Eos, Vol. 94, No. 15, 9 April 2013

PAGE 148
Understanding the structureof subducting plates

Low density of Earth’s core due to oxygen and silicon impurities

Seismic studies are helping scientists
learn more about the structure of subducting oceanic plates. Using an air gun arrayand 80 ocean bottom seismometers spaced
along a 500- kilometer profile, Fujie et al.
conducted a seismic reflection andrefraction survey at the Kuril trench in thenorthwestern Pacific margin, where part of
the Pacific plate is subducting beneath theOkhotsk plate. They estimated the watercontent of the subducting plate by measuring the velocity of seismic waves—bothP waves and S waves—through the plate.
The ratio of seismic wave velocities (Vp /Vs )is an indicator of the lithology, porosity, andpresence of fluid in the plate. Their findings
showed that the water content in the plateincreased toward the trench, along withgreater bending and fracturing, suggestingthat water enters the plate through thefractures. The authors conclude that thebending and fracturing of the plate as it
subducts play an important role in thewater cycle in subduction zones.
(Geophysical Research Letters, doi:10.1029/
2012GL054340, 2013)—EB

Depth relative tothe basement (km)Trench
00100

200

300

400

500
1.90

21.85

41.75

61.80
1.70

Vp/Vs within the oceanic crust

The ratio of different seismic wave velocities( Vp/ Vs ) within the uppermost oceanic crust
increases toward the oceanic trench, suggesting seawater percolation into the oceanic
plate in the trench–outer rise region.

From a national collaboration among
citizen scientists, educational institutions,environmental organizations, and federal
agencies, Jeong et al. developed the firstcountrywide assessment of springtime tree
budding based on in-the-field measurements.
The phenological assessment, whichmeasured the timing of bud burst for five tree
species, was used to build a model ofspecies- specific budding across the continental United States. Using their tree budding
model in forecasts of future climate change,
the authors found that by the end of thecentury, tree budding across the United States
will occur more than 1 month earlier in someplaces.

and oxygen into the core, creating a light,
element-rich, buoyant layer on top of theliquid outer core. They suggest that
evidence for such a layer may have been
observed in seismic studies.
Using a multianvil press, the authors
drove a mixture of iron, magnesium
silicate, silicon dioxide, and the iron oxide
wüstite to 25 gigapascals of pressure andtemperatures from 2700 to 3080 Kelvin.
They found that at temperatures below
3000 Kelvin, silicon and oxygen in the ironmelt were mutually exclusive, withconcentrations not rising above the lowlevels identified in previous research.
Above 3000 Kelvin, however, they foundthat the presence of oxygen actuallyenhanced the partitioning of silicon into theiron, with the concentrations of both silicon
and oxygen increasing. (GeophysicalResearch Letters, doi:10.1029/2012GL054116,2013) —CS

From observations of tree leaf emergencecollected by citizen and professional
scientists working together as part of the USA
National Phenology Network, the authors
assessed projected changes in tree bud burst
using seven Earth system or coupled globalmodels. The authors’ forecasts drew on four
future emissions scenarios, or representative
concentration pathways, that are to be usedas the guiding scenarios for research to be
included in the upcoming fifth assessment
report of the Intergovernmental Panel on
Climate Change.
The authors found that the earlier budding
will predominantly affect northern trees.
Under the climate scenario with the mostwarming, they found that trees in Maine, NewYork, Michigan, and Wisconsin will bud up to38 days earlier. In southern states such as
Florida, Alabama, and Georgia, however, budburst will occur 10 days earlier under thesame climate scenario. The result is that theannual latitudinal “green wave” of tree
budding could take just 59.1 days by the endof the century, as compared with the 74.7 days
that it takes now. The authors suggest that thechanging bud burst dates could affectinterspecies competition and the global
carbon budget. (Geophysical Research Letters,
doi:10.1029/2012GL054431, 2013) —CS

Brian Powell, USA NPN

Citizen science finds spring could comea month early to the United States

During accretion and differentiation ofthe Earth, chemical interactions in a silicate
magma ocean and liquid iron drove silicon
and oxygen impurities into what went on tobecome the liquid outer core. Contrastingwith previous research, which suggested
that silicon and oxygen would only appear
in very low concentrations (less than 1%by weight) in the liquid iron, Tsuno et al.
found that at the base of a magma ocean
1200 kilometers deep, these light elements
could simultaneously reach concentrations
as high as 5% oxygen and 8% silicon byweight. Such impurity levels would
decrease the density of the outer core,accounting for the so- called “densitydeficit” identified in previous research,whereby the outer core is roughly 10% lessdense than a pure iron-nickel alloy.
The researchers also propose that at thepresent- day core-mantle boundary, hightemperatures would drive additional silicon

A volunteer in a leaf phenology program.

© 2013. American Geophysical Union. All Rights Reserved.

Is there a link between Arctic
amplification and midlatitude weather?
As the Earth’s climate warms, the Arctic
has been warming more rapidly than the restof the globe. Some previous studies havesuggested that this Arctic amplification ofwarming is causing an increase in extreme
weather events in the midlatitudes. They
proposed that this occurs because Arctic
warming induces changes in atmosphericcirculation, including changes in atmosphericplanetary waves that tend to promotepersistent extreme weather conditions, suchas heat waves, cold spells, droughts, andfloods.
To evaluate this possibility, Screen et al.
examined changes in atmospheric planetary
wave amplitude over recent decades. Unlike
previous studies, they considered two
different descriptions of planetary wave
amplitude: meridional amplitude, a measureof north-south meandering, and zonal
amplitude, a measure of the intensity (height)of atmospheric ridges and troughs at 45°N.
The authors also examined the planetary
wave amplitude changes at all longitudes andover three separate longitudinal sectors toidentify possible impacts of Arctic amplification over regions such as Europe or Asia.

