Madeline E. Schreiber

Sulfur geochemical constraints on mesoproterozoic restricted marine deposition: lower Belt Supergroup, northwestern United States

Abstract Despite nearly a century of study by a diverse group of Precambrian workers, competing interpretations have persisted that favor both marine and nonmarine depositional settings for the Mesoproterozoic Belt Supergroup of the northwestern United States. Our data for sedimentary pyrite formation in unmetamorphosed organic-rich shales of the Newland Formation of the lower Belt Supergroup, western Montana, argue that at least a portion of the Belt basin, during its early history, was characterized by restricted marine conditions with only episodic incursions of seawater. This model is supported by ratios of organic carbon (Corg) to pyrite sulfur that, once corrected for selective diagenetic and thermal loss of Corg and the inferred high reactivity of the Corg reservoir in the absence of vascular land-plant remains, suggest degrees of sulfate limitation consistent with a restricted marine setting. The isotopically heavy disseminated pyrite that dominates the black shales shows a broad range of δ34S values of bacterial origin (δ34S ranges from −8.7 to +36.3‰; mean = +7.6‰, n = 41). The pyrite data overlap with the δ34S range of coeval (barite) sulfate within the basin and are similar to independent estimates for the sulfate of Mesoproterozoic seawater. These relationships have allowed us to delineate a record of bacteriogenic pyrite that formed in a modified marine reservoir where the supply of sulfate was limited. A bacterial mechanism is further suggested by the morphological character of this early-formed pyrite, including the ubiquitous framboids. After comparing our results with those from present-day localities, such as the Black Sea and sites of lacustrine deposition, we conclude that the abundant 34S-enriched bacteriogenic disseminated pyrite observed in black shales of the Newland Formation must record a style of sulfate limitation possible only within an isolated marine basin receiving an intermittent flux of seawater. Although the entire Belt basin likely remained a restricted marine setting throughout deposition of the lower Belt Supergroup, sulfur isotope data from the Newland Formation at two localities separated by more than 150 km in the eastern Belt basin reveal stratigraphic trends in the strength of the marine connection. Specifically, the connection evolved such that inputs of seawater sulfate increased progressively in frequency and possibly strength during deposition of the Newland Formation. Overall, the hypothesized restricted marine setting, when viewed in light of independent tectonic, sedimentologic and geochronologic evidence, suggests syndepositional and likely episodic continental rifting, which helps constrain the Proterozoic paleotectonic history of western Laurentia.

Development of bacteria-based bioassays for arsenic detection in natural waters.

Arsenic contamination of natural waters is a worldwide concern, as the drinking water supplies for large populations can have high concentrations of arsenic. Traditional techniques to detect arsenic in natural water samples can be costly and time-consuming; therefore, robust and inexpensive methods to detect arsenic in water are highly desirable. Additionally, methods for detecting arsenic in the field have been greatly sought after. This article focuses on the use of bacteria-based assays as an emerging method that is both robust and inexpensive for the detection of arsenic in groundwater both in the field and in the laboratory. The arsenic detection elements in bacteria-based bioassays are biosensor–reporter strains; genetically modified strains of, e.g., Escherichia coli, Bacillus subtilis, Staphylococcus aureus, and Rhodopseudomonas palustris. In response to the presence of arsenic, such bacteria produce a reporter protein, the amount or activity of which is measured in the bioassay. Some of these bacterial biosensor–reporters have been successfully utilized for comparative in-field analyses through the use of simple solution-based assays, but future methods may concentrate on miniaturization using fiberoptics or microfluidics platforms. Additionally, there are other potential emerging bioassays for the detection of arsenic in natural waters including nematodes and clams.

Nitrification rates in a headwater stream: influences of seasonal variation in C and N supply

Abstract Nitrification, the chemoautotrophic process by which NH4-N is converted to NO3-N, is an integral biogeochemical transformation in stream ecosystems. Previous research has shown that experimental addition of dissolved organic C inhibits rates of nitrification, and that NH4-N addition stimulates rates of nitrification. In many streams, large amounts of C and N may be present in particulate and sorbed forms. Hugh White Creek, a headwater stream located in the southern Appalachian mountains of North Carolina, USA, has very low concentrations of dissolved N and receives large inputs of allochthonous particulate organic matter (POM) each autumn. We conducted a seasonal survey of organic matter (OM) standing stocks and nitrification rates, and we experimentally manipulated dissolved C and N supplies in stream-sediment microcosms to determine: 1) how rates of nitrification compare across seasons, and 2) to what extent nitrification rates are influenced by seasonal changes in standing stocks and relative abundances of particulate, sorbed, and dissolved forms of C and N. Rates of nitrification were closely and positively related to rates of ammonification which, in turn, were negatively related to C:N ratios of fine benthic organic matter (FBOM). Uniform additions of dissolved C and N had varying effects on sediment N-transformation rates during different seasons. Variable responses to experimental additions probably reflected the changing relative importance of C and N as sediment OM stocks were depleted and underwent changes in quality. Slow rates of nitrification for much of the year may be attributed to colder temperatures and large quantities of particulate C relative to N. To the extent that changes in POM stocks dictate changes in C and N availability, seasonal OM dynamics are closely linked to rates of nitrification.

