The timing of oxygenation events throughout Earth’s history remains controversial. Oceans and the atmosphere were essentially devoid of oxygen until the ‘great oxygenation event’ around 2.4 billion years ago that raised oxygen levels to 1-10% of todays present atmospheric levels (Holland, 2006). Earth’s second canonical oxygenation event, that raised atmospheric levels to near present, is thought to have occurred in the Neoproterozoic prior to the diversification of animal life. Using geochemical proxies, two frameworks of oxygenation have been developed for the timing of the Neoproterozoic event. The first suggests oxygenation of the deep oceans early in the Ediacaran succeeding the Marinoan Glaciation (~635Ma) (Shen et al., 2008; Sahoo et al., 2012), whereas the second places the event roughly coincident with the first large multicellular eukaryotes following the Gaskiers Glaciation (~580 Ma) (Canfield et al., 2007; Fike et al., 2006). Understanding the timing of oxygenation is critical in understanding the mechanisms and feedbacks associated with the diversification of animal life. A multi-proxy geochemical approach on black shale is useful to investigate the redox conditions in the Early Ediacaran following the Marinoan glaciation. Black shale samples are from well-exposed sections of the Sheepbed Formation, of known early Ediacaran age, at its type locality in the Mackenzie Mountains, Northwest Territories, Canada. Shen et al. (2008), Canfield et al. (2008), and Johnston et al. (2013) all conducted geochemical redox studies on the formation in different localities with mixed results. The current study extends the geologic coverage of redox data in the complex and laterally variable Northwest Canada Neoproterozoic strata. High-resolution iron speciation data in this study suggests a transition from ferruginous to oxygenated ocean waters with episodic ferruginous conditions. These data support the conclusions of Shen et al. (2008) that oxygenation is recorded within the Sheepbed. They do not suggest a “great event” occurred due to fluctuations in the redox stat following the oxygenation signal. Furthermore, in light of the studies at other localities, these data suggest heterogeneity in redox conditions across the early Ediacaran basin strata of Northwest Canada. Redox sensitive trace element data in ferruginous bottom waters of the basal Sheepbed Formation do not support the conclusion of Sahoo et al. (2012) that global ocean oxygenation directly followed the Marinoan Glaciation. Rather, moderate enrichments later in the section could suggest a possible global ocean oxygenation signal. More redox sensitive geochemical studies in post- Marinoan transgressive units are necessary to elucidate a clear understanding of global ocean redox conditions in the early Ediacaran and overcome the noise associated with localized effects on redox proxies.
An assemblage of massive remobilized sandstone bodies and subsidiary sedimentary dikes, hosted by Pikes Peak Granite and older Proterozoic rocks, exists in the Colorado Front Range (CFR). Detrital zircon (DZ) zircon provenance analysis reveals the Tava sandstone is Neoproterozoic (800-680 Ma). Tava contains multiple DZ ages including a broad ‘Grenville’ age plateau (Gehrels and Pecha, 2014) and sharp, narrow age peaks at ~1400 Ma and ~1700 Ma (Figure 2) correlating to known Neoproterozoic age samples. There are no other non-metamorphosed sedimentary rocks of Proterozoic age known in Colorado, excluding the Uintah Mountains. Field-based research, U-Pb DZ geochronology and scanning electron microscope (SEM) imaging of quartz grains were the research methods used. Field-based research entailed observation of Tava sandstone sedimentology and variation in sandstone characteristics at sites along the Ute Pass Fault. U-Pb DZ geochronology produced minimum ages of deposition for Tava sandstone sediment. SEM imaging determined if some quartz and zircon grains were a result of aeolian transport. U-Pb DZ geochronology produced DZ spectra for the eleven sample sites. Within these sites that are ~11 km apart, the DZ spectra varied in its major peaks. Differences in DZ age abundances between Tava sample sites are reflected in varying P-values that are used to assess statistical probability of correlation among Tava samples (Siddoway and Gehrels, 2014). These variations in sedimentary and vii DZ provenances indicate the presence of topographic relief at the time of Tava sandstone formation. Evidence of extensive river systems in the Neoproterozoic provides a mechanism for zircon transport and a paleoenvironment to transport zircons to Colorado from the opposite side of Laurentia (Figure 4, Yonkee et al., 2014; Dehler et al., 2010; Rainbird et al., 2012). The Tava sandstone is a test for existing hypotheses about the region’s paleogeography. Comparing the weighted means of the detrital zircon populations in each Tava sample to the age of igneous sources and detrital zircon populations of Proterozoic igneous rocks of Colorado, reveals potential local igneous sources for the zircons. Cross cutting relationships indicate a generation younger than the Tava parent bodies. The ~1.4 Ga and ~1.7 Ga aged peaks from bedrock exposure suggests potential erosion of uplifted bedrock caused by tectonism. These differences in DZ spectra provide evidence for the presence of an intracontinental rift across Colorado.
