in South Dakota

Infestations of corn rootworms (Coleoptera: Chrysomelidae) create economic and environmental concerns in the Corn Belt region of the United States. To supplement the population control tactics of areawide pest management programs, we believe that a better understanding of the spatial relationships between biotic and abiotic or physical factors at the landscape scale is needed. Our research used several geographical information systems (GIS) and spatial analytical techniques to examine relationships between corn rootworm metapopulation dynamics, soil texture, and elevation. Within GIS, several spatially explicit procedures were used that include an interpolation technique, spatial autocorrelation analysis, and contingency analysis. Corn rootworm metapopulation distributions were found to be aggregated and related to soil texture and elevation. We review techniques and discuss our preferences for using particular spatially explicit procedures. The information derived from the spatial analyses demonstrates how GIS can be used in areawide pest management to provide inputs for spatially explicit models to predict future pest populations and formulate more well‐informed pest management decisions. The techniques described in this paper could easily be extended to study the spatial dynamics between other pest populations in agricultural landscapes.     

Further testing of the Integrated Hydrology Model (InHM): event‐based simulations for a small rangeland catchment located near Chickasha,Oklahoma

In the paper that is the foundation for this study, VanderKwaak and Loague (2001. Water Resources Research 37 : 999–1013) reported a demonstration of a fully coupled comprehensive physics‐based hydrologic‐response model, InHM (Integrated Hydrology Model), for two rainfall‐runoff events from the small rangeland catchment known as R‐5. The InHM simulations reported herein address (in three phases) limitations in the VanderKwaak and Loague (2001. Water Resources Research 37 : 999–1013) simulations. In Phase I, a new finite‐element mesh was selected to represent R‐5. In Phase II, with the new mesh in place, evaporation was considered for the R‐5 events. In Phase III, with the new mesh in place and evaporation considered, the geology of R‐5 was approximated. Each phase, compared with the results reported by VanderKwaak and Loague (2001. Water Resources Research 37 : 999–1013), shows a change in the simulated near‐surface response. The performance of InHM for 15 R‐5 events is also reported herein. The results from two stages of model calibration are presented. The uncertainty in initial soil‐water content estimates for event‐based simulation is shown to be a major limitation for physics‐based models. The performance of InHM, relative to past event‐based simulation efforts with a quasi‐physically based rainfall‐runoff model, is better for both peak stormflow and the time to peak stormflow, but worse for stormflow depth. The InHM simulations reported here set the stage for continuous simulation of near‐surface response for the R‐5 catchment with InHM. Copyright © 2004 John Wiley & Sons, Ltd.     

“These Maps Talk for Us:” Participatory Action Mapping as Civic Engagement Practice

Geographers have long been associated with mapping and cartography, because the visual representation of space fits neatly into the wide-ranging discipline that engages both the physical and the social worlds. Mapmaking remained in the domain of experts for centuries until the advent of new mapping technologies, which have widened the possibilities for mapmaking from experts and nonexperts alike. Simply widening participation in mapmaking does not necessarily democratize the knowledge-production process, however, as scholars have recently argued. What is required, we suggest, are critically trained geographers who take seriously both the conventions of professional cartography and the power relations embedded in and reflected in the map-making process and in maps themselves. We name participatory action mapping (PAM) as a methodology that seeks to be as effective in advancing the mapping needs of the public as it is critical in evaluating the processes through which maps are produced. PAM is a practice of civic engagement that borrows from community mapping and public participatory geographic information systems and that is deeply informed by participatory action research. We highlight the contours of PAM through a case study of our work with the Westside Atlanta Land Trust in Atlanta, Georgia.     

Slow dissent and the emotional geographies of resistance

Anger, grief, regret and shame are some of the myriad ways that people narrate a decade of life along the Chad‐Cameroon Oil Pipeline. These stories reveal a perceptive and collective socio‐political awareness situated within multifaceted emotional geographies of resistance. In spite of resistance narratives, explicit and collective resistance practices remain uncommon. As people struggle and live within composite landscapes of structural violence along the pipeline, particular processes and mechanisms of uneven power influence the tendency for resistance struggles to be slow, impromptu or labour based. In this comparative ethnographic analysis, I consider the political environment that shapes socio‐political emotional ties in Nanga, particularly its socio‐political positioning as ‘the village of the First Lady’. In this case, people in proximate positions vis‐à‐vis the ruling family experience heightened oppression(s) and dispossession(s), at the same time that they report feeling little political recourse. In Kribi, on the other hand, responses to the pipeline can be described as defiant withdrawals, demonstrated through a series of unconnected refusals. Although resistance practices along the pipeline have not been visible or successful in an established sense, emotional geographies of resistance elucidate long‐term struggles to survive (i.e., slow dissent), including the accumulation of a collective emotional consciousness grounded in an awareness of historical patterns of injustice.     

Early Eocene magnetostratigraphy and tectonic evolution of the Xining Basin,NE Tibet

The Cenozoic strata of the Xining Basin, NE Tibet, have provided crucial records for understanding the tectonic and palaeo-environmental evolution of the region. Yet, the age of the lower part of the sedimentary stratigraphy and, consequently, the early tectonic evolution of the basin remain debated. Here, we present the litho- and magnetostratigraphy of various early Eocene sections throughout the Xining Basin independently constrained by the U–Pb radiometric age of a carbonate bed. Our study extends the dated stratigraphy down to 53.0 Ma (C24n.1r) and reveals highly variable accumulation rates during the early Eocene ranging from 0.5 to 8 cm/ka. This is in stark contrast to the low but stable accumulation rates (2–3 cm/ka) observed throughout the overlying Palaeogene and Neogene strata. Such a pattern of basin infill is not characteristic of flexural subsidence as previously proposed, but rather supports an extensional origin of the Xining Basin with multiple depocentres, which subsequently coalesced into a more stable and slowly subsiding basin. Whether this extension was related to the far-field effects of the subducting Pacific Plate or the India–Asia collision remains to be confirmed by future studies.     

Community geography for precarious researchers: examining the intricacies of mutually beneficial and co-produced knowledge

At the center of community geography is a commitment to mutually beneficial and co-produced knowledge. While the intricacies of managing these two commitments are often well-articulated for community partners, university faculty and their undergraduate students, the experiences of precariously positioned researchers (such as graduate students or those who work outside the university) remain under-examined. Therefore, through a reflection on the authors’ personal experiences facilitating community geography projects, this paper takes seriously the experiences of precariously positioned researchers. We highlight how the privileging of co-production creates moments of dissonance for precariously placed researcher’s experiences of mutually beneficial research. We argue that precarity, particularly when paired with privilege, results in heightened feelings of risk that may lead researchers to compromise their own ethics or values to ensure both the ongoing continuation of the partnership and the desired goals of the community partners. As we work to further establish community geography, we call for more nuanced considerations of how the entanglements fostered through co-production impact experiences of mutually beneficial research for differently positioned researchers, particularly for those situated within in the neoliberal university.

     

Development of methods to collect and analyze gasoline range (C5–C12) hydrocarbons from seabed sediments as indicators of subsurface hydrocarbon generation and entrapment

Gasoline range hydrocarbons (C₅–C₁₂) are usually associated with petroleum generation, yet few surface geochemical surveys have attempted to evaluate the gasoline range hydrocarbons in near-surface marine sediments. This is due to the difficulty in capturing and analyzing this volatile range of hydrocarbons with minimum loss and evaporative fractionation. In this study, a Headspace Solid Phase Microextraction (HSPME) method was developed and evaluated for the purpose of capturing the gasoline range of hydrocarbons within unconsolidated sediment using a solventless protocol.