• Assessing Long‐Term Postseismic Transients From GPS Time Series in Southern California

      Guns, K. A.; Bennett, R. A.; Univ Arizona, Dept Geosci (AMER GEOPHYSICAL UNION, 2020-03-03)
      In order to gain insight into long-term plate boundary motion and to shed light on geodetic-based fault slip rates and the seismic hazards they inform, we apply a forward modeling strategy to identify and reduce the short- and long-term effects of viscoelastic postseismic deformation on modern GPS observations following large magnitude earthquakes in Southern California. We assess ongoing postseismic deformation in the southwestern United States by analyzing all magnitude >= M(w)6.0 earthquakes that have occurred there and in Baja California and Sonora, Mexico, since year 1800, finding that ongoing postseismic displacements from 12 events are potentially contributing to the modern day deformation field in Southern California. With a forward modeling step, we calculate postseismic displacements associated with these 12 events using a reference model consisting of a layered, laterally homogeneous, viscoelastic Earth structure; these displacements are then subtracted from processed horizontal GPS coordinate time series data to produce a postseismic-reduced data set. In order to quantify the success of this forward modeling in reducing the postseismic signal, we estimate parameters representing logarithmic decay associated with the 2010 M(w)7.2 El Mayor-Cucapah earthquake using two different time series analysis methods. Variance reduction indicates we were able to reduce postseismic deformation in this test case by up to 60%. Anomaly maps produced using our assessment of deformation around the El Mayor-Cucapah event highlight hot spots in which secondary processes may be occurring or where a more complex viscosity structure may be necessary. Plain Language Summary Earthquakes relieve the shear stresses that build up on crustal faults, but at the same time induce other stresses in the nearby crust and mantle. Relaxation of earthquake-induced stresses can cause Earth's surface to move in predictable patterns for decades or longer. However, the accuracy of such predictions is hampered by our limited understanding of how the lower crust and upper mantle relax as a function of depth and tectonic setting. Here, we apply a regionally optimized Earth model to calculate displacements caused by all historic earthquakes that may be contributing to the contemporary crustal motion field in Southern California. Our modeling assesses all magnitude 6.0 and larger earthquakes since 1800 in California, Nevada, and Baja California, and Sonora, Mexico, to evaluate ongoing relaxation and associated crustal motion. We devise a method for determining ongoing postseismic motions that overcomes limitations in our knowledge of crust and mantle relaxation properties. We discuss the implications of our modeling for crustal strain accumulation and seismic hazards in Southern California. Our new modeling approach may help improve knowledge of the relaxation properties of the lower crust and upper mantle.
    • Assessing XMT‐Measurement Variability of Air‐Water Interfacial Areas in Natural Porous Media

      Araujo, Juliana B.; Brusseau, Mark L.; Univ Arizona, Dept Environm Sci (AMER GEOPHYSICAL UNION, 2020-01-07)
      This study investigates the accuracy and reproducibility of air-water interfacial areas measured with high-resolution synchrotron X-ray microtomography (XMT). Columns packed with one of two relatively coarse-grained monodisperse granular media, glass beads or a well-sorted quartz sand, were imaged over several years, encompassing changes in acquisition equipment, improved image quality, and enhancements to image acquisition and to processing software. For the glass beads, the specific solid surface area (SSSA-XMT) of 31.6 +/- 1 cm(-1) determined from direct analysis of the segmented solid-phase image data is statistically identical to the independently calculated geometric smooth-sphere specific solid surface area (GSSA, 32 +/- 1 cm(-1)) and to the measured SSSA (28 +/- 3 cm(-1)) obtained with the N-2-Brunauer, Emmett, and Teller method. The maximum specific air-water interfacial area (A(max)) is 27.4 (+/- 2) cm(-1), which compares very well to the SSSA-XMT, GSSA, and SSSA-N-2-Brunauer, Emmett, and Teller values. For the sand, the SSSA-XMT (111 +/- 2 cm(-1)) and GSSA (113 +/- 1 cm(-1)) are similar. The mean A(max) is 96 +/- 5 cm(-1), which compares well to both the SSSA and the GSSA values. The XMT-SSSA values deviated from the GSSA values by 7-16% for the first four experiments but were essentially identical for the later experiments. This indicates that enhancements in image acquisition and processing improved data accuracy. The Amax values ranged from 74 cm(-1) to 101 cm(-1), with a coefficient of variation (COV) of 9%. The maximum capillary interfacial area ranged from 12 cm(-1) to 19 cm(-1), for a COV of 10%. The COVs for both decreased to 5-6% for the latter five experiments. These results demonstrate that XMT imaging provides accurate and reproducible measurements of total and capillary interfacial areas.
    • Assessment of Satellite and Reanalysis Cold Season Snowfall Estimates Over Arctic Sea Ice

