Corners Area Research Articles

Probabilistic Determination of the Role of Faults and Intrusions in Helium‐Rich Gas Fields Formation

AbstractNatural gas fields with economic helium (>0.3 He %) require the radioactive decay of crustal uranium (U) and thorium (Th) to generate He and tectonic/structural regimes favorable to releasing and concentrating He. An unknown is determining the role of faults and structural features in focusing deep‐seated He sources on shallow accumulations. We tested the correlation between high‐He wells (n = 94) and structural features using a new high‐resolution aeromagnetic survey in the Four Corners area, USA. A depth‐to‐basement map with basement lineaments/faults, an intrusion map, and a flattened basement structural high map were created using Werner deconvolution algorithms by combining magnetic, gravity, and topography data with magnetic and gravity depth profiles. We show quantitatively (via analysis of variance) that a non‐random process controls the relationship between He (>0.3%) and both basement faults and intrusions: 88% of high‐He wells occur <1 km of basement faults; and 85% of high‐He wells occur <1 km of intrusions. As He % increases, the distance to the structural features decreases. Strong spatial/statistical correlations of He wells to both basement faults and intrusions suggest that advective transport via faults/intrusions facilitates He migration. The role of gas phase buoyancy and structural trapping is confirmed: 88% of high‐He occurs within basement structural highs, and 91% of the remaining wells are <1 km from intrusions (potential structural high). We present a composite figure to illustrate how a probabilistic approach can be used as a predictive model to improve He exploration success by targeting zones of intersection of basement faults and intrusions within basement structural highs.

Regional Engagement, Identity, and Exchange Across an Indigenous Borderland: An Investigation of Ceramic Artifacts in the International Four Corners Area of the American Southwest/Mexican Northwest

Archaeologists traditionally assess the archaeological record of the International Four Corners area through a perspective focused on external cultural traditions and processes. In particular, prior interpretations view the history of this portion of the American Southwest/Mexican Northwest as either the northern periphery of the Casas Grandes culture or the southern extent of the Salado culture. However, there are few systematic investigations of excavated sites and individual site excavation reports that offer competing views of a highly connected frontier or a weakly integrated, independent series of residential sites. To better understand the dynamics of the region, I analyzed ca. 80,000 ceramic artifacts from 11 sites and compared the assemblages. Results demonstrate that sites in the area have dynamic internal histories that do not readily fit within existing models for peripheral zones. I interpret the collective assemblages as indicative of a borderlands zone wherein mixed identities, exchange networks, and ethnicities abounded.

Constructing a borderlands in the ancient international four corners: Settlement layout, architecture, and mortuary practices in thirteenth through fifteenth century CE villages along the contemporary united states-Mexico border

Archaeological interpretations for the seemingly sudden introduction of new types of material culture or cultural practice often include attribution to the arrival of a migrant population as part the construction of a periphery or frontier zone. In the International Four Corners area of the American Southwest/Mexican Northwest, archaeologists often correlate the ascendancy of Paquimé in the late thirteenth century CE with the development of a northern periphery in southwestern New Mexico. Simultaneously, sites in far southeastern Arizona became partially integrated into the Salado phenomenon. I evaluate architecture, settlement, and mortuary data from 26 sites with respect to existing models. Given ongoing historian discourse regarding Indigenous borderlands during European colonization, I advocate a model enabling the occurrence of borderlands construction prior to colonization and lacking a predominate hierarchical society. I conclude that the inhabitants of the International Four Corners region situated themselves within multiple inter- and intra-regional zones of interaction and that existing models of frontiers and edge regions are inadequate to address the variability present, but that of the borderlands does as it recognizes relationships to adjacent culture cores as influential but also centers the local inhabitants and their agency.