Moulins are a crucial but poorly understood aspect of the glacial hydrological system, providing a channel for proglacial meltwater to drain into englacial conduits. This project examined moulin microclimatology utilizing 16 temperature data loggers emplaced in 7 moulins on the Root and Kennicott Glaciers, Alaska, from June 22nd to July 17th, 2015. These loggers were attached to the walls of the moulin and recorded air temperature every 16 minutes. Using this data set, the project examined the depth of penetration, timing, and magnitude of diurnal temperature fluctuations within moulins on a temperate glacier. Diurnal temperature variations penetrated to depths up to 10 meters into the moulins, with the magnitude of variation generally decreasing with increasing depth. Sensors close to the ice surface recorded temperature peaks in the early afternoon, while deeper sensors (3+ meters) had weak diurnal peaks between 4-8 am. Additionally, this project examined correlations between external weather characteristics (ambient temperatures, incoming shortwave radiation, wind speeds, and precipitation) and internal moulin temperatures. Shortwave radiation had the highest correlation to moulin temperatures (r2= 0.6-0.8), with weaker correlation between external temperature and moulin temperature (r2= 0.2-0.3). No significant correlation was found between wind speeds, rain, and moulin temperatures. Finally the data were analyzed to identify lag periods between changes in ambient temperature and correlated swings in moulin temperatures. My deepest sensors (>6 meters) seemed to have a lag period of between 3 and 8 hours compared to ambient temperature peaks.
Fire is a natural process, influenced by climate and vegetation (fuel source). The relationships between fire, climate and vegetation under changing climatic conditions are important to understand. Fire frequency in the western United States has significantly increased in the past three decades. This study examined the top two meters of a sediment core taken from the northern sub-basin of Swiftcurrent Lake, Glacier National Park, Montana, with the goal of using variability in charcoal flux into the lake as a proxy for fire history in the Grinnell Glacier and Swiftcurrent Valley drainage basins. Previous work suggests that an increase in fire-frequency occurred in the western United States over the more recent record, and because this core site has a higher sedimentation rate, the fire record will be more finely resolved in time. A preliminary model suggests that the core represents the last ~1700 years of sedimentation, at an average sedimentation rate of 1.2 mm/yr. Based on the sedimentation rates and the charcoal concentrations, charcoal accumulation rates (CHAR) range from 0 to 72 grains cm-2 yr-1. The 1700-year fire record shows a higher amount of fire events than previously found in a nearby, upvalley core. The fire return interval of the northern Swiftcurrent sub-basin is roughly 46 years between fires, whereas the southern sub-basin had an average return interval of 363 years between fires. The lower accumulation rates in the southern sub-basin could be due to the two large lakes (Lake Josephine and Lower Grinnell Lake) that serve as sediment sinks in the watershed, or it could be the result of the smaller drainage area. This shorter, more detailed record provides a new evaluation of the fire record of the Swiftcurrent Lake drainage basin.