      Song, Yang; Behrangi, Ali; Blanchard-Wrigglesworth, E.; Univ Arizona, Dept Hydrol & Atmospher Sci Engn (AMER GEOPHYSICAL UNION, 2020-08)
      This work presents a systematic assessment of precipitation estimates from satellite and reanalysis products over Arctic sea ice by reconstructing snow depths from precipitation products and comparing them with snow depth observations from National Aeronautics and Space Administration (NASA)'s Operation IceBridge (OIB). Results show that the observed snow depth pattern is generally captured through reconstruction of snow depth using various precipitation products, but the use of passive microwave precipitation estimates results in significant underestimation of the snow depth. By using CloudSat monthly precipitation rate, to adjust the Global Precipitation Climatology Product (GPCP V1.3), the modified product (GPCP V1.3-mod) shows improved statistics over GPCP V1.3 as compared with OIB snow depth observations. Snow depth reconstructed from ERA-Int precipitation rate outperformed other products by showing the highest correlation coefficient and lowest root-mean-square error (RMSE). ERA5 shows largerRMSEthan ERA-Int, while MERRA-2 results in large overestimation of snow depth and largerRMSEcompared to GPCP and other reanalysis products.
    • Assessment of the Carbonate Chemistry Seasonal Cycles in the Southern Ocean From Persistent Observational Platforms

      Williams, N. L.; Juranek, L. W.; Feely, R. A.; Russell, Joellen; Johnson, K. S.; Hales, B.; Univ Arizona, Dept Geosci (AMER GEOPHYSICAL UNION, 2018-07)
      Observations from Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) biogeochemical profiling Argo floats are used to characterize the climatological seasonal cycles and drivers of dissolved inorganic carbon, total alkalinity, pH, the partial pressure of carbon dioxide (CO2), and the saturation state of aragonite at the surface and at 200 m across five Southern Ocean frontal regimes, including under sea ice. The Southern Ocean ranges from a temperature-dominated system in the northernmost Subtropical Zone to a biologically dominated system in the most poleward Seasonal Sea Ice Zone. In all zones, the ingassing or outgassing of CO2 must be balanced by geostrophic and Ekman transport, mixing from below, and particle transport of carbon into or out of the euphotic zone. The climatological seasonal cycles spanning the period from 2014 to 2017 compare favorably with existing climatologies in spring and summer and less so during winter months, at higher latitudes, and in ice-covered regions due, in part, to limited wintertime observations before SOCCOM. We observe increases in the carbon and nutrient content of surface waters south of the Subantarctic Front between climatological data products and the SOCCOM float climatologies, even after adjusting for anthropogenic change, suggesting a large-scale increase in the amount of upwelled carbon- and nutrient-rich deep waters. This increased upwelling corresponds to a positive Southern Annular Mode Index over 2014-2017 and likely acts to decrease the magnitude of the Southern Ocean sink of total carbon by increasing outgassing of natural CO2, especially during winter months.
    • Assimilating satellite-based canopy height within an ecosystem model to estimate aboveground forest biomass

      Joetzjer, E.; Pillet, M.; Ciais, P.; Barbier, N.; Chave, J.; Schlund, M.; Maignan, Fabienne; Barichivich, J.; Luyssaert, S.; Hérault, B.; et al. (AMER GEOPHYSICAL UNION, 2017-07-16)
      Despite advances in Earth observation and modeling, estimating tropical biomass remains a challenge. Recent work suggests that integrating satellite measurements of canopy height within ecosystem models is a promising approach to infer biomass. We tested the feasibility of this approach to retrieve aboveground biomass (AGB) at three tropical forest sites by assimilating remotely sensed canopy height derived from a texture analysis algorithm applied to the high-resolution Pleiades imager in the Organizing Carbon and Hydrology in Dynamic Ecosystems Canopy (ORCHIDEE-CAN) ecosystem model. While mean AGB could be estimated within 10% of AGB derived from census data in average across sites, canopy height derived from Pleiades product was spatially too smooth, thus unable to accurately resolve large height (and biomass) variations within the site considered. The error budget was evaluated in details, and systematic errors related to the ORCHIDEE-CAN structure contribute as a secondary source of error and could be overcome by using improved allometric equations.
    • The association of hydrogen with sulfur on Mars across latitudes, longitudes, and compositional extremes