High helium reservoirs in the Four Corners area of the Colorado Plateau, USA

Radiogenic 4He is naturally produced in Earth's crust due to alpha decay of Uranium (U) and Thorium (Th). Helium has unique thermodynamic properties required for the medical imaging industry, aerospace and other fields of high-tech manufacturing, and currently is in increasingly high demand. Despite its economic value, the mechanisms of helium migration and retention in sedimentary basins remain poorly understood.Oil and gas fields with economic helium (>0.3%) concentrations have been discovered in Paleozoic intervals in the Colorado Plateau, southwestern USA. Here we report new noble gas isotope and abundance data for gas samples (n = 31), from actively producing Paleozoic formations within five fields: Ratherford, Tocito Dome, Navajo Springs, Pinta Dome, and Dineh-Bi-Keyah. Helium concentrations range from 0.01% to 7.9% with varying amounts of liquid and gaseous hydrocarbons, N2, and CO2. We present multi-stage gas, water, and oil equilibration models to account for the observed noble gas elemental and isotopic signatures. Oil-dominated systems are explained by a closed system oil/water equilibration and subsequent admixture of air. He-rich dry gas samples exhibit uniform 4He/N2 ratios consistent with the regional mean values, suggesting a common crustal source and no subsequent fractionation. In contrast, air-derived 20Ne/36Ar ratios are highly fractionated. These observations are consistent with a tectonically controlled crustal gas release from the basement, groundwater saturation with 4He and N2, and subsequent degassing. Extensive gas-water interaction (i.e., migration) leads to extreme fractionation of 20Ne/36Ar, but does not affect 4He/N2 due to water saturation with crustal gases released from the basement.We show the volume of rock required to have produced helium in the reservoir to be significantly larger than the current reservoir volume immediately beneath the field. Therefore, the reservoir helium concentration cannot be sourced by in-reservoir decay of U and Th and instead requires a process to incorporate exogenous sources of helium in the reservoir without significant dilution from hydrocarbons. For helium-rich fields, excess helium is sourced from the Precambrian granitic basement likely utilizing a large area beneath the field area (i.e., crustal gas mobilization and transport via fracture zones), consistent with the degree of water contact. Deep crustal faults in the Precambrian basement are in close proximity to the high helium fields, indicating that these structures are potentially serving as primary migration conduits via advective fluid flow.

Systemic Disease and Ocular Comorbidity Analysis of Geographically Isolated Federally Recognized American Indian Tribes of the Intermountain West.

Background: The American Indian Navajo and Goshute peoples are underserved patient populations residing in the Four Corners area of the United States and Ibupah, Utah, respectively. Methods: We conducted a cross-sectional study of epidemiological factors and lipid biomarkers that may be associated with type II diabetes, hypertension and retinal manifestations in tribal and non-tribal members in the study areas (n = 146 participants). We performed multivariate analyses to determine which, if any, risk factors were unique at the tribal level. Fundus photos and epidemiological data through standardized questionnaires were collected. Blood samples were collected to analyze lipid biomarkers. Univariate analyses were conducted and statistically significant factors at p < 0.10 were entered into a multivariate regression. Results: Of 51 participants for whom phenotyping was available, from the Four Corners region, 31 had type II diabetes (DM), 26 had hypertension and 6 had diabetic retinopathy (DR). Of the 64 participants from Ibupah with phenotyping available, 20 had diabetes, 19 had hypertension and 6 had DR. Navajo participants were less likely to have any type of retinopathy as compared to Goshute participants (odds ratio (OR) = 0.059; 95% confidence interval (CI) = 0.016–0.223; p < 0.001). Associations were found between diabetes and hypertension in both populations. Older age was associated with hypertension in the Four Corners, and the Navajo that reside there on the reservation, but not within the Goshute and Ibupah populations. Combining both the Ibupah, Utah and Four Corners study populations, being American Indian (p = 0.022), residing in the Four Corners (p = 0.027) and having hypertension (p < 0.001) increased the risk of DM. DM (p < 0.001) and age (p = 0.002) were significantly associated with hypertension in both populations examined. When retinopathy was evaluated for both populations combined, hypertension (p = 0.037) and living in Ibupah (p < 0.001) were associated with greater risk of retinopathy. When combining both American Indian populations from the Four Corners and Ibupah, those with hypertension were more likely to have DM (p < 0.001). No lipid biomarkers were found to be significantly associated with any disease state. Conclusions: We found different comorbid factors with retinal disease outcome between the two tribes that reside within the Intermountain West. This is indicated by the association of tribe and with the type of retinopathy outcome when we combined the populations of American Indians. Overall, the Navajo peoples and the Four Corners had a higher prevalence of chronic disease that included diabetes and hypertension than the Goshutes and Ibupah. To the best of our knowledge, this is the first study to conduct an analysis for disease outcomes exclusively including the Navajo and Goshute tribe of the Intermountain West.

