Physical Sciences and Mathematics Commons^™

Refractory Black Carbon (Rbc) Variability In A 47-Year West Antarctic Snow And Firn Core, Marquetto Luciano, Susan Kaspari, Jefferson Cardia Simões

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Black carbon (BC) is an important climate-forcing agent that affects snow albedo. In this work, we present a record of refractory black carbon (rBC) variability, measured from a 20m deep snow and firn core drilled in West Antarctica (79°55’34.600” S, 94°21’13.3”W, 2122m above sea level) during the 2014–2015 austral summer. This is the highest elevation rBC record from West Antarctica. The core was analyzed using the Single Particle Soot Photometer (SP2) coupled to a CETAC Marin-5 nebulizer. Results show a well-defined seasonality with geometric mean concentrations of 0.015 μg L^-1 for the wet season (austral summer–fall) and 0.057 μg …

A New Early Occurrence Of Cervidae In North America From The Miocene-Pliocene Ellensburg Formation In Washington, Usa, Meaghan M. Emery-Wetherell, Joseph F. Schilter

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A new fossil cervid from the Craig’s Hill locality of the Miocene-Pliocene Ellensburg Formation in the State of Washington, USA, may be one of the oldest fossil deer yet found in North America, underlying a date of 4.9 Ma ± 0.1 Ma. This mandible fragment with m2, m3, and associated p2 has a size that does not distinguish it from Bretzia pseudalces, Odocoileus hemionus, or Capreolus constantini, and distinguishes it from Eocoileus gentryorum and Odocoileus lucasi only in having a thinner p2. A strong paraconid on the p2, and ectostylids and cingulids on the m2 and m3 link it most …

Geochronology Of The Middle Eocene Purple Bench Locality (Devil’S Graveyard Formation), Trans-Pecos Texas, Usa, Amy L. Atwater, Kelly D. Thomson, E. Christopher Kirk, Meaghan Emery-Wetherell, Logan Wetherell, Daniel F. Stockli

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Purple Bench is a middle Eocene fossil locality in the Devil’s Graveyard Formation of the Trans-Pecos region of West Texas. In addition to yielding a range of taxa characteristic of the Uintan North American Land Mammal Age, the Purple Bench locality is noteworthy in documenting a number of endemic species that are known only from the site. Despite the Uintan character of the mammalian fauna, the absolute age of Purple Bench is a matter of debate. This uncertainty stems from the wide interval of time encompassed by current radiometric dates bracketing the Purple Bench locality and from conflicting magnetostratigraphic correlations …

Mapping Crustal Shear Wave Velocity Structure And Radial Anisotropy Beneath West Antarctica Using Seismic Ambient Noise, J. P. O'Donnell, Audrey D. Huerta, J. Paul Winberry

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Using 8- to 25-s-period Rayleigh and Love wave phase velocity dispersion data extracted from seismic ambient noise, we (i) model the 3-D shear wave velocity structure of the West Antarctic crust and (ii) map variations in crustal radial anisotropy. Enhanced regional resolution is offered by the UK Antarctic Seismic Network. In the West Antarctic Rift System (WARS), a ridge of crust ∼26–30km thick extending south from Marie Byrd Land separates domains of more extended crust (∼22km thick) in the Ross and Amundsen Sea Embayments, suggesting along-strike variability in the Cenozoic evolution of the WARS. The southern margin of the WARS …

Deep Open Storage And Shallow Closed Transport System For A Continental Flood Basalt Sequence Revealed With Magma Chamber Simulator, Jussi S. Heinonen, Arto V. Luttinen, Frank J. Spera, Wendy A. Bohrson

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The Magma Chamber Simulator (MCS) quantitatively models the phase equilibria, mineral chemistry, major and trace elements, and radiogenic isotopes in a multicomponent–multiphase magma + wallrock + recharge system by minimization or maximization of the appropriate thermodynamic potential for the given process. In this study, we utilize MCS to decipher the differentiation history of a continental flood basalt sequence from the Antarctic portion of the ~ 180 Ma Karoo large igneous province. Typical of many flood basalts, this suite exhibits geochemical evidence (e.g., negative initial εNd) of interaction with crustal materials. We show that isobaric assimilation-fractional crystallization models fail to …