      Karunatillake, Suniti; Wray, James J.; Gasnault, Olivier; McLennan, Scott M.; Deanne Rogers, A.; Squyres, Steven W.; Boynton, William V.; Skok, J. R.; Button, Nicole E.; Ojha, Lujendra; et al. (AMER GEOPHYSICAL UNION, 2016-07)
      Midlatitudinal hydrated sulfates on Mars may influence brine pH, atmospheric humidity, and collectively water activity. These factors affect the habitability of the planetary subsurface and the preservation of relict biomolecules. Regolith at grain sizes smaller than gravel, constituting the bulk of the Martian subsurface at regional scales, may be a primary repository of chemical alteration, mechanical alteration, and biosignatures. The Mars Odyssey Gamma Ray Spectrometer with hundreds of kilometers of lateral resolution and compositional sensitivity to decimeter depth provides unique insight into this component of the regolith, which we call soil. Advancing the globally compelling association between H2O and S established by our previous work, we characterize latitudinal variations in the association between H and S, as well as in the hydration state of soil. Represented by H2O:S molar ratios, the hydration state of candidate sulfates increases with latitude in the northern hemisphere. In contrast, hydration states generally decrease with latitude in the south. Furthermore, we observe that H2O concentration may affect the degree of sulfate hydration more than S concentration. Limited H2O availability in soil-atmosphere exchange and in subsurface recharge could explain such control exerted by H2O on salt hydration. Differences in soil thickness, ground ice table depths, atmospheric circulation, and insolation may contribute to hemispheric differences in the progression of hydration with latitude. Our observations support chemical association of H2O with S in the southern hemisphere as suggested by Karunatillake et al. (2014), including the possibility of Fe sulfates as a key mineral group.
    • Atmospheric Waves and Their Possible Effect on the Thermal Structure of Saturn's Thermosphere

      Müller‐Wodarg, I. C. F.; Koskinen, T. T.; Moore, L.; Serigano, J.; Yelle, R. V.; Hörst, S.; Waite, J. H.; Mendillo, M.; Univ Arizona, Lunar & Planetary Lab (AMER GEOPHYSICAL UNION, 2019-03-11)
      Atmospheric waves have been discovered for the first time in Saturn's neutral upper atmosphere (thermosphere). Waves may be generated from instabilities, convective storms or other atmospheric phenomena. The inferred wave amplitudes change little with height within the sampled region, raising the possibility of the waves being damped, which in turn may enhance the eddy friction within the thermosphere. Using our Saturn Thermosphere Ionosphere General Circulation Model, we explore the parameter space of how an enhanced Rayleigh drag in different latitude regimes would affect the global circulation pattern within the thermosphere and, in turn, its global thermal structure. We find that Rayleigh drag of sufficient magnitude at midlatitudes may reduce the otherwise dominant Coriolis forces and enhance equatorward winds to transport energy from poles toward the equator, raising the temperatures there to observed values. Without this Rayleigh drag, energy supplied into the polar upper atmosphere by magnetosphere-atmosphere coupling processes remains trapped at high latitudes and causes low-latitude thermosphere temperatures to remain well below the observed levels. Our simulations thus suggest that giant planet upper atmosphere global circulation models need to include additional Rayleigh drag in order to capture the effects of physical processes otherwise not resolved by the codes. Plain Language Summary Atmospheric waves have been discovered for the first time in Saturn's neutral upper atmosphere (thermosphere) with typical vertical wavelengths ranging from 100-200 km and density amplitudes reaching around 10%. Amplitudes are roughly constant over this height range, implying that wave damping occurs, which in turn is expected to enhance eddy friction (Rayleigh drag) within the thermosphere. Using the Saturn Thermosphere Ionosphere General Circulation Model (STIM), we explore the response of Saturn's thermosphere to a range of possible Rayleigh drag profiles. We find that the introduction of momentum dissipation equatorward of 60 degrees latitude will slow down the zonal winds on Saturn sufficiently to enhance equatorward winds and thereby allow energy propagation from the poles toward the equator. Under the assumption that sufficiently strong Rayleigh drag is present in Saturn's thermosphere, large temperatures at low latitudes may result from wind-driven global redistribution of energy from the polar regions.
    • Autonomous Detection of Particles and Tracks in Optical Images