Ground-penetrating radar (GPR) images of Taylor Glacier, in the McMurdo Dry Valleys, Antarctica, reveal an englacial drainage system near this polar glacier’s terminus at Blood Falls. Blood Falls is the surface manifestation of episodic releases of subglacial, iron-rich, hypersaline, microorganism-bearing brine. Locating englacial brine near Blood Falls would allow us to extract unoxidized brine in order to better understand the geochemistry and microorganisms as a proxy for life on other planets. In the current study, we collected a grid of GPR transects immediately upglacier from Blood Falls to locate the path by which brine surfaces and to inform future drilling operations in search of subsurface brine. Additionally, this study explores the extent of the subglacial and englacial brine reservoirs and seeks to refine hypotheses about the mechanisms driving the brine to the surface. In each of the GPR profiles, we found an englacial scattering zone located above a break in the basal-ice reflection. Downwarping of the basal-ice reflection on either side of the break and the break itself indicate that the scattering zone has slowed down the electromagnetic waves and prevented their further propagation into the glacier. We interpret this scattering zone as evidence of water-saturated and/or salty ice. A three-dimensional plot of the scattering zones visible on the profiles reveals a linear trend upglacier from Blood Falls nearly paralleling previously active brine-releasing cracks. Our evidence suggests that the zone is a recently or currently active englacial brine reservoir. In 2014, a team drilled near the area and successfully extracted pressurized brine ~16 m deep and upstream from Blood Falls at -7.1°C within surrounding ice of ~-17 °C. This brine temperature is consistent with the theoretical basal temperature of -7.8 °C that Hubbard et al. (2004) modeled near their hypothesized brine source 3-6 km upglacier from the terminus using geothermal heat flux and friction caused by ice deformation. Further study of the GPR data has allowed us to better understand the extent and movement of subglacial brine to the surface. Our cross-terminus traverse GPR transect shows that the subglacial brine reservoir may, in some form, extend all the way to the terminus and allow continuous brine release into Lake Bonney. As the subglacial brine surfaces, our data and analyses confirm hypotheses that it follows favorable pressure gradients up through surface cracks that penetrate the brine reservoir.
The timing and uplift of the Laramide province in Western North America to current mean elevations of 2500 m continues to be a subject of great debate among geologists. Producing estimates of paleoelevation since the Laramide orogeny is one way to constrain possible histories of uplift. This study utilizes hydrogen isotope ratios of ancient meteoric water in latest Eocene and earliest Oligocene rhyolitic glass as a proxy for elevation. The proxy relies on the relationship between changes in elevation and the temperature-driven rainout of heavier isotopes from air masses as they are moved up and over orographic barriers. Glass samples were collected from the Central Rocky Mountains, Great Plains, and coastal Texas. Results of this study suggest that mixing of Pacific and Gulf coast air masses took place over these areas during the latest Eocene, and thus previous interpretations of hydrogen isotope data that assume rainout of air masses from a single source are not meaningful. The question of absolute elevations over these regions during the latest Eocene/earliest Oligocene remains open. It does, however, appear possible to investigate topographic relief and to conclude that the Central Rocky Mountain region was not significantly higher than the surrounding Great Plains.
Despite superb exposure of the Ordovician strata in the region, and detailed knowledge of the eastern and northeastern margins, the western margin of the North China Block (NCB) lacks an integrated study of sedimentology, biostratigraphy, and chemostratigraphy. This study presents such information, elucidating the depositional history and sequence stratigraphic framework of Middle Ordovician strata in Inner Mongolia, China. Two sections of the Middle Ordovician Kelimoli Formation contain the positive Middle-Darriwilian Carbon Isotope Excursion (MDICE), the first recorded evidence of MDICE in the region. Additionally, biostratigraphic data and previous research in the area allow for the proposal of a sequence stratigraphic framework for the NCB. Specifically, the disconformity at the base of the lowest Middle Ordovician Sandaokan Formation contains a sequence boundary between two megasequences, megasequences 1 and 2, in the lowermost Paleozoic (Myrow et al, in press; Meng et al., 1997). The Sandaokan, Zhuozhishan, and Kelimoli formations represent the first, second, and third sequences of megasequence 2, respectively. The Kelimoli Formation is overlain by a disconformity, which likely corresponds with the upper boundary of this megasequence and with globally widespread nondeposition according to biostratigraphic and chemostratigraphic correlation (Schmitz et al., 2010). This hypothesized sequence stratigraphic framework will be tested in the future with high-resolution biostratigraphic and chemostratigrpahic analysis of the Zhuozhishan Formation.