Dextral, Normal, And Sinistral Faulting Across The Eastern California Shear Zone-Mina Deflection Transition, California-Nevada, Usa, Kevin Delano, Jeffrey Lee, Rachelle Roper, Andrew Calvert

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Strike-slip faults commonly include extensional and contractional bends and stepovers, whereas rotational stepovers are less common. The Volcanic Tableland, Black Mountain, and River Spring areas (California and Nevada, USA) (hereafter referred to as the VBR region) straddle the transition from the dominantly NW-striking dextral faults that define the northwestern part

of the eastern California shear zone into a rotational stepover characterized by dominantly NE-striking sinistral faults that define the southwestern Mina deflection. New detailed geologic mapping, structural studies, and 40Ar/39Ar geochronology across the VBR region allow us to calculate Pliocene to Pleistocene fault slip rates and test predictions for the …

Tidal And Spatial Variability Of Flow Speed And Seismicity Near The Grounding Zone Of Beardmore Glacier, Antarctica, Jade Cooley, J. Paul Winberry, Michelle Koutnik, Howard Conway

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GPS measurements of tidal modulation of ice flow and seismicity within the grounding zone of Beardmore Glacier show that tidally induced fluctuations of horizontal flow are largest near the grounding line and decrease downstream. Seismic activity is continuous, but peaks occur on falling and rising tides. Beamforming methods reveal that most seismic events originate from two distinct locations, one on the grid-north side of the grounding zone, and one on the grid-south side. The broad pattern of deformation generated as Beardmore Glacier merges with the Ross Ice Shelf results in net extension along the grid-north side of the grounding zone …

Radiative Forcing By Dust And Black Carbon On The Juneau Icefield, Alaska, Sonia A. Nagorski, Susan D. Kaspari, Eran Hood, Jason B. Fellman, S. Mckenzie Skiles

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Here we present the first known data set on black carbon (BC) and mineral dust concentrations in snow from the Juneau Icefield (JIF) in southeastern Alaska, where glacier melt rates are among the highest on Earth. In May 2016, concentrations of BC (0.4–3.1 μg/L) and dust (0.2–34 mg/L) were relatively low and decreased toward the interior of the JIF. The associated radiative forcing (RF) averaged 4 W/m2. In July, after 10 weeks of exposure, the aged snow surface had substantially higher concentrations of BC (2.1–14.8 μg/L) and dust (11–72 mg/L) that were not spatially distributed by elevation or distance from …

Seismic Tremor Reveals Spatial Organization And Temporal Changes Of Subglacial Water System, Margot E. Vore, Timothy C. Bartholomaus, J. Paul Winberry, Jacob I. Walter, Jason M. Amundson

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Subglacial water flow impacts glacier dynamics and shapes the subglacial environment. However, due to the challenges of observing glacier beds, the spatial organization of subglacial water systems and the time scales of conduit evolution and migration are largely unknown. To address these questions, we analyze 1.5‐ to 10‐Hz seismic tremor that we associate with subglacial water flow, hat is, glaciohydraulic tremor, at Taku Glacier, Alaska, throughout the 2016 melt season. We use frequency‐dependent polarization analysis to estimate glaciohydraulic tremor propagation direction (related to the subglacial conduit location) and a degree day melt model to monitor variations in melt‐water input. We …

The Three-Stage Petrochemical Evolution Of The Steens Basalt [Southeast Oregon, Usa] Compared To Large Igneous Provinces And Layered Mafic Intrusions, N. E. Moore, A. L. Grunder, Wendy A. Bohrson

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The Steens Basalt, southeast Oregon, USA, initiated at 17 Ma as the earliest pulse of the Columbia River Flood Basalt of the northwestern USA. New and existing stratigraphically controlled data reveal temporal changes in lava flow character, and whole-rock and mineral compositions, which we use to evaluate how the balance of magma differentiation processes change in time. Temporal petrochemical variations in the Steens Basalt are analogous to the transition from Imnaha Basalt to Grande Ronde Basalt units of the Columbia River Flood Basalt and have parallels to the temporal evolution of the Deccan and Siberian traps, in India and Russia, …