      Liounis, Andrew J.; Small, Jeffrey L.; Swenson, Jason C.; Lyzhoft, Joshua R.; Ashman, Benjamin W.; Getzandanner, Kenneth M.; Moreau, Michael C.; Adam, Coralie D.; Leonard, Jason M.; Nelson, Derek S.; et al. (AMER GEOPHYSICAL UNION, 2020-08)
      When optical navigation images acquired by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) mission revealed the periodic ejection of particles from asteroid (101955) Bennu, it became a mission priority to quickly identify and track these objects for both spacecraft safety and scientific purposes. The large number of particles and the mission criticality rendered time-intensive manual inspection impractical. We present autonomous techniques for particle detection and tracking that were developed in response to the Bennu phenomenon but that have the capacity for general application to particles in motion about a celestial body. In an example OSIRIS-REx data set, our autonomous techniques identified 93.6% of real particle tracks and nearly doubled the number of tracks detected versus manual inspection alone.
    • Bayesian Calibration of the Mg/Ca Paleothermometer in Planktic Foraminifera

      Tierney, Jessica E.; Malevich, Steven B.; Gray, William; Vetter, Lael; Thirumalai, Kaustubh; Univ Arizona, Dept Geosci (AMER GEOPHYSICAL UNION, 2019-12-13)
      The Mg/Ca ratio of planktic foraminifera is a widely used proxy for sea-surface temperature but is also sensitive to other environmental factors. Previous work has relied on correcting Mg/Ca for nonthermal influences. Here, we develop a set of Bayesian models for Mg/Ca in four major planktic groups-Globigerinoides ruber (including both pink and white chromotypes), Trilobatus sacculifer, Globigerina bulloides, and Neogloboquadrina pachyderma (including N. incompta)-that account for the multivariate influences on this proxy in an integrated framework. We use a hierarchical model design that leverages information from both laboratory culture studies and globally distributed core top data, allowing us to include environmental sensitivities that are poorly constrained by core top observations alone. For applications over longer geological timescales, we develop a version of the model that incorporates changes in the Mg/Ca ratio of seawater. We test our models-collectively referred to as BAYMAG-on sediment trap data and on representative paleoclimate time series and demonstrate good agreement with observations and independent sea-surface temperature proxies. BAYMAG provides probabilistic estimates of past temperatures that can accommodate uncertainties in other environmental influences, enhancing our ability to interpret signals encoded in Mg/Ca.
    • A Bayesian hierarchical nonhomogeneous hidden Markov model for multisite streamflow reconstructions

      Bracken, C.; Rajagopalan, B.; Woodhouse, C.; Univ Arizona, Sch Geog & Dev; Department of Civil, Environmental and Architectural Engineering; University of Colorado at Boulder; Boulder Colorado USA; Department of Civil, Environmental and Architectural Engineering; University of Colorado at Boulder; Boulder Colorado USA; School of Geography and Development; University of Arizona; Tucson Arizona USA (AMER GEOPHYSICAL UNION, 2016-10)
      In many complex water supply systems, the next generation of water resources planning models will require simultaneous probabilistic streamflow inputs at multiple locations on an interconnected network. To make use of the valuable multicentury records provided by tree-ring data, reconstruction models must be able to produce appropriate multisite inputs. Existing streamflow reconstruction models typically focus on one site at a time, not addressing intersite dependencies and potentially misrepresenting uncertainty. To this end, we develop a model for multisite streamflow reconstruction with the ability to capture intersite correlations. The proposed model is a hierarchical Bayesian nonhomogeneous hidden Markov model (NHMM). A NHMM is fit to contemporary streamflow at each location using lognormal component distributions. Leading principal components of tree rings are used as covariates to model nonstationary transition probabilities and the parameters of the lognormal component distributions. Spatial dependence between sites is captured with a Gaussian elliptical copula. Parameters of the model are estimated in a fully Bayesian framework, in that marginal posterior distributions of all the parameters are obtained. The model is applied to reconstruct flows at 20 sites in the Upper Colorado River Basin (UCRB) from 1473 to 1906. Many previous reconstructions are available for this basin, making it ideal for testing this new method. The results show some improvements over regression-based methods in terms of validation statistics. Key advantages of the Bayesian NHMM over traditional approaches are a dynamic representation of uncertainty and the ability to make long multisite simulations that capture at-site statistics and spatial correlations between sites.
    • BAYSPLINE: A New Calibration for the Alkenone Paleothermometer