Heat Flow Data in an Area of the Eastern Southern Basin and Range in Arizona Contribute to an Analysis of Neogene Lithosphere Thinning Greater than 100 Km

Abstract Heat flow data and thermochronologic derived paleotemperature gradient data are examined to calculate heat flow ~25 Ma and, at present, for a southern Basin and Range location north of Tucson, Arizona. An increase in the surface heat flow is estimated from ~25 Ma to the present; changing from ~47 to ~83 mW m-2. Steady-state conduction temperature vs. depth profiles provide estimates of lithosphere thicknesses both for the present and for ~25 Ma. Different heat transfer models for present heat flow predict present LAB depth that agrees with seismic studies. From these temperature profiles, lithosphere thinning from ~184 km to ~70 km is suggested during the Neogene. Mantle lithosphere thinning caused by thermal phenomena is likely a fundamental driving force for southern Basin and Range extension. Because the mantle lithosphere has likely thinned much more than the crust, it is shown that additional vertical advection, such as an asthenosphere plume, delaminating part of the mantle lithosphere, convection cells, and rising magmas along conduits, add to the vertical advection component of upper mantle lithosphere extension. Interestingly, values of heat flow 25 Ma, lithosphere thicknesses 25 Ma, and Neogene lithosphere thinning are somewhat similar for the Four Corners area of the Colorado Plateau and the southern Basin and Range, even though Neogene tectonic development was quite different, i.e., no Neogene extension in the Colorado Plateau vs. ~57% in the southern Basin and Range. Neogene lithosphere thinning phenomena are likely different in the two regions.

Gaseous Oxidized mercury dry deposition measurements in the Four Corners area, U.S.A., after large power plant mercury emission reductions

Gaseous oxidized mercury (GOM) dry deposition measurements using surrogate surface passive samplers were collected at six sites in the Four Corners area, U.S.A., for the two-year period August 2017–August 2019, after the implementation of large power plant mercury emission reductions across the U.S.A. Two-year baseline GOM dry deposition measurements at the same six sites in the Four Corners area, taken before the implementation of U.S.A. power plant mercury control regulations, were conducted earlier from August 2009–August 2011. The GOM dry deposition rate estimate decreased at the Four Corners area high elevation remote mountain site of Molas Pass, Colorado (3249 m asl) from 0.4 ng/m2h for August 2009–August 2011 to 0.3 ng/m2h for August 2017–August 2019. In contrast, GOM dry deposition rate estimates for the remaining five sites increased for August 2017–August 2019, ranging from 0.8 to 1.3 ng/m2h, up from the August 2009–August 2011 range of 0.6–1.0 ng/m2h. Comparisons of median GOM dry deposition values showed a statistically significant decrease of 17 ng/m2 at the Molas Pass site between August 2009–August 2011 and August 2017–August 2019, and a statistically significant increase of 66 ng/m2 and 64 ng/m2, respectively, at the Mesa Verde National Park and Farmington Substation sites between August 2009–August 2011 and August 2017–August 2019. For the four years of GOM dry deposition data collected in the Four Corners area annual GOM dry deposition levels ranged from 2237 ng/m2yr (at the Molas Pass high elevation remote mountain site) to 11542 ng/m2yr (at the Mesa Verde National Park site), and the estimates were generally higher in magnitude in the spring and summer compared to the fall and winter. In light of the unexpected increases in GOM dry deposition rates at the non-remote sites, it is suggested that large regional wildfires and local anthropogenic mercury emission sources from cities and oil/gas production areas are possible notable contributors to the GOM dry deposition measurements collected in the Four Corners area.