The Crust And Upper Mantle Structure Of Central And West Antarctica From Bayesian Inversion Of Rayleigh Wave And Receiver Functions, Weisen Shen, Audrey D. Huerta, J. Paul Winberry

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We construct a new seismic model for central and West Antarctica by jointly inverting Rayleigh wave phase and group velocities along with P wave receiver functions. Ambient noise tomography exploiting data from more than 200 seismic stations deployed over the past 18 years is used to construct Rayleigh wave phase and group velocity dispersion maps. Comparison between the ambient noise phase velocity maps with those constructed using teleseismic earthquakes confirms the accuracy of both results. These maps, together with P receiver function waveforms, are used to construct a new 3-D shear velocity (Vs) model for the crust and uppermost mantle …

The 2015 Landslide And Tsunami In Taan Fiord, Alaska, Bretwood Higman, Breanyn Macinnes, Colin Bloom

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Glacial retreat in recent decades has exposed unstable slopes and allowed deep water to extend beneath some of those slopes. Slope failure at the terminus of Tyndall Glacier on 17 October 2015 sent 180 million tons of rock into Taan Fiord, Alaska. The resulting tsunami reached elevations as high as 193 m, one of the highest tsunami runups ever documented worldwide. Precursory deformation began decades before failure, and the event left a distinct sedimentary record, showing that geologic evidence can help understand past occurrences of similar events, and might provide forewarning. The event was detected within hours through automated seismological …

On The Interaction Of The North Andes Plate With The Caribbean And South American Plates In Northwestern South America From Gps Geodesy And Seismic Data, Omar J. Pérez, Steven G. Wesnousky, Roberto De La Rosa, Julio Márquez, Redescal Uzcátegui, Christian Quintero, Luis Liberal, Héctor Mora-Páez, Walter Szeliga

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We examine the hypocentral distribution of seismicity and a series of geodetic velocity vectors obtained from Global Positioning System observations between 1994 and 2015 both offshore and mainland northwestern South America (66°W–77°W; 8°N–14°N). Our analysis, that includes a kinematic block modelling, shows that east of the Caribbean–South American–North Andes plates’ triple junction at ∼68°W; 10.7°N, right-lateral easterly oriented shear motion (∼19.6 ± 2.0 mm yr⁻¹) between the Caribbean and South America plates is split along two easterly striking, right-lateral strike-slip subparallel fault zones: the San Sebastián fault that runs off-shore the Venezuelan coast and slips about 17.0±0.5mm …

Phase Equilibrium Modelling And Implications For P-T Determinations Of Medium-Temperature Uhp Eclogites, North Qaidam Terrane, China, David Hernández‐Uribe, Chris G. Mattinson, Jianxin Zhang

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In contrast to low-T eclogites (garnet growth zoning preserved) or high-T eclogites (garnet diffusionally homogenized at peak conditions), medium-temperature eclogites pose additional challenges to P-T determinations due to the partial preservation of garnet zoning. The Dulan area, in the southeastern part of North Qaidam ultrahigh-pressure terrane, exposes minor eclogites hosted by ortho- and paragneisses. Four fresh, medium-temperature eclogites contain the paragenesis Grt+Omp+Rt+Qz/Coe+Ph +/- Ky +/- Zo. In all samples, garnet X-Mg shows little zoning, suggesting diffusional modification, and precludes the use of pyrope+almandine+grossular isopleth intersections to determine a P-T path. However, in one sample, sharp zoning in grossular content suggests …

Gps Vertical Land Motion Corrections To Sea-Level Rise Estimates In The Pacific Northwest, Jean-Philippe Montillet, Timothy I. Melbourne, Walter M. Szeliga