Nitrate-enhanced bioremediation of BTEX-contaminated groundwater: parameter estimation from natural-gradient tracer experiments.

Two natural-gradient pulse tracer tests were conducted in a petroleum-contaminated aquifer to evaluate the potential for benzene, toluene, ethylbenzene, and xylenes (BTEX) biodegradation under enhanced nitrate-reducing conditions. Addition of nitrate resulted in loss of toluene, ethylbenzene, and m,p-xylenes (TEX) after an initial lag period of approximately 9 days. Losses of benzene were not observed over the 60-day monitoring period. Tracer breakthrough curves (BTCs) were analyzed to derive transport and biodegradation parameters, including advective velocities, retardation factors, dispersion coefficients, biodegradation rate constants, and nitrate utilization ratios. Using the parameters derived from the BTC analysis, numerical simulations of one of the tracer experiments were conducted using BIONAPL/3D [Molson, J., BIONAPL/3D User Guide, A 3D Coupled Flow and Multi-Component Reactive transport model. University of Waterloo, Waterloo, Ontario, Canada]. Simulations using the BTC-derived transport and biodegradation parameters successfully reproduced benzene, TEX, and nitrate concentrations measured during the tracer experiment. Comparisons of observed and simulated nitrate concentrations indicate that the mass ratio of nitrate-N utilized to TEX degraded increased over time during the experiment, reaching values many times that expected based on stoichiometry of TEX oxidation coupled to nitrate reduction. Excess nitrate loss is likely due to oxidation of other organics in addition to TEX.

Mechanisms of Arsenic Release to Ground Water from Naturally Occurring Sources, Eastern Wisconsin

Arsenic concentrations up to 12,000 μg/l have been measured in ground water from a sandstone aquifer in the Fox River valley in eastern Wisconsin, USA. In addition to a sulfide-bearing secondary cement horizon (SCH), which is present at the top of the aquifer, sulfide mineralization is also present throughout the aquifer. Within the SCH, arsenic occurs in pyrite and marcasite, and in iron hydroxides, but not as a separate arsenopyrite phase. Geologic, hydrogeologic, and geochemical data were used to characterize the arsenic source and the predominant geochemical process that controls its release to ground water. Several lines of evidence suggest that oxidation of sulfides is the cause of high (>100 μg/l) concentrations of arsenic in ground water, including 1) the presence of the arsenic-bearing sulfides in the aquifer; 2) water chemistry data that show a positive correlation between arsenic, iron, and sulfate and negative correlation between arsenic and pH; and 3) similar sulfur isotopic signatures in sulfides of the SCH and dissolved sulfate in ground water. We propose that atmospheric oxygen, introduced to the SCH through well boreholes, provides an oxidant to the system. This hypothesis is supported by the occurrence of high arsenic concentrations where water levels within the well intersect the SCH. However, the data do not unequivocally show sulfide oxidation to be the cause of the moderate (10−100 μg/l) and low (<10 μg/l) arsenic concentrations measured in ground water in the study area. The variability in thickness of the SCH and the concentration of arsenic within the sulfides, as well as the local availability of oxygen to the SCH, likely contribute to the spatial variability of ground water arsenic concentrations.

Influence of elevated As on leaf breakdown in an Appalachian headwater stream

Abstract A headwater stream adjacent to an abandoned As mine was investigated to determine the influence of As on stream biota and organic matter processing using an upstream (reference reach) and downstream (mine-influenced reach) comparative approach. Field assessments of stream chemistry, macroinvertebrate abundance and composition, and leaf breakdown were coupled with laboratory experiments addressing As influences on leaf biofilm respiration. Streamwater As concentrations varied from below detection limit (2.5μg/L) in the upstream reach to >12,000 μg/L. Concentrations were low in the reference reach, increased immediately adjacent to tailing piles, and climbed significantly with distance along the mine-influenced reach. Compared to the reference reach, macroinvertebrate density (7869 vs 154 individuals/m2), shredder abundance (3340 vs 22 individuals/m2), and species richness (11.9 vs 0.8 species/sample) were significantly lower in the mine-influenced reach. For both white oak and red maple leaf packs, breakdown rates in the reference reach (k = 0.0048 and 0.009/d, respectively) were significantly greater than in the mine-influenced reach immediately downstream of waste piles (k = 0.0019 and 0.003/d) and further downstream (k = 0.0014 and 0.005/d). In one experiment, laboratory assays showed that short-term exposure to elevated As concentrations did not alter leaf biofilm respiration rates. In a 2nd experiment addressing chronic exposure, respiration rates for extant leaf biofilms in the reference reach (0.37 ± 0.01 μg O2 mg ash-free-dry-mass [AFDM]−1 h−1) were significantly greater than in the mine-influenced reach (0.29 ± 0.01 μg O2 mg AFDM−1 h−1), but rates in both reaches were typical of forested headwater streams not exposed to elevated As concentrations. Together, these data suggest that elevated As concentrations in the stream have led to altered organic matter processing not by reducing microbial activity but primarily by decreasing invertebrate densities, limiting shredder abundance, and decreasing litter breakdown rates.