Identification of the natural origin of waterproofing pine pitch in historical Southwest Native American basketry through gas chromatography-mass spectrometry

Archaeological and ethnographic records document a long and rich history of plant-based resource use by Indigenous peoples of the American Southwest. The history of this resource use tells a complex story of human-environment relationships in this arid landscape. Indigenous peoples of the American Southwest have historically made use of pine pitch to waterproof their basketry. Here, we have identified the pine species origin of the pitch used to construct historical baskets utilizing gas chromatography-mass spectrometry. Resin samples were collected from the three main resinous plants in the Four Corners area: Pinus edilus, Pinus ponderosa, and Juniperus scoplulorum, and their GC chromatograms were analyzed for characteristic molecular components. Furthermore, some of these resin samples were heated to simulate the pitch-application process to ensure the characteristic molecular components were still present after baking. We have found that the ratio of γ-muurolene to longifolene provides a reliable indication of the pines species origin of the pitch, even after being exposed to elevated temperatures, and that all 14 of the historical baskets analyzed were waterproofed primarily with resinous material collected from the Pinus edilus species.

An Optimistic Engineer

An Optimistic Engineer Nick Arvin (bio) They depart in the early morning hours in a rainstorm, and as they drive north the sheets of falling water turn to windblown snow. The client leads in an SUV with a couple of his employees; Jake follows in his own SUV, Reggie beside him. Despite the weather, the client presses the speed limit, 75 mph in Colorado, 80 mph in Wyoming. Two hours. Three hours. "We're going to die," Jake says, repeatedly. "We're going to die in flames and snow." The wipers labor, and the tires carry forward on the icy lanes as if floating. In the passenger seat, Reggie sleeps. Reggie is hard of hearing. It is late May; five days ago it was 90 degrees. Despite the specter of imminent death, it is a long drive, and Jake's thoughts stray to other depressing topics. When they finish their work today, they will spend the night in Gillette. Jake's ex-wife, Deb, is from Gillette, her father a welder and rodeo cowboy, but she moved to Denver the moment she finished high school, and she always said she loathed the wind, dust, and emptiness of Wyoming. Then, as soon as she and Jake were divorced, she moved back to Gillette. Jake notes gloomily that it would be a terrible mistake to go see her. Fiascos will ensue. Yet, of course, while he's in Gillette, he will try to see her. It's doom. But maybe she can help him with the boys. Maybe there's some tiny chance. Reggie in the passenger seat wakes and tells a story about a compressor station that he helped design years ago, which later caught fire. "Of course, a fire starts, they just get everyone out, close the block valves, and let the place burn to the ground! Wasn't my fault, thank God! Bad relief valve!" Due to his difficulty hearing, Reggie speaks as if yelling to someone in another room. The snow turns back to rain. Perhaps they won't die after all. And maybe it doesn't matter anyway, Jake thinks, since every day is either a slow death or a fast death. That's the only real choice. He tells Reggie—because it's necessary during this trip, and he wants to get it out of the way—that he'll probably have to shut down the office next week and file for bankruptcy. "You've said that a hundred times!" Reggie says. [End Page 18] This time it's absolutely true, but Jake doesn't say that. Instead he says, "The thing everyone forgets about the boy who cried wolf is that eventually the wolf did come and eat everything up." ________ At a gas station, they meet several men who work for the Arroyo Pipeline Company. In a convoy of SUVs, they pass over a cattle guard and proceed by gravel backroads and two-tracks into the rolling empty expanse of prairie. Eventually, they arrive at a complex of machinery on a large square of gravel. Massive engines power reciprocating compressors interconnected by steel pipes with huge vessels half buried in the earth and a 25-foot flare stack flaming at the top with a continuous ripping sound. The machines here pull natural gas from wellheads, knock out some of the liquids that come with the gas, and then push the gas on to other facilities for more cleanup, more compression, and finally to factories, power plants, homes, and kitchen stovetops. Jake has spent a lot of time in places like this, and still it seems a miracle that it all doesn't blow up and kill him. Who knows when the relief valves were last tested? The operators mention that these engines have more than 100,000 operating hours each. They mention "deferred maintenance." The client draws Jake away from the others. The client's name is Samuel, and Jake worked with him years ago, when they were young engineers, designing and building natural gas liquids terminals in the Four Corners area. He wants to know what Jake thinks. "What are you hoping I'll think?" Jake asks. "I think it looks fine," Samuel...