      Tierney, Jessica E.; Tingley, Martin P.; Univ Arizona, Dept Geosci (AMER GEOPHYSICAL UNION, 2018-03)
      The alkenone-based U-37(K') proxy is a cornerstone of paleoclimatology, providing insight into the temperature history of the Earth's surface ocean. Although the relationship between U-37(K') and sea surface temperatures (SSTs) is robust and well supported by experimental data, there remain outstanding issues regarding the seasonality of production of alkenones and the response of U-37(K') at very warm and cold SSTs. Using a data set of over 1,300 core-top U-37(K') measurements, we find compelling evidence of seasonal production in the North Atlantic, North Pacific, and Mediterranean Oceans. We also find significant attenuation of the U-37(K') response to SST at warm temperatures (> 24 degrees C), with the slope reduced by nearly 50% as U-37(K') approaches unity. To account for these observations in a calibration, we develop a new Bayesian B-spline regression model, BAYSPLINE, for the U-37(K') paleothermometer. BAYSPLINE produces similar estimates as previous calibrations below similar to 24 degrees, but above this point it predicts larger SST changes, in accordance with the attenuation of the U-37(K') response. Example applications of BAYSPLINE demonstrate that its treatment of seasonality and slope attenuation improves paleoclimatic interpretations, with important consequences for the inference of SSTs in the tropical oceans. BAYSPLINE facilitates a probabilistic approach to paleoclimate, building upon growing efforts to develop more formalized statistical frameworks for paleoceanographic reconstruction. Plain Language Summary "Alkenones" are lipids (fats) made by marine phytoplankton. The plankton alter the degree of unsaturation in these lipids in response to sea surface temperature (SST), producing more unsaturated compounds in colder water. These lipids are well preserved in marine sediments, such that paleoclimatologists can measure the unsaturation and determine what SSTs were in the past. In this manuscript, we review the calibration of this powerful "paleothermometer" and propose a new model that uses spline fits and Bayesian regression. We find that the new model improves our ability to estimate past SSTs, especially in the tropical oceans.
    • Big Jump of Record Warm Global Mean Surface Temperature in 2014-2016 Related to Unusually Large Oceanic Heat Releases

      Yin, J.; Overpeck, Jonathan T.; Peyser, Cheryl; Stouffer, Ronald; Univ Arizona, Dept Geosci; Department of Geosciences; University of Arizona; Tucson AZ USA; School for Environment and Sustainability; University of Michigan; Ann Arbor MI USA; Department of Geosciences; University of Arizona; Tucson AZ USA; Department of Geosciences; University of Arizona; Tucson AZ USA (AMER GEOPHYSICAL UNION, 2018-01-28)
      A 0.24 degrees C jump of record warm global mean surface temperature (GMST) over the past three consecutive record-breaking years (2014-2016) was highly unusual and largely a consequence of an El Nino that released unusually large amounts of ocean heat from the subsurface layer of the northwestern tropical Pacific. This heat had built up since the 1990s mainly due to greenhouse-gas (GHG) forcing and possible remote oceanic effects. Model simulations and projections suggest that the fundamental cause, and robust predictor of large record-breaking events of GMST in the 21st century, is GHG forcing rather than internal climate variability alone. Such events will increase in frequency, magnitude, and duration, as well as impact, in the future unless GHG forcing is reduced.
    • Biologically‐Relevant Trends in Springtime Temperatures Across the United States

      Crimmins, Theresa M.; Crimmins, Michael A.; Univ Arizona, Dept Environm Sci; Univ Arizona, Sch Nat Resources & Environm (AMER GEOPHYSICAL UNION, 2019-11-06)
      Long‐term trends in temperature—a primary driver of phenology—are typically evaluated using monthly or seasonal averages. However, accumulated warmth, rather than average temperature, cues phenological events; further, the amount of heat necessary to trigger activity is species‐specific. We evaluated trends in the timing of three heat accumulation thresholds encompassing spring‐season biological activity in the conterminous United States over a 70‐year period to document changes from a biologically relevant perspective. The Southwest, Northeast, and Northwest regions exhibit the strongest advancements. Rates of change vary among thresholds within many regions, resulting in temporal compression and lengthening within the season. Further, in the Eastern United States, the days between when a single threshold is met in the south and north are decreasing; in the West, the opposite pattern is occurring. These trends generally match long‐term observations of species' phenology, underscoring the value of this approach for documenting biologically relevant changes in temperature.
    • Biomass Burning Plumes in the Vicinity of the California Coast: Airborne Characterization of Physicochemical Properties, Heating Rates, and Spatiotemporal Features