Spatial Electron Acceptor Variability: Implications for Assessing Bioremediation Potential

An extensive network of multilevel samplers was established in a hydrocarbon-contaminated wetland aquifer. Results of groundwater sampling for benzene, toluene, ethylbenzene, and xylenes (BTEX), and electron acceptors show that both pristine and contaminated groundwater have spatially variable chemical signatures, owing primarily to microbially mediated oxidation-reduction reactions. Due to these spatial variations, estimates of the efficiency of intrinsic bioremediation can vary significantly depending on how geochemical data are collected. Use of data collected from monitoring wells with screens longer than the vertical extent of the plume will generally underestimate the potential for intrinsic bioremediation for the most chemically active horizon of the plume. A comparison of pristine and contaminated redox patterns demonstrates that, although BTEX exerts the highest demand for electron acceptors, oxidation of natural organic matter also contributes to electron acceptor utilization. If natural and oth...

Pilot testing of a new design for presentation slides to teach science and engineering

Pilot testing in a large geology course shows that a new sentence-headline design of presentation slides was more effective than the traditional phrase-headline design at teaching science to undergraduates. Rather than having a phrase headline supported by a bullet list, the new design relies on a succinct sentence headline supported by visual evidence. In the new design, bullets are not used. The testing of the new slide design involved comparing test scores between a prior section that viewed the traditional design and the test scores of a current section that viewed the new design. On 7 of the 20 questions, the section viewing the new design achieved improvements in test scores that were statistically significant at the 99.9% confidence level. On no question did the prior section viewing the traditional design achieve a higher test score that was statistically significant. The mean test scores on the 20 questions were 71% correct for the earlier group (traditional design) and 82% correct for the latter group (new design). In addition to analyzing the test scores, we conducted surveys that indicate the students preferred the new design over the traditional design by a more than 7-to 1 ratio. These test scores and survey results have implications in the way that presentation slides should be designed not only for science and engineering classrooms, but also for forums of scientific and engineering research

Quantifying Potential Recharge in Mantled Sinkholes Using ERT

Potential recharge through thick soils in mantled sinkholes was quantified using differential electrical resistivity tomography (ERT). Conversion of time series two-dimensional (2D) ERT profiles into 2D volumetric water content profiles using a numerically optimized form of Archies law allowed us to monitor temporal changes in water content in soil profiles up to 9 m in depth. Combining Penman-Monteith daily potential evapotranspiration (PET) and daily precipitation data with potential recharge calculations for three sinkhole transects indicates that potential recharge occurred only during brief intervals over the study period and ranged from 19% to 31% of cumulative precipitation. Spatial analysis of ERT-derived water content showed that infiltration occurred both on sinkhole flanks and in sinkhole bottoms. Results also demonstrate that mantled sinkholes can act as regions of both rapid and slow recharge. Rapid recharge is likely the result of flow through macropores (such as root casts and thin gravel layers), while slow recharge is the result of unsaturated flow through fine-grained sediments. In addition to developing a new method for quantifying potential recharge at the field scale in unsaturated conditions, we show that mantled sinkholes are an important component of storage in a karst system.

Increased Student Learning and Attendance in Resources Geology through the Combination of Sentence-Headline Slides and Active Learning Measures

This study assessed how combining active learning measures with teaching slides that relied on sentence headlines affected test scores and attendance in a large resources geology course. For all six sections of the course, the following remained the same: instructor, classroom, semester time slot, and number of teaching slides. For two sections, though, we transformed the existing teaching slides that followed the traditional design of a phrase headline supported by a bulleted list (and often an image) to a sentence-headline design supported by visual evidence. In this new design, the sentence headline stated the main assertion of the slide. To increase active participation, we posted these sentence-headline slides as fill-in-the-blank notes, as opposed to complete slides as posted for the other sections. To increase attendance in these two sections, we administered a graded activity in each class period. These two sections had statistically significant increases (p < 0.001) in both class attendance and test scores on identical questions.