W. Gerald Rainer, MD, November 13, 1927 to November 14, 2017

W. Gerald Rainer, the 25th president of The Society of Thoracic Surgeons and a founding member of The Society of Thoracic Surgeons, died suddenly on November 14, 2017, the day after celebrating his 90th birthday with his family. He will be remembered as a superb private practice cardiothoracic surgeon who also participated in research, had a strong commitment to the community and the University of Colorado, and a superb record of service to The Society of Thoracic Surgeons and other professional societies.

Providing a model for estimating the compressive strength of square and rectangular columns confined with a variety of fibre-reinforced polymer sheets

One of the most common ways of strengthening the columns is the confinement of the reinforced concrete columns. So far, several experiments have been conducted on concrete columns confined with fibre-reinforced polymer sheets and the results show that the use of fibre-reinforced polymer sheets increases the compressive strength of the concrete columns effectively. Different models in order to determine the compressive strength of the fibre-reinforced-polymer-confined concrete columns are provided in the previous researches. In this study, a wide range of experimental data for square and rectangular columns confined with a variety of fibre-reinforced polymer sheets has been collected. In order to increase the accuracy in the existing models, a modified model for predicting the compressive strength of square and rectangular columns confined with different fibre-reinforced sheets has been proposed by using the collected data. In the current study, the fibre-reinforced polymer strain efficiency factor and the section shape factor are composed and considered as a unique factor. Considering the fibre-reinforced polymer hoop strain concentration at the affected area including the corners area and the surrounding area of them, this factor has been adopted as a fibre-reinforced polymer averaged hoop strain factor for the whole circumference of the section. Based on the published experimental data, a comparison between the analytical results obtained with the present model and the results of the other models, shows considerably better results of the proposed model described in this study. Moreover, application of the proposed model for behaviour prediction of concrete-filled fibre-reinforced polymer tubes is evaluated in this study and the results show that the proposed model is more accurate than the other models.

Evaluation of trade and interaction between Chaco Canyon and Chaco outlier sites in the American Southwest by investigating trachybasalt temper in pottery sherds

Deciphering trade and interaction in prehistory is central for archeologists to understand social, economic, political, and religious ties among groups in a particular area. Constraint on the provenance of raw materials used for manufacturing pottery vessels and their original sources is critical to these goals. In this study, trade patterns and interaction between Pueblo Bonito in Chaco Canyon and Pueblo groups in the Chuska Mountains, along with four Chaco outliers in the Four Corners area of the American Southwest, were investigated using chemical analysis of sanidine crystal inclusions identified in three groups of trachybasalt pottery temper and rocks from the Navajo volcanic field. Initial petrographic analyses were then complemented with chemical analyses using EMP and LA-ICP-MS to determine the concentrations of major and rare trace elements of the sanidine identified in trachybasalt rocks and temper sherds. Partial least squares regression on trace element concentrations from sanidine analyses for the pottery sherds and rock samples demonstrate that although the majority of sanidine tempered materials was procured from Narbona Pass in the Chuska Mountains, New Mexico, some ceramic samples were still procured from other areas of the Navajo volcanic field. The results of our provenience and geochemical study indicate that a strong relationship between Chaco Canyon and Chaco outliers should be reconsidered, and that a political and religious model of exchange—people in Chaco Canyon and Chuska Mountains controlled production and exchange systems in the San Juan Basin—should be reevaluated.

The nutritional context of the Pueblo III depopulation of the northern San Juan: Too much maize?

