AGU 期刊一周Research Spotlights (Nov 25 ~Nov 30, 2016)

发布时间:2016-12-2 12:28:50 点击次数:599

I. Geochemistry, Mineralogy, Volcanology

1. Deciphering the Cosmogenic Code to Learn Earth's Surface History

Third Nordic Workshop on Cosmogenic Nuclide Techniques; Stockholm, Sweden, 8–10 June 2016

https://eos.org/meeting-reports/deciphering-the-cosmogenic-code-to-learn-earths-surface-history

2Earth's Carbon-Climate Feedbacks Varied in Past Warming Episodes

Records from drill holes in the eastern equatorial Pacific indicate that Earth's orbital eccentricity played an important role in controlling climate as the planet warmed.

https://eos.org/research-spotlights/earths-carbon-climate-feedbacks-varied-in-past-warming-episodes

II.Hazards & Disasters

1. First of Next Generation of U.S. Weather Satellites Hits the Sky

The novel spacecraft and three sister satellites to follow are expected to vastly improve storm forecasting.

https://eos.org/articles/first-of-next-generation-of-u-s-weather-satellites-hits-the-sky

2. When Might the Campi Flegrei Caldera Erupt Again?

The clock may be ticking for Italy's Campi Flegrei caldera, a region with a pattern of numerous and sometimes large explosive eruptions. The next explosion could be less than 100 years away.

https://eos.org/research-spotlights/when-might-the-campi-flegrei-caldera-erupt-again

III. Climate Change

1. EPA Head Calls Climate Change Biggest Threat to U.S. Progress

McCarthy says that despite anxiety at the agency about the election results, she is confident in EPA’s work and that efforts to control climate change will continue.

https://eos.org/articles/epa-head-calls-climate-change-biggest-threat-u-s-progress

IV. Geology & Geophysics

1. Laboratory Sharing to Improve Rock Deformation Research

An ever-growing group of scientists seeks to integrate rock deformation labs from across the United States into one shared national facility.

https://eos.org/opinions/laboratory-sharing-to-improve-rock-deformation-research

V. Geophysical Research Letters

1. Ionospheric conductances and currents of a morning sector auroral arc from Swarm-A electric and magnetic field measurements

We show the first ionospheric Hall and Pedersen conductances derived from Swarm magnetic and electric field measurements during a crossing of a morning sector auroral arc. Only Swarm-A was used, with assumptions of negligible azimuthal gradients and vanishing eastward electric field. We find upward field-aligned current, enhanced Hall and Pedersen conductances, and relatively weak electric field coincident with the arc. Poleward of the arc, the field-aligned current was downward, conductances lower, and the electric field enhanced. The arc was embedded in a westward electrojet, immediately equatorward of the peak current density. The equatorward portion of the electrojet could thus be considered conductance dominant and the poleward portion electric field dominant. Although the electric field measured by Swarm was intense, resulting in conductances lower than those typically reported, comparable electric fields have been observed earlier. These results demonstrate how Swarm data can significantly contribute to our understanding of the ionospheric electrodynamics.

http://onlinelibrary.wiley.com/doi/10.1002/2016GL070248/full

2. Mapping of hydrocarbons and H3+emissions at Jupiter's north pole using Galileo/NIMS data

In this paper we report the mapping of H3+, C2H2, and CH4 as derived by an unexploited Galileo/Near-Infrared Mapping Spectrometer (NIMS) data set. As previously observed, hydrocarbons emissions appear to be located in the internal part of the auroral main oval, where CH4 3 μm vibrational band intensity ratios suggest that nonthermal excitation mechanisms, such as auroral particle precipitation and/or Joule heating, are responsible for the observed emissions. Temperature estimation are in good agreement for the CH4-emitting region on the hot spot, while the values obtained for H3+ are lower in comparison with Cassini/visual and infrared mapping spectrometer and ground-based data. C2H2emission overlaps the CH4 one only at higher latitudes >75°N, indicating that different energetic particles are at work inside the main oval polar ward. CH4 is also found on the northern section of the main oval (135°< longitude <190°, 60°< latitude <90°N). The present investigation results have implications on the Juno/Jovian InfraRed Auroral Mapper observation planning as well as on the codes that will be used to retrieve temperatures and densities of all the emitting species involved in the Jupiter auroral processes.

http://onlinelibrary.wiley.com/doi/10.1002/2016GL070787/full

3. Accelerated ice shelf rifting and retreat at Pine Island Glacier, West Antarctica

Pine Island Glacier has undergone several major iceberg calving events over the past decades. These typically occurred when a rift at the heavily fractured shear margin propagated across the width of the ice shelf. This type of calving is common on polar ice shelves, with no clear connection to ocean-ice dynamic forcing. In contrast, we report on the recent development of multiple rifts initiating from basal crevasses in the center of the ice shelf, resulted in calving further upglacier than previously observed. Coincident with rift formation was the sudden disintegration of the ice mélange that filled the northern shear margin, resulting in ice sheet detachment from this margin. Examination of ice velocity suggests that this internal rifting resulted from the combination of a change in ice shelf stress regime caused by disintegration of the mélange and intensified melting within basal crevasses, both of which may be linked to ocean forcing.