      Mardi, Ali Hossein; Dadashazar, Hossein; MacDonald, Alexander B.; Braun, Rachel A.; Crosbie, Ewan; Xian, Peng; Thorsen, Tyler J.; Coggon, Matthew M.; Fenn, Marta A.; Ferrare, Richard A.; et al. (AMER GEOPHYSICAL UNION, 2018-12-16)
      This study characterizes in situ airborne properties associated with biomass burning (BB) plumes in the vicinity of the California coast. Out of 231 total aircraft soundings in July-August 2013 and 2016, 81 were impacted by BB layers. A number of vertical characteristics of BB layers are summarized in this work (altitude, location relative to cloud top height, thickness, number of vertically adjacent layers, interlayer distances) in addition to differences in vertical aerosol concentration profiles due to either surface type (e.g., land or ocean) or time of day. Significant BB layer stratification occurred, especially over ocean versus land, with the majority of layers in the free troposphere and within 100m of the boundary layer top. Heating rate profiles demonstrated the combined effect of cloud and BB layers and their mutual interactions, with enhanced heating in BB layers with clouds present underneath. Aerosol size distribution data are summarized below and above the boundary layer, with a notable finding being enhanced concentrations of supermicrometer particles in BB conditions. A plume aging case study revealed the dominance of organics in the free troposphere, with secondary production of inorganic and organic species and coagulation as a function of distance from fire source up to 450km. Rather than higher horizontal and vertical resolution, a new smoke injection height method was the source of improved agreement for the vertical distribution of BB aerosol in the Navy Aerosol Analysis and Prediction System model when compared to airborne data.
    • Birth, life, and demise of the Andean-syn-collisional Gissar arc: Late Paleozoic tectono-magmatic-metamorphic evolution of the southwestern Tian Shan, Tajikistan

      Worthington, James R.; Kapp, Paul; Minaev, Vladislav; Chapman, James B.; Mazdab, Frank K.; Ducea, Mihai N.; Oimahmadov, Ilhomjon; Gadoev, Mustafo; Univ Arizona, Dept Geosci; Department of Geosciences; University of Arizona; Tucson Arizona USA; et al. (AMER GEOPHYSICAL UNION, 2017-10)
      The amalgamation of the Central Asian Orogenic Belt in the southwestern Tian Shan in Tajikistan is represented by tectono-magmatic-metamorphic processes that accompanied late Paleozoic ocean closure and collision between the Karakum-Tarim and Kazakh-Kyrgyz terranes. Integrated U-Pb geochronology, thermobarometry, pseudosection modeling, and Hf geochemistry constrain the timing and petro-tectonic nature of these processes. The Gissar batholith and the Garm massif represent an eastward, along-strike increase in paleodepth from upper-batholith (similar to 21-7km) to arc-root (similar to 36-19km) levels of the Andean-syn-collisional Gissar arc, which developed from similar to 323-288Ma in two stages: (i) Andean, I-type granitoid magmatism from similar to 323-306Ma due to northward subduction of the Gissar back-arc ocean basin under the Gissar microcontinent, which was immediately followed by (ii) syn-collisional, I-S-type granitoid magmatism in the Gissar batholith and the Garm massif from similar to 304-288Ma due to northward subduction/underthrusting of Karakum marginal-continental crust under the Gissar microcontinent. A rapid isotopic pull-up from similar to 288-286Ma signals the onset of juvenile, alkaline-syenitic, post-collisional magmatism by similar to 280Ma, which was driven by delamination of the Gissar arclogite root and consequent convective asthenospheric upwelling. Whereas M-HT/LP prograde metamorphism in the Garm massif (650-750 degrees C/6-7kbar) from similar to 310-288Ma was associated with subduction-magma inundation and crustal thickening, HT/LP heating and decompression to peak-metamorphic temperatures (similar to 800-820 degrees C/6-4kbar) at similar to 2886Ma was driven by the transmission of a post-collisional, mantle-derived heat wave through the Garm-massif crust.
    • Buried Ice and Sand Caps at the North Pole of Mars: Revealing a Record of Climate Change in the Cavi Unit With SHARAD