The abandonment of the Four Corners area in the U.S. Southwest is a long-standing problem in American archeology. Recent work has shown that the terminal occupation was concentrated into a limited number of large defensive sites. This resulted in an extreme emphasis on maize, which was nutritionally unsustainable because of its low lysine and tryptophan content. I describe the processes that led to this settlement pattern and the limitations of the Pueblo II–III maize-based diet. I then explain how the combination of the settlement pattern and the extreme reliance on maize resulted in a nutritionally fragile situation that collapsed.

Heat-flow data in the Four Corners area suggest Neogene crustal warming resulting from partial lithosphere replacement in the Colorado Plateau interior, southwest USA

The uplift of the Colorado Plateau is of great geologic interest, and low-noise terrestrial heat-flow data can provide boundary conditions that should be met by proposed models of uplift. The structural continuity of the plateau interior with respect to the neighboring Basin and Range and Southern Rocky Mountains implies a distinctive Neogene lithosphere-crustal evolution. The Four Corners area represents a unique location in the Colorado Plateau interior, and likely the world, where both heat-flow data with little variability, and mineralogy studies relating to pre-Neogene lithosphere temperatures, are available. Together these studies allow one to calculate data-derived heat-flow values at two different times, ca. 25 Ma and the present. Using a first-order model for temperature-depth profiles, these two heat-flow values imply ∼100 km of Neogene lithosphere-asthenosphere boundary (LAB) shallowing in the study area. If crustal radiogenic heat remains constant, the calculated Neogene increase in heat flow of ∼17 mW m –2 will be produced below the Moho. Several mechanisms are considered that might cause the LAB to shallow and the near-surface heat flow to increase. Derived Neogene lithosphere thinning of ∼100 km leads one to consider lithosphere displacement of the same order. If the presented finite-difference model approximates Neogene thermal phenomena in the Four Corners, then partial lithosphere replacement of the upper-mantle lithosphere appears to be the most suitable cause of the heat-flow rise in the area, as well as the near-uniform heat flow over large regions of the plateau interior identified by low-noise heat-flow data.

Tribe at a Crossroads: The Navajo Nation Purchases a Coal Mine

Tribe at a Crossroads: The Navajo Nation Purchases a Coal Mine

Gaseous oxidized mercury dry deposition measurements in the southwestern USA: a comparison between Texas, eastern Oklahoma, and the Four Corners area.

Gaseous oxidized mercury (GOM) dry deposition measurements using aerodynamic surrogate surface passive samplers were collected in central and eastern Texas and eastern Oklahoma, from September 2011 to September 2012. The purpose of this study was to provide an initial characterization of the magnitude and spatial extent of ambient GOM dry deposition in central and eastern Texas for a 12-month period which contained statistically average annual results for precipitation totals, temperature, and wind speed. The research objective was to investigate GOM dry deposition in areas of Texas impacted by emissions from coal-fired utility boilers and compare it with GOM dry deposition measurements previously observed in eastern Oklahoma and the Four Corners area. Annual GOM dry deposition rate estimates were relatively low in Texas, ranging from 0.1 to 0.3 ng/m2h at the four Texas monitoring sites, similar to the 0.2 ng/m2h annual GOM dry deposition rate estimate recorded at the eastern Oklahoma monitoring site. The Texas and eastern Oklahoma annual GOM dry deposition rate estimates were at least four times lower than the highest annual GOM dry deposition rate estimate previously measured in the more arid bordering western states of New Mexico and Colorado in the Four Corners area.

Towards a Common Understanding: A Revision of Forurche Maline Chronology in Oklahoma

Answering the question of what Fourche Maline is has long been hampered by lack of consensus on the terminology and chronology within the ancestral Caddoan area of the four corners area of southeast Oklahoma, southwest Arkansas, northeast Texas and northwest Louisiana. To address this problem an evolution of the Oklahoma concept of Fourche Maline is presented, and developing a new chronology based on seriation of temporally sensitive artifact categories is discussed. It is concluded that Fourche Maline is a solely Woodland period culture, with a chronology that makes identifying changes over time possible. This conclusion will facilitate the understanding of Fourche Maline within the Caddon homeland.