Eos, Vol. 94, No. 15, 9 April 2013
NASA Goddard Space Flight Center Scientifi c Visualization Studio

Japan, during the summer of 2012. From thesampled storms, the authors identified
232 instances of positive narrow bipolar
lightning and 22 negative discharges. Theauthors found that positive narrow bipolar
discharges are typically located deep within
the cloud, either in or surrounding the regionof deepest convection. Negative NBEs, on theother hand, almost exclusively occur near thecloud tops, with altitudes from 14 to 16 kilometers, they found.
Based on their observations, the authors
suggest that there is a critical cloud height—
around 15 kilometers altitude—below which
negative narrow bipolar discharges will notoccur. As a result of this finding, the authors
suggest that the detection of negative NBEs
could be used to estimate cloud top height
remotely. Barring that, they say that the mere
detection of negative NBEs can be used for aquick rough assessment of thundercloudheight and hence of its likely severity.
(Geophysical Research Letters, doi:10.1002/grl.50112, 2013) —CS

They found that from 1979 to 2011,
statistically significant changes in either
measure were observed during only a few
seasons, and the two measures showed
different changes. For instance, in summers
the authors found increases in meridional
amplitude over Europe but found decreases
in zonal amplitude. They concluded that any
possible connection between Arctic amplification and planetary wave amplitudedepends on how the atmospheric waves aredefined, and thus, studies of a possible
impact of Arctic amplification on weather
extremes need to take this into account.
(Geophysical Research Letters, doi:10.1002/
grl.50174, 2013) —EB

Guiding future researchon terrestrial ecosystem disturbanceWith North American ecosystems responsible for drawing hundreds of teragrams ofcarbon from the atmosphere each year, thetenuous balance of the terrestrial carbon
budget can be upset for decades by disturbances such as fires, storms, diseaseoutbreaks, insect infestations, and logging.
Research cataloging the effects of such
disturbances on regional carbon cycling
tends to be sporadic or of limited scope. Most
research has focused on forests but is lessextensive for other important ecosystems
such as grasslands or permafrost peatlands.
With climate change set to alter disturbance patterns—and in turn the balance ofthe carbon budget—a more thoroughunderstanding of the effects of disturbanceson terrestrial ecosystems is needed. To drive
this effort, Kasischke et al. provide an
overview of an attempt by more than
100 scientists to synthesize existing research,and they have identified steps that can betaken to direct the future course of researchin this field.
Drawing on research conducted under thebanner of the North American Carbon
Program and the CarboNA project, theauthors identified trends in ecosystem

disturbances over the past 2 decades. They
found that across North America 32.8 million
hectares of forest were disturbed to someextent, in particular by a widespreadinfestation of bark beetles in western North
American and by tree harvesting.
The authors emphasize that to understandthe full impact of a disturbance on the carbon
cycle, scientists must consider the direct
effects, such as tree mortality and emissionsof carbon during fires, as well as the indirecteffects, such as changes in the local climate
or in abiotic ecosystem properties. To start,
the authors suggest that an integrated
database that includes all disturbance types
should be developed, using a combination of
information from satellite remote sensing
systems and from disturbance- specificdatabases that currently exist. (Journal ofGeophysical Research- Biogeosciences,doi:10.1002/jgrg.20027, 2013) —CS

Negative narrow bipolar lightning
needs critical cloud height
High within towering thunderclouds, adistinct form of intracloud lightning, known
as “narrow bipolar pulse” discharges, canoccur. Like other forms of lightning, narrow
bipolar events (NBE) can be either negativeor positive discharges. These events are
known for their high-powered, short- distanceelectrical discharges that produce strong
emissions of very high frequency radio waves.
Previous research has found that since NBEs
take place at relatively high altitudes, it ispossible to detect them remotely using
satellites. To be able to use the detection ofnarrow bipolar events to measure cloud
behavior or storm dynamics, however,requires a better understanding of therelationship between cloud properties andNBEs.
Using a high spatial resolution radar array
and a low-frequency lightning location
system, Wu et al. measured the properties ofNBEs produced by 10 storms near Osaka,© 2013. American Geophysical Union. All Rights Reserved.

Ting Wu

A visualization of atmospheric waves. Screen et al. examined whether atmospheric waves could
provide a link between Arctic amplification and midlatitude weather extremes.

Researchers used the phased array radar at
Osaka University to observe narrow bipolar
lightning.

Arctic sea ice thickness
and volume declining
The record low extent of Arctic sea ice areain September 2012 made headlines. Studies
using models have suggested that sea volume
has also been declining. A new study byLaxon et al. confirms this with observational
data. The authors used satellite radarobservations from the European Space
Agency CryoSat-2 mission to estimate Arctic
sea ice thickness and volume for the falls andwinters of 2010–2011 and 2011–2012. Bycomparing these new data with earlier
estimates from NASA’s ICESat satellite, theyfound that the average ice volume declined
by 4291 cubic kilometers in fall and1479 cubic kilometers in winter between theperiod from 2003 to 2008 and the 2010–2012
period. The average volume of ice loss overboth the fall and winter periods was found to
be about 500 cubic kilometers per year,
equivalent to a 0.075 meter per year decreasein thickness. (Geophysical Research Letters,doi:10.1002/grl.50193, 2013) —EB—ERNIE BALCERAK, Staff Writer, and COLIN
SCHULTZ, Writer