http://onlinelibrary.wiley.com/doi/10.1002/2016GL071360/full

4. Deep carbon cycle recorded by calcium-silicate rocks (rodingites) in a subduction-related ophiolite

Carbon cycling in subduction zones remains poorly constrained due to the lack of relevant geological records. Here we report magnesium isotope data (δ26MgDSM3) from calcium-silicate rocks (rodingites) from the Xigaze ophiolite, southern Tibet, which is thought to represent remnants of Neo-Tethyan oceanic lithosphere. Behaviors of immobile trace elements in rodingites resemble those of their mafic dike protoliths, showing subduction-related signatures. The majority of rodingites exhibits low δ26Mg values of −0.72‰ to −0.33‰ with a weighted average of −0.47 ± 0.11‰ (2 SD), significantly lighter than that of their protoliths (−0.31 ± 0.03‰). This difference likely reflects the interaction of the protolith with isotopically light carbonate fluids. Modeling indicates that this hypothesis requires the input of 5 to 15 wt % carbonates during rodingitization. Our study suggests that rodingite may represent a previously unrecognized reservoir of dissolved Ca from subducted carbonates.

http://onlinelibrary.wiley.com/doi/10.1002/2016GL070474/full

5. Low radiation efficiency of the intermediate-depth earthquakes in the Japan subduction zone

Robust determination of earthquake source parameters over a continuous depth range is central to inferring rupture mechanisms dominant at different depths. We employed a cluster-event method to constrain the source parameters as well as along-path attenuation for earthquakes over 0–150 km depths and 4 orders of seismic moments in the Japan subduction zone. We found that corner frequency and stress drop increase with depth, whereas the radiated energy scaled by seismic moment declines with depth slightly. As a result, the radiation efficiency exhibits a notable deficit for events deeper than 60 km. Together these suggest an increased energy dissipation during faulting in ductile deformation regime, consistent with shear heating instability as an important faulting mechanism for intermediate-depth earthquakes.

http://onlinelibrary.wiley.com/doi/10.1002/2016GL070993/full

6. Mantle plumes beneath the South Pacific superswell revealed by finite frequency Ptomography using regional seafloor and island data

We present a new tomographic image beneath the South Pacific superswell, using finite frequency P wave travel time tomography with global and regional data. The regional stations include broadband ocean-bottom seismograph stations. The tomographic image shows slow anomalies of 200-300 km in diameter beneath most hot spots in the studied region, extending continuously from the shallow upper mantle to 400 km depth. Narrow and weak slow anomalies are detected at depths of 500–1000 km, connecting the upper mantle slow anomalies with large-scale slow anomalies with lateral dimension of 1000–2000 km prevailing below 1000 km depth down to the core-mantle boundary. There are two slow anomalies around the Society hot spot at depths shallower than 400 km, which both emerge from the same slow anomaly at 500 km depth. One of them is located beneath the Society hot spot and the other underlies 500 km east of the Society hot spot, where no volcanism is observed.

http://onlinelibrary.wiley.com/doi/10.1002/2016GL070793/full

7. Van Allen Probes observations of cross-scale coupling between electromagnetic ion cyclotron waves and higher-frequency wave modes

We present observations of higher-frequency (~50–2500 Hz, ~0.1–0.7 fce) wave modes modulated at the frequency of colocated lower frequency (0.5–2 Hz, on the order of fci) waves. These observations come from the Van Allen Probes Electric Field and Waves instrument's burst mode data and represent the first observations of coupling between waves in these frequency ranges. The higher-frequency wave modes, typically whistler mode hiss and chorus or magnetosonic waves, last for a few to a few tens of seconds but are in some cases observed repeatedly over several hours. The higher-frequency waves are observed to be unmodulated before and after the presence of the electromagnetic ion cyclotron (EMIC) waves, but when the EMIC waves are present, the amplitude of the higher-frequency waves drops to the instrument noise level once every EMIC wave cycle. Such modulation could significantly impact wave-particle interactions such as acceleration and pitch angle scattering, which are crucial in the formation and depletion of the radiation belts. We present one case study with broadband, high-frequency waves observed to be modulated by EMIC waves repeatedly over a 2 h time span on both spacecraft. Finally, we show two additional case studies where other high-frequency wave modes exhibit similar modulation.