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We construct coastal Pacific Northwest profiles of vertical land motion (VLM) known to bias long-term tide-gauge measurements of sea-level rise (SLR) and use them to estimate absolute sea-level rise with respect to Earth’s center of mass. Multidecade GPS measurements at 47 coastal stations along the Cascadia subduction zone show VLM varies regionally but smoothly along the Pacific coast and inland Puget Sound with rates ranging from +4.9 to –1.2 mm/yr. Puget Sound VLM is characterized by uniform subsidence at relatively slow rates of +0.1 to –0.3 mm/yr. Uplift rates of 4.5 mm/yr persist along the western Olympic Peninsula of northwestern …

High Throughput Petrochronology And Sedimentary Provenance Analysis By Automated Phase Mapping And Laicpms, Pieter Vermeesch, Martin Rittner, Ethan Petrou, Jenny Omma, Chris Mattinson, Eduardo Garzanti

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The first step in most geochronological studies is to extract dateable minerals from the host rock, which is time consuming, removes textural context, and increases the chance for sample cross contamination. We here present a new method to rapidly perform in situ analyses by coupling a fast scanning electron microscope (SEM) with Energy Dispersive X-ray Spectrometer (EDS) to a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LAICPMS) instrument. Given a polished hand specimen, a petrographic thin section, or a grain mount, Automated Phase Mapping (APM) by SEM/EDS produces chemical and mineralogical maps from which the X-Y coordinates of the datable …

Exploring The Historical Earthquakes Preceding The Giant 1960 Chile Earthquake In A Time-Dependent Seismogenic Zone, Marco Cisternas, Matias Carvajal, Robert Wesson, Lisa L. Ely, Nicolás Gorigoitia

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New documentary findings and available paleoseismological evidence provide both new insights into the historical seismic sequence that ended with the giant 1960 south‐central Chile earthquake and relevant information about the region’s seismogenic zone. According to the few available written records, this region was previously struck by earthquakes of varying size in 1575, 1737, and 1837. We expanded the existing compilations of the effects of the two latter using unpublished first‐hand accounts found in archives in Chile, Peru, Spain, and New England. We further investigated their sources by comparing the newly unearthed historical data and available paleoseismological evidence with the effects …

Bedrock Geology Of Dfdp-2b, Central Alpine Fault, New Zealand, Virginia Gail Toy, Angela Halfpenny

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During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. …

Origin Of Primitive Ocean Island Basalts By Crustal Gabbro Assimilation And Multiple Recharge Of Plume-Derived Melts, Anastassia Y. Borisova, Wendy A. Bohrson, Michel Grégoire

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Chemical Geodynamics relies on a paradigm that the isotopic composition of ocean island basalt (OIB) represents equilibrium with its primary mantle sources. However, the discovery of huge isotopic heterogeneity within olivine‐hosted melt inclusions in primitive basalts from Kerguelen, Iceland, Hawaii and South Pacific Polynesia islands implies open‐system behavior of OIBs, where during magma residence and transport, basaltic melts are contaminated by surrounding lithosphere. To constrain the processes of crustal assimilation by OIBs, we employed the Magma Chamber Simulator (MCS), an energy‐constrained thermodynamic model of recharge, assimilation and fractional crystallization. For a case study of the 21–19 Ma basaltic series, the …

Timing Of Mid-Crustal Ductile Extension In The Northern Snake Range Metamorphic Core Complex, Nevada: Evidence From U/Pb Zircon Ages, Jeffrey Lee, Terrence Blackburn, Scott Johnston

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Metamorphic core complexes within the western U.S. record a history of Cenozoic ductile and brittle extensional deformation, metamorphism, magmatism, and exhumation within the footwalls of high-angle Basin and Range normal faults. In models proposed for the formation of metamorphic core complexes there is a close temporal and spatial link between upper crustal normal faulting, lower crustal ductile extension and flow, and detachment faulting. To provide constraints on the timing of ductile extension in the northern Snake Range metamorphic core complex (Nevada) and thereby test these models, we present new 238U-206Pb dates on zircons from both deformed and undeformed rhyolite dikes …

Unusual Geologic Evidence Of Coeval Seismic Shaking And Tsunamis Shows Variability In Earthquake Size And Recurrence In The Area Of The Giant 1960 Chile Earthquake, M. Cisternas, E. Garrett, R. Wesson, T. Dura, Lisa L. Ely

