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Geology

Stratigraphy and preliminary geochemical analysis of Lake Bonneville Marl, Tule Valley, UT

Laura Haynes (2013); Mentor(s): Robert Gaines

Abstract: During the Last Glacial maximum, increased precipitation in the Great Basin created large networks of pluvial lakes, such as Lake Bonneville, in which finegrained carbonate sediments were deposited out of the water column. These deposits of lacustrine marl have enormous geochemical potential for the reconstruction of regional paleoclimate in this critical period of earth's climatic history by using carbonate mineralogy, stable isotope ratios, and sedimentary analysis. Fieldwork was conducted in the Tule Valley of Southwestern Utah, where two >1m marl outcrops were continuously sampled at the opposite ends of the valley in order to investigate intra-basinal climate effects. The Constitution marl outcrop, which likely retains its pelagic sequence, is directly overlain by Provo-aged beach gravels, indicating the quick decline in lake level following the Bonneville flood at 14.5 kya. The aragonite to calcite ratio of the Constitution marl, which may be used as a proxy for lake level and water conditions (Oviatt 1997), increases continuously until ~60cm above base, where it falls and stays relatively constant. X-radiographs of the sequence show alternating periods of finely laminated and massive marl, which will be analyzed in thin section for variations in Ca/Mg ratios. Radiocarbon dates of gastropods and δ18O, δ13C, and clumped oxygen isotope measurements will further illuminate the late Pleistocene paleoclimate record retained by this detailed sequence.
Funding Provided by: Pomona College SURP; Pomona College Geology Department

Evidence for microbial liberation of structurally bound iron in clay minerals as a source of nutrient in the world ocean

Kyle Metcalfe (2014); Mentor(s): Robert Gaines; E.J. Crane

Abstract: Clay minerals are the most abundant materials found at the surface of earth and are the primary constituents of marine sediments. Iron, a limiting nutrient in many marine settings, is a common constituent of clay minerals. Recent experimental evidence has shown that lab cultures of Fe-reducing bacteria are able to utilize structurally-bound Fe from the crystal lattice of nontronite, an uncommon and Fe-rich smectite. Reduction of structurally-coordinated Fe results in liberation of Fe(II) to solution, where it is available for other biotic processes. However, it has remained unclear: 1. if Fe-reducers are able to access structurally coordinated Fe found in low wt.% in common clay minerals; 2. if naturally occurring populations of Fe-reducers are able to reduce structurally coordinated Fe as some lab strains are. In order to address these questions, we conducted experiments using a suite of 16 clay minerals with iron contents ranging from low to high weight percents. Clays were treated with Na-dithionite solution to remove surface Fe. Experimental evidence indicates that Fe(III) bound in common clay minerals is available for reduction by the lab strain S. oneidensis MR-1 as well as by a naturally occurring consortia of Fe-reducers cultured from the San Pedro and Santa Monica Basins. Our findings suggest that common clay minerals may represent a large, previously unrecognized pool of bioavailable Fe in the world ocean that contributes to biogeochemical cycling of Fe and C.
Funding Provided by: Sherman Fairchild Foundation

The Sedimentary Rocks of Long Valley Caldera: A Record of the Post-Eruption History of a Restless Caldera

Benjamin Murphy (2013); Mentor(s): Robert Gaines; Jade Star Lackey

Abstract: 760,000 years ago, caldera collapse after a huge volcanic eruption on the eastern flank of the Sierra Nevada created the large (17 x 32 km) valley north of Bishop, California, that is now known as Long Valley. Almost immediately after formation of this valley, a lake filled the topographic depression. Sedimentary rocks that record the post-eruption history of this volcanic system were subsequently deposited within this lake; the continuing goal of this project is to document and interpret these sedimentary rocks in order to better understand the post-eruption history of what continues to be one of the most restless volcanic systems in the United States. This summer, more than twenty measured stratigraphic sections, over one hundred rock samples, and countless pages of detailed field observations were collected. Preliminary analysis of this information indicates several depositional environments within the caldera lake. Spatially pervasive coarse-grained sandstones and conglomerates indicate that a large delta system existed within the lake. In specific areas, siltstones that include fossilized plant remains record a calm marsh environment that would have existed along shore. Very fine sediments that indicate deep-water deposition contain diatom plankton and ostracods. Cemented and non-cemented beach and terrace gravels record lake level over time. Continuing analysis throughout the upcoming school year will provide a more detailed insight into the history of Long Valley Lake.
Funding Provided by: Sherman Fairchild Foundation; Pomona College Geology Department; American Chemical Society Petroleum Research Fund - 50152- UN18

Probing the Origins of the Kaweah Peaks Volcanics, South-Central Sierra Nevada

James Gordon (2015); Mentor(s): Jade Star Lackey

Abstract removed upon request.

Surface Wave Dispersion from Recent Australian Earthquakes

Dulcie Head (2014); Additional Collaborator(s): Kazunori Yoshizawa†; Mentor(s): Hrvoje Tkalcic*
*The Australian National University; †Hokkaido University

Abstract: Various imaging techniques have been deployed recently to make maps of the subsurface of Australia. These maps contribute to the understanding of structure and evolution of the Australian lithosphere, and can also improve understanding of the associated geohazards. Surface wave group velocity dispersion is one of the tools used to image the shallow subsurface, especially the crust and the uppermost mantle. Most such velocity maps of Australia’s subsurface were made using data from earthquakes that occurred at teleseismic distances. The longer travel paths used in previous studies mean that the average velocity information inevitably included structures from offshore and away from the landmass. Data from recent earthquakes large enough to have excited surface waves that occurred on the continental crust in Australia are thus, arguably, a unique tool to scrutinize the current maps and provide additional information. This study incorporates data from 6 earthquakes and about 200 new paths within the Australian continent. The newly obtained dispersion curves are compared with dispersion curves and tomographic maps obtained using long source-receiver paths from the updated model of Yoshizawa and Kennett (2004) and with the PREM predictions (Dziewonski and Anderson, 1981). This allows for insight into how the offshore distance affects the results and presents the potential to create a better, higher resolution map of crustal and lithoshperic properties of Australia.
Funding Provided by: National Science Foundation; Incorporated Research Institutions for Seismology

Research at Pomona