May 6, 2014

Category: News

This year’s Montgomery Prizes were recently awarded at two annual events designed to recognize excellence in student research: El Día del Agua and GeoDaze. The El Día winner is Dan Ritter, a Ph.D. candidate in the Department of Hydrology & Water Resources at the University of Arizona (UA). Dan will be sharing his research on coalbed methanogenesis with us as part of a brown bag at the Tucson office on Wednesday, May 14. The GeoDaze winner is Kendra Murray, whose research focuses on Colorado Plateau laccoliths. She is a Ph.D. candidate in the UA’s Department of Geosciences.

The award, a longstanding M&A tradition, consists of a $2,000 cash prize. The winners are selected from a pool of UA graduate students based on the recommendations of a panel of faculty members and professionals.

El Día del Agua is 1-day symposium that serves as a research showcase for the Department of Hydrology & Water Resources. GeoDaze is a student-run symposium organized each year by the Department of Geosciences. It gives students in the geosciences an opportunity to showcase their research and receive valuable feedback from faculty, alumni, and peers. Both events are held in April each year.

Abstract: Relationship between recharge, redox conditions and microbial methane generation in coalbeds

—Daniel Ritter, Jennifer C. McIntosh, and David Vinson

Coalbed methane (CBM) represents a significant portion of the world’s natural gas reserves, and approximately 20% of the world’s natural gas is thought to be microbial in origin.  Laboratory and field experiments have shown that microbes are actively generating CBM in many sedimentary basins worldwide.

In order to better understand in situ conditions that lead to microbial CBM generation, samples were collected in areas of sulfate reduction and methanogenesis in the Powder River Basin. Results from the Powder River Basin were also compared with results from other studies in the Williston Basin, Elk Valley Coalfield, Manville Coalfield, and the Illinois Basin to investigate what impact groundwater flow and recharge might have on methanogenesis.

It was expected that there would be different pathways and conditions for methanogenesis in different areas of the basins (i.e. basin margins versus basin centers). Coal waters associated with methanogenesis have relatively consistent major ion chemistry, with SO42- concentrations < 0.1 mM. The relationship between ?13C of CO2 and CH4 varies by sample location in the basins, and to a lesser extent between basins. These variations could indicate a lower extent of methanogenesis at basin margins and higher extent of methanogenesis at basin centers relative to non-methanogenic processes such as sulfate reduction.

Organic carbon analysis showed greater biodegradation in sulfate reducing coals than in methanogenic coals. Microbiology results showed that samples collected from sulfate reducing areas in different basins were more similar to each other than to samples collected from methanogenic areas in the same basin.

Abstract: Oligocene laccoliths on the Colorado Plateau: A key to understanding Cenozoic cooling and canyon cutting

—Kendra E. Murray, Peter W. Reiners, Stuart N. Thomson

The timing, mechanisms, and magnitude of Colorado Plateau uplift and erosion during the Cenozoic have been among the great puzzles of North American geology since the first observations of Powell, Gilbert, and Dutton, and remain controversial today. Erosion of Utah’s iconic canyon country is unconstrained compared to the region along the Colorado River downstream of Lee’s Ferry, Arizona, including the Grand Canyon. In part, this is because many low-temperature thermochronometers in rocks exposed in canyon country were not sufficiently reset during Mesozoic burial to clearly constrain Cenozoic exhumation patterns.

Here, we demonstrate that the magmatic history of the Henry, La Sal, and Abajo mountains provides unique constraints on the middle and late Cenozoic cooling and exhumation history across the Colorado Plateau. In these mountains, shallowly intruded Oligocene laccoliths heated late Paleozoic and Mesozoic country rocks ~26 Ma, locally resetting apatite fission track and (U-Th)/He ages so they record Miocene-Quaternary cooling and exhumation. Sandstone samples from within 3 km of laccoliths in each mountain range yield single-grain apatite He ages that vary with the effective uranium concentration (eU). In these key samples, minimum He ages are ~5 Ma in grains with eU < 10 ppm, and maximum ages are ~25-20 Ma with eU > 60 ppm.

We model and interpret these apatite He age-eU patterns together with (1) with stratigraphic constraints, (2) the timing of laccolith intrusion determined by zircon U/Pb geochronology, and (3) the extent of resetting temperatures >100 ?C by apatite fission track analysis. Results require prolonged sample residence in the apatite He partial retention zone (40-60 ?C, ~1-3 km depth) from 25-10 Ma, suggesting there was minimal early and middle Miocene exhumation. Best fit time-temperature solutions show onset of rapid cooling to surface temperatures in the Plio-Pleistocene; this erosional event removed ~1.5 km at time-averaged rates between 0.3 and >1.0 km/Myr.

These results agree with previous studies that link regional late Cenozoic exhumation to the ~6 Ma integration of the Colorado River through the western Grand Canyon. More broadly, the Oligocene laccoliths are one expression of a regional thermo-magmatic event that likely transiently raised the Plateau’s geothermal gradient. Therefore, previous interpretations of cooling in the late Oligocene as an erosional event in the Grand Canyon and elsewhere need to be re-evaluated.

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