      Nerozzi, S.; Holt, J. W.; Univ Arizona, Dept Planetary Sci, Lunar & Planetary Lab (AMER GEOPHYSICAL UNION, 2019-05-22)
      The cavi unit at the north pole of Mars is a deposit of aeolian sand and water ice underlying the Late Amazonian north polar layered deposits. Its strata of Middle to Late Amazonian age record wind patterns and past climate. The Mars Reconnaissance Orbiter Shallow Radar (SHARAD) reveals extensive internal and basal layering within the cavi unit, allowing us to determine its general structure and relative permittivity. Assuming a basalt composition for the sand (ε′ = 8.8), results indicate that cavi contains an average ice fraction between 62% in Olympia Planum and 88% in its northern reaches beneath the north polar layered deposits and thus represents one of the largest water reservoirs on the planet. Internal reflectors indicate vertical variability in composition, likely in the form of alternating ice and sand layers. The ice layers may be remnants of former polar caps and thus represent a unique record of climate cycles predating the north polar layered deposits.
    • Calculating surface ocean pCO(2) from biogeochemical Argo floats equipped with pH: An uncertainty analysis

      Williams, N. L.; Juranek, L. W.; Feely, R. A.; Johnson, K. S.; Sarmiento, J. L.; Talley, L. D.; Dickson, A. G.; Gray, A. R.; Wanninkhof, R.; Russell, Joellen; et al. (AMER GEOPHYSICAL UNION, 2017-03)
      More than 74 biogeochemical profiling floats that measure water column pH, oxygen, nitrate, fluorescence, and backscattering at 10 day intervals have been deployed throughout the Southern Ocean. Calculating the surface ocean partial pressure of carbon dioxide (pCO(2sw)) from float pH has uncertainty contributions from the pH sensor, the alkalinity estimate, and carbonate system equilibrium constants, resulting in a relative standard uncertainty in pCO(2sw) of 2.7% (or 11 mu atm at pCO(2sw) of 400 mu atm). The calculated pCO(2sw) from several floats spanning a range of oceanographic regimes are compared to existing climatologies. In some locations, such as the subantarctic zone, the float data closely match the climatologies, but in the polar Antarctic zone significantly higher pCO(2sw) are calculated in the wintertime implying a greater air-sea CO2 efflux estimate. Our results based on four representative floats suggest that despite their uncertainty relative to direct measurements, the float data can be used to improve estimates for air-sea carbon flux, as well as to increase knowledge of spatial, seasonal, and interannual variability in this flux. Plain Language Summary The Southern Ocean is a key player in the global flow of carbon, yet it is hard to reach, and there are relatively few measurements there, especially in winter. Measuring the amount of carbon dioxide gas in seawater is key to advancing our understanding of the Southern Ocean. More than 74 robotic floats that use sensors to measure seawater properties have been deployed throughout the Southern Ocean, and each has a lifetime of around 5 years. It is currently not possible to directly measure carbon dioxide gas from these floats; however, it is possible to estimate carbon dioxide from things that the float can measure, like pH, a measure of ocean acidity. Here surface ocean carbon dioxide is estimated from several floats and compared to two ship-based estimates. In some locations, the floats closely match the existing estimates, but in other locations the floats see significantly higher surface ocean carbon dioxide in the wintertime, reinforcing the idea that the Southern Ocean's role in the global carbon cycle needs a closer look. Our results show that despite not measuring carbon dioxide directly, these floats will help scientists learn a lot about the Southern Ocean's part in the global flow of carbon.
    • Can Convection‐Permitting Modeling Provide Decent Precipitation for Offline High‐Resolution Snowpack Simulations Over Mountains?