Little was previously known for certain about the age and depositional history of the strata exposed in Picket Wire Canyon in Comanche National Grasslands, CO, as prior work in developing a comprehensive geochronology and stratigraphy of the area has been minimal. Previous age constraints for the strata have been largely speculative, as the canyon contains the only outcrop of Paleozoic¬–Mesozoic strata within 100 km, and correlating local strata to regional formations is troublesome. Prior chronostratigraphic constraints were based entirely on fossils in Late Triassic and younger strata, but there have not previously been age constraints for strata older than the Late Triassic Chinle Formation. Radiometric U/Pb dating of detrital and volcanic zircon grains, along with carbon and oxygen stable isotope geochemistry, were employed in this study to constrain ages of the strata in Picket Wire Canyon, as well as determine depositional conditions. A 77 m thick eolianite unit disconformably underlies the Chinle Formation and was of unknown age; however, detrital zircons extracted from the base of the unit yielded a youngest grain of 245.5 ± 5.9 Ma, indicating the eolianite is correlative to the Triassic Red Draw Member of Jelm Formation. Additionally, this data suggests the oldest strata in the canyon, those stratigraphically below the eolianite, correlate to the Permian¬–Triassic Lykins Formation. U/Pb dating of volcanic ash in the Ralston Creek Member of the Morrison Formation yields an age of 152.987 ± 0.063 Ma, which is the most precise age for the Morrison Formation known to date. δ13C and δ18O values of Morrison carbonate beds indicate deposition in an arid, hydraulically open lacustrine setting, and are consistent with previous results from other Morrison localities. The Lower Morrison Formation at this locality contains one of the world’s largest continuously mapped dinosaur trackways. These footprints are atypical in that they are preserved in oolite, which due to its granular texture should not theoretically preserve footprints. Thin section analysis of the oolite depicts microbial films both within and surrounding the ooids, along with cementation structures indicative of meteoric vadose diagensis. It is hypothesized that the presence of microbial films and diagenetic cements allowed for the preservation of the footprints by increasing cohesion between grains, thus permitting the imprints to maintain their mold indefinitely.
Aden Crater and the Albuquerque volcanoes are spatter and cinder cone systems with minor lava flows. They developed within the Rio Grande rift, a wedge shaped extensional feature in the southwestern United States, in the late Tertiary. Both systems are thus hypothesized to have a similar geochemistry that reflects upon comparable tectonic processes, settings, and time periods of eruption. The Aden volcanic field is a <80 Ka volcanic system in southern New Mexico, situated within the Rio Grande rift, approximately 25 miles north of the Mexican border. The eleven samples collected at the Aden system are from the spatter wall, lava lake feeder vent, and flow features. In thin section, plagioclase composition of An60 occurs alongside olivine crystals without reaction rims in a plagioclase, clinopyroxene, brown glass, and olivine groundmass. Major element geochemistry, determined by X-ray fluorescence (XRF), reveals an SiO2 content of 50-55% and an MgO content of 10-11%. The Aden basalts are classified as alkaline, within-rift basalts on IgPet classification diagrams. In-depth geochemical study, focusing on trace element geochemistry, reveals a close match on spider-type plots with Ocean Island Basalt and internal variation in heavy rare earth element (HREE) contents. Ytterbium divides the Aden basalts into three groups on rare earth element (REE) plots based on location, but there is not enough evidence in other elements or classification diagrams to support differentiation. There are no distinct differentiation or fractionation trends on SiO2 vs. V or Ni vs. Rb diagrams. The unusually high magnesium content and the pattern of spider diagrams suggest the presence of garnet in the source melt retaining these HREEs. Furthermore, the lack of distinguishable trends indicates a relatively short eruption from a single source without time for fractionation. The Albuquerque volcanoes lie eight kilometers to the southwest of Albuquerque, New Mexico, situated near the Albuquerque basin, within the Rio Grande rift. The volcanoes are situated in the center of the Rio Grande rift both from a north to south and east to west perspective. The eight samples collected at the Albuquerque volcanoes are include three from surrounding flows and five samples from the three main volcanoes: JA cone, Black Cone, and Vulcan. Phenocrysts of plagioclase (An69-65) and olivine occur in a grey groundmass. The basalts are classified as subalkaline, and their geochemical data plots most similarly to Enriched Mid Ocean Ridge Basalt (EMORB). Interpreting these data with relation to the geochemistry of the Albuquerque volcanoes, and other volcanic fields in the Rio Grande rift area, suggests that Aden Crater is not representative of central Rio Grande rift geochemistry. The geochemistry of the Albuquerque volcanoes is similar to the Servilleta Basalt and Taos Volcanic field, both within the Rio Grande rift, while the Aden basalts are most similar to the Ocate volcanic field and the Zuni-Bandera volcanic field, both outside the boundaries of the Rio Grande rift. This difference in classification reflects differences in levels of crustal trace elements like Zr, Nb, and K, yet Aden Crater and the Albuquerque volcanoes have a similar overlying crustal thickness. I believe that Aden Crater was sourced from a metasomatized garnet bearing mantle, such metasomatism caused by overlying remnants of the Farallon plate enriching Aden Crater in crustal trace elements not seen in other within-rift basalts.