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An uncommon coastal sedimentary record combines evidence for seismic shaking and coincident tsunami inundation since AD 1000 in the region of the largest earthquake recorded instrumentally: the giant 1960 southern Chile earthquake (Mw 9.5). The record reveals significant variability in the size and recurrence of megathrust earthquakes and ensuing tsunamis along this part of the Nazca-South American plate boundary. A 500-m long coastal outcrop on Isla Chiloé, midway along the 1960 rupture, provides continuous exposure of soil horizons buried locally by debris-flow diamicts and extensively by tsunami sand sheets. The diamicts flattened plants that yield geologically precise ages to correlate …

Gps Constraints On Interplate Locking Within The Makran Subduction Zone, Elyse Frohling, Walter Szeliga

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The Makran subduction zone is one of the last convergent margins to be investigated using space-based geodesy. While there is a lack of historical and modern instrumentation in the region, a sparse sampling of continuous and campaign measurements over the past decade has allowed us to make the first estimates of convergence rates. We combine GPS measurements from 20 stations located in Iran, Pakistan and Oman along with hypocentral locations from the International Seismological Centre to create a preliminary 3-D estimate of the geometry of the megathrust, along with a preliminary fault-coupling model for the Makran subduction zone. Using a …

A Seismic Transect Across West Antarctica: Evidence For Mantle Thermal Anomalies Beneath The Bentley Subglacial Trench And The Marie Byrd Land Dome, Andrew J. Lloyd, Douglas A. Wiens, Andrew A. Nyblade, Sridhar Anandakrishnan, Richard C. Aster, Audrey D. Huerta, Terry J. Wilson, Ian W. D. Dalziel, Patrick J. Shore, Dapeng Zhao

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West Antarctica consists of several tectonically diverse terranes, including the West Antarctic Rift System, a topographic low region of extended continental crust. In contrast, the adjacent Marie Byrd Land and Ellsworth-Whitmore mountains crustal blocks are on average over 1km higher, with the former dominated by polygenetic shield and stratovolcanoes protruding through the West Antarctic ice sheet and the latter having a Precambrian basement. The upper mantle structure of these regions is important for inferring the geologic history and tectonic processes, as well as the influence of the solid earth on ice sheet dynamics. Yet this structure is poorly constrained due …

Slip Pulse And Resonance Of The Kathmandu Basin During The 2015 Gorkha Earthquake, Nepal, John Galetzka, Walter Szeliga

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Detailed geodetic imaging of earthquake ruptures enhances our understanding of earthquake physics and associated ground shaking. The 25 April 2015 moment magnitude 7.8 earthquake in Gorkha, Nepal was the first large continental megathrust rupture to have occurred beneath a high-rate (5-hertz) Global Positioning System (GPS) network. We used GPS and interferometric synthetic aperture radar data to model the earthquake rupture as a slip pulse ~20 kilometers in width, ~6 seconds in duration, and with a peak sliding velocity of 1.1 meters per second, which propagated toward the Kathmandu basin at ~3.3 kilometers per second over ~140 kilometers. The smooth slip …

Dramatic Loss Of Glacier Accumulation Area On The Tibetan Plateau Revealed By Ice Core Tritium And Mercury Records, S. Kang, F. Wang, U. Morgenstern, Y. Zhang, B. Grigholm, Susan D. Kaspari, M. Schwikowski, J. Ren, T. Yao, D. Qin, P. A. Mayewski

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Two ice cores were retrieved from high elevations (~5800 m a.s.l.) at Mt. Nyainqêntanglha and Mt. Geladaindong in the southern and central Tibetan Plateau region. The combined tracer analysis of tritium (³H), ²¹⁰Pb and mercury, along with other chemical records, provided multiple lines of evidence supporting that the two coring sites had not received net ice accumulation since at least the 1950s and 1980s, respectively. These results implied an annual ice loss rate of more than several hundred millimeter water equivalent over the past 30–60 years. Both mass balance modeling at the sites and in situ data …