CHACOAN AND POST-CHACO OCCUPATIONS IN THE MIDDLE SAN JUAN REGION: CHANGES IN SETTLEMENT AND POPULATION

The Middle San Juan region occupies a critical location between Chaco Canyon to the south and Mesa Verde to the north. Site-settlement analysis and demographic reconstruction reveal continuous occupation across the Pueblo II–III transition and steady population growth for 200 years prior to regional depopulation in the late A.D. 1200s. These findings challenge previous research which postulated two distinct occupations: Chacoan domination during the Pueblo II period, followed by abandonment and subsequent immigration from Mesa Verde comprising reoccupation during the Pueblo III period. Our research indicates that late Pueblo II Chacoan colonization at Salmon Pueblo (along the San Juan River) and Aztec Ruins (on the Animas River) occurred abruptly in areas with little previous settlement. In contrast, much smaller Chacoan outliers were built within dense, extant communities in the La Plata Valley and lower San Juan River between A.D. 1075 and 1125. Instead of abandonment coinciding with the region-wide drought during the mid-1100s, we argue that demographic trends and other archaeological evidence indicate sustained population growth coinciding with changes in social identity during the post-Chaco era. Depopulation did not occur until the late thirteenth century when a similar process affected the entire Four Corners area.

Gaseous oxidized mercury dry deposition measurements in the Four Corners area and Eastern Oklahoma, U.S.A.

Gaseous oxidized mercury (GOM) dry deposition measurements using surrogate surface passive samplers were collected in the Four Corners area and eastern Oklahoma from August, 2009–August, 2011. Using data from a six site area network, a characterization of the magnitude and spatial extent of ambient mercury pollution in the arid Four Corners area was accomplished, which included the observation of a strong regional signature in the GOM dry deposition data set. GOM dry deposition rate estimates ranged from 0.4–1.0 ng/m2 h at the six Four Corners area monitoring sites, while the GOM dry deposition rate estimate at the eastern Oklahoma monitoring site was lower at 0.2 ng/m2 h. The highest GOM dry deposition estimates were recorded during the spring and summer while the lowest GOM dry deposition estimates were recorded during the fall and winter. During the second year of this study the highest annual GOM dry deposition estimate so far measured in the United States (U.S.) with smooth–edge surrogate surface passive samplers, 10 889 ng/m2, was recorded at the Mesa Verde National Park site, a site at which the two– year cumulative GOM dry deposition estimate exceeded the mercury wet deposition estimate. GOM dry deposition estimates during the second year of the study were statistically significantly higher than the first year of the study at six of the seven sites. The data from this study provide a two–year baseline of GOM dry deposition data in the Four Corners area and eastern Oklahoma immediately before the current implementation of new U.S. power plant and boiler mercury control regulations which will significantly reduce mercury emissions from those two sectors of local and regional anthropogenic mercury emission sources.

Influence of Relictual Species on the Morphology of a Hybridizing Oak Complex: An Analysis of theQuercusX UndulataComplex in the Four Corners Region

ABSTRACT. The genus Quercus represents an important component of the scrublands and low forests of the Four Corners area of northeastern Arizona, southeastern Utah, southwestern Colorado, and northwestern New Mexico. The area is dominated by the widespread Quercus gambelii, although relictual and marginal populations of Q. turbinella and Q. havardii are also present. Hybridization among these 3 species is common, leading to much variability in phenotypic traits, and this hybridization forms part of the widespread complex of hybrid oaks recognized as Q. x undulata. Multivariate analysis of leaf morphology was performed in 9 populations, representing the various taxa and levels of hybridization. This analysis was intended to determine the distinctiveness conferred by, and extent of influence from, hybridization and successive introgression. It was found that, though common, hybridization seems to act at very restricted spatial scales and not in the production of a region-wide hybrid swarm. Most hybridizatio...