      He, Cenlin; Chen, Fei; Barlage, Michael; Liu, Changhai; Newman, Andrew; Tang, Wenfu; Ikeda, Kyoko; Rasmussen, Roy; Univ Arizona, Dept Hydrol & Atmospher Sci (AMER GEOPHYSICAL UNION, 2019-11-22)
      Accurate precipitation estimates are critical to simulating seasonal snowpack evolution. We conduct and evaluate high-resolution (4-km) snowpack simulations over the western United States (WUS) mountains in Water Year 2013 using the Noah with multi-parameterization (Noah-MP) land surface model driven by precipitation forcing from convection-permitting (4-km) Weather Research and Forecasting (WRF) modeling and four widely used high-resolution datasets that are derived from statistical interpolation based on in situ measurements. Substantial differences in the precipitation amount among these five datasets, particularly over the western and northern portions of WUS mountains, significantly affect simulated snow water equivalent (SWE) and snow depth (SD) but have relatively limited effects on snow cover fraction (SCF) and surface albedo. WRF generally captures observed precipitation patterns and results in an overall best-performed SWE and SD in the western and northern portions of WUS mountains, where the statistically interpolated datasets lead to underpredicted precipitation, SWE, and SD. Over the interior WUS mountains, all the datasets consistently underestimate precipitation, causing significant negative biases in SWE and SD, among which the results driven by the WRF precipitation show an average performance. Further analysis reveals systematic positive biases in SCF and surface albedo across the WUS mountains, with similar bias patterns and magnitudes for simulations driven by different precipitation datasets, suggesting an urgent need to improve the Noah-MP snowpack physics. This study highlights that convection-permitting modeling with proper configurations can have added values in providing decent precipitation for high-resolution snowpack simulations over the WUS mountains in a typical ENSO-neutral year, particularly over observation-scarce regions.
    • Can Leaf Spectroscopy Predict Leaf and Forest Traits Along a Peruvian Tropical Forest Elevation Gradient?

      Doughty, Christopher E.; Santos-Andrade, P. E.; Goldsmith, G. R.; Blonder, B.; Shenkin, A.; Bentley, L. P.; Chavana-Bryant, C.; Huaraca-Huasco, W.; Díaz, S.; Salinas, N.; et al. (AMER GEOPHYSICAL UNION, 2017-11)
      High-resolution spectroscopy can be used to measure leaf chemical and structural traits. Such leaf traits are often highly correlated to other traits, such as photosynthesis, through the leaf economics spectrum. We measured VNIR (visible-near infrared) leaf reflectance (400-1,075nm) of sunlit and shaded leaves in similar to 150 dominant species across ten, 1ha plots along a 3,300m elevation gradient in Peru (on 4,284 individual leaves). We used partial least squares (PLS) regression to compare leaf reflectance to chemical traits, such as nitrogen and phosphorus, structural traits, including leaf mass per area (LMA), branch wood density and leaf venation, and higher-level traits such as leaf photosynthetic capacity, leaf water repellency, and woody growth rates. Empirical models using leaf reflectance predicted leaf N and LMA (r(2)>30% and %RMSE<30%), weakly predicted leaf venation, photosynthesis, and branch density (r(2) between 10 and 35% and %RMSE between 10% and 65%), and did not predict leaf water repellency or woody growth rates (r(2)<5%). Prediction of higher-level traits such as photosynthesis and branch density is likely due to these traits correlations with LMA, a trait readily predicted with leaf spectroscopy.
    • Cassini UVIS Detection of Saturn's North Polar Hexagon in the Grand Finale Orbits

      Pryor, W. R.; Esposito, L. W.; Jouchoux, A.; West, R. A.; Grodent, D.; Gérard, J.‐C.; Radioti, A.; Lamy, L.; Koskinen, T.; Univ Arizona, Lunar & Planetary Lab (AMER GEOPHYSICAL UNION, 2019-07-29)
      Cassini's final orbits in 2016 and 2017 provided unprecedented spatial resolution of Saturn's polar regions from near-polar spacecraft viewing geometries. Long-wavelength channels of Cassini's Ultraviolet Imaging Spectrograph instrument detected Saturn's UV-dark north polar hexagon near 180 nm at planetocentric latitudes near 75 degrees N. The dark polar hexagon is surrounded by a larger, less UV-dark collar poleward of planetocentric latitude 65 degrees N associated with the dark north polar region seen in ground-based images. The hexagon is closely surrounded by the main arc of Saturn's UV aurora. The UV-dark material was locally darkest on one occasion (23 January 2017) at the boundary of the hexagon; in most Ultraviolet Imaging Spectrograph images the dark material more uniformly fills the hexagon. The observed UV-dark stratospheric material may be a hydrocarbon haze produced by auroral ion-neutral chemistry at submicrobar pressure levels. Ultraviolet Imaging Spectrograph polar observations are sensitive to UV-absorbing haze particles at pressures lower than about 10-20 mbar.