The Mantle Transition Zone Beneath West Antarctica: Seismic Evidence For Hydration And Thermal Upwellings, E. L. Emry, A. A. Nyblade, J. Juliá, S. Anandakrishnan, R. C. Aster, D. A. Wiens, Audrey D. Huerta, T. J. Wilson

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Although prior work suggests that a mantle plume is associated with Cenozoic rifting and volcanism in West Antarctica, the existence of a plume remains conjectural. Here we use P wave receiver functions (PRFs) from the Antarctic POLENET array to estimate mantle transition zone thickness, which is sensitive to temperature perturbations, throughout previously unstudied parts of West Antarctica. We obtain over 8000 high‐quality PRFs using an iterative, time domain deconvolution method filtered with a Gaussian width of 0.5 and 1.0, corresponding to frequencies less than ∼0.24 and ∼0.48 Hz, respectively. Single‐station and common conversion point stacks, migrated to depth using the …

Clockwise Rotation Of The Brahmaputra Valley Relative To India: Tectonic Convergence In The Eastern Himalaya, Naga Hills, And Shillong Plateau, ‪Philippe Vernant, R. Bilham, Walter Szeliga, D. Drupka, S. Kalita, A. K. Bhattacharyya, V. K. Gaur

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GPS data reveal that the Brahmaputra Valley has broken from the Indian Plate and rotates clockwise relative to India about a point a few hundred kilometers west of the Shillong Plateau. The GPS velocity vectors define two distinct blocks separated by the Kopili fault upon which 2–3 mm/yr of dextral slip is observed: the Shillong block between longitudes 89 and 93°E rotating clockwise at 1.15°/Myr and the Assam block from 93.5°E to 97°E rotating at ≈1.13°/Myr. These two blocks are more than 120 km wide in a north‐south sense, but they extend locally a similar distance beneath the Himalaya and …

Seismic And Geodetic Evidence For Grounding-Line Control Of Whillans Ice Stream Stick-Slip Events, Martin J. Pratt, J. Paul Winberry, Douglas A. Wiens, Sridhar Anandakrishnan, Richard B. Alley

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The tidally modulated, stick‐slip events of Whillans Ice Stream in West Antarctica produce seismic energy from three locations near the grounding line. Using ice velocity records obtained by combining time series from colocated broadband seismometers and GPS receivers installed on the ice stream during the 2010–2011 and 2011–2012 austral summers, along with far‐field seismic recordings of elastic waves, we locate regions of high rupture velocity and stress drop. These regions, which are analogous to “asperities” in traditional seismic fault studies, are areas of elevated friction at the base of the ice stream. Slip events consistently initiate at one of two …

Glacier Slip And Seismicity Induced By Surface Melt, Peter L. Moore, J. Paul Winberry, Neal R. Iverson, Knut A. Christianson, Sridhar Anandakrishnan, Miriam Jackson, Mark E. Mathison, Denis Cohen

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Many of the key processes governing fast glacier flow involve interaction between a glacier and its basal hydrological system, which is hidden from direct observation. Passive seismic monitoring has shown promise as a tool for remotely monitoring basal processes, but lack of glacier-bed access prevents clear understanding of the relationships between subglacial processes and corresponding seismic emissions. Here we describe direct measurements of basal hydrology, sliding, and broadband seismicity made in a unique subglacial facility in Norway during the onset of two summer melt seasons. In the most pronounced of these episodes, rapid delivery of surface meltwater to the bed …

Alluvial Sediment Or Playas: What Is The Dominant Source Of Sand And Silt In Desert Soil Vesicular A Horizons, Southwest Usa, Mark R. Sweeney, Eric V. Mcdonald, Christohper E. Markley

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Vesicular A (Av) soil horizons form beneath desert pavements from the accretion of aeolian sediment (dust) commonly thought to be derived primarily from desiccating pluvial lakes and playas, with contributions from ephemeral washes and alluvial fans. Particle size distributions of Av horizons are typically bimodal with primary modes of very fine silt and fine sand, suggesting that the horizon matrix is derived from multiple sources. Here we conduct detailed chemical and physical analysis of both Av horizon soil samples and potential sources of aeolian sediment to better constrain the relative contributions of dust associated with the development of Av horizons. …