http://onlinelibrary.wiley.com/doi/10.1002/2016GL071566/full

8. River-discharge dynamics in the Southern Central Andes and the 1976–77 global climate shift

Recent studies have shown that the 1976–77 global climate shift strongly affected the South American climate. In our study, we observed a link between this climate shift and river-discharge variability in the subtropical Southern Central Andes. We analyzed the daily river-discharge time series between 1940 and 1999 from small to medium mountain drainage basins (102–104 km2) across a steep climatic and topographic gradient. We document that the discharge frequency distribution changed significantly, with higher percentiles exhibiting more pronounced trends. A change point between 1971 and 1977 marked an intensification of the hydrological cycle, which resulted in increased river discharge. In the upper Rio Bermejo basin of the northernmost Argentine Andes, the mean annual discharge increased by 40% over 7 years. Our findings are important for flood risk management in areas impacted by the 1976–77 climate shift; discharge frequency distribution analysis provides important insights into the variability of the hydrological cycle in the Andean realm.

http://onlinelibrary.wiley.com/doi/10.1002/2016GL070868/full

9. Using GPS radio occultations to infer the water vapor feedback

The air refractive index at L-band frequencies depends on the air's water vapor content and density. Exploiting this relationship, we derive for the first time a theoretical model to infer the specific humidity response to surface temperature variations, dq/dTs, given knowledge of how the air refractive index and temperature vary with surface temperature. We validate this model by using 1.2–1.6 GHz Global Positioning System Radio Occultation (GPS RO) observations from 2007 to 2010 at 250 hPa, where the water vapor feedback on surface warming is strongest. The dq/dTs estimation from GPS RO observations shows excellent agreement with previously published results and the responses estimated by using the Atmospheric Infrared Sounder and the NASA's Modern-Era Retrospective Analysis for Research and Applications data sets. Because of their high sensitivity to fractional changes in water vapor, current and future GPS RO observations show great promise in monitoring climate feedback and their trends.

http://onlinelibrary.wiley.com/doi/10.1002/2016GL071017/full

10. A global view of atmospheric ice particle complexity

Atmospheric ice particles exist in a variety of shapes and sizes. Single hexagonal crystals like common hexagonal plates and columns are possible, but more frequently, atmospheric ice particles are much more complex. Ice particle shapes have a substantial impact on many atmospheric processes through fall speed, affecting cloud lifetime, to radiative properties, affecting energy balance to name a few. This publication builds on earlier work where a technique was demonstrated to separate single crystals and aggregates of crystals using particle imagery data from aircraft field campaigns. Here data from 10 field programs have been analyzed and ice particle complexity parameterized by cloud temperature for arctic, midlatitude (summer and frontal), and tropical cloud systems. Results show that the transition from simple to complex particles can be as small as 80 μm or as large as 400 μm depending on conditions. All regimes show trends of decreasing transition size with decreasing temperature.

http://onlinelibrary.wiley.com/doi/10.1002/2016GL071267/full

VI.AGU News

1. WEST ANTARCTIC ICE SHELF BREAKING UP FROM THE INSIDE OUT


WASHINGTON, DC — A key glacier in Antarctica is breaking apart from the inside out, suggesting that the ocean is weakening ice on the edges of the continent.

The Pine Island Glacier, part of the ice shelf that bounds the West Antarctic Ice Sheet, is one of two glaciers that researchers believe are most likely to undergo rapid retreat, bringing more ice from the interior of the ice sheet to the ocean, where its melting would flood coastlines around the world.

https://news.agu.org/press-release/west-antarctic-ice-shelf-breaking-up-from-the-inside-out/

VII.AGU Blogs

1. Avalanches more complex than previously thought

Avalanches can throw up a powdery cloud of snow as they violently charge down mountains, obscuring their inner workings from scientists. But new observations of artificially-triggered avalanches in Switzerland’s Vallée de la Sionne have penetrated this powdery veil.

http://blogs.agu.org/geospace/2016/11/22/avalanches-complex-previously-thought/

2.A conversation with Zack Labe

Yesterday, I mentioned climate change visualizer extraordinaire Zack Labe. As delineated then, he’s a PhD student at U.C. – Irvine in the Earth Systems Science department. He’s producing some really excellent #dataviz on climate change. Today, I’d like to share a short exchange I had with Zack about his work. 1)      Please give Mountain Beltway readers a sense of your background, leading up to what you’re working on …

http://blogs.agu.org/mountainbeltway/2016/11/23/conversation-zack-labe/

3. What’s Happening in the Arctic is Astonishing

I’ve been busy with the GOES-R launch, but am also following the incredible situation in the High Arctic, where Arctic Sea ice continues to run at record low levels. More like falling of a cliff actually, and the only word I can come up with is astonishing! There is a real temperature dipole showing up between the warm Arctic and the very cold areas of Russia/Asia where the snowfall was …

http://blogs.agu.org/wildwildscience/2016/11/24/whats-happening-arctic-astonishing/