© 2013. American Geophysical Union. All Rights Reserved.

Eos, Vol. 94, No. 22, 28 May 2013

PAGE 204

Climatic controls on carbon storage in seasonally frozen soils

When soil goes through an annual

freeze-thaw cycle, the expansion and

shrinkage of underground water causes a

second cycle, one of heaving and sinking,

that can produce unusual geometric patterns

on the surface. Known as cryoturbation, this

process drives buried boulders to the surface

and lets fine particles settle in the holes left

behind. In some regions, cryoturbation gives

rise to circles on the surface (some a few

meters wide): patches of bare soil ringed by

rocks. In others, such as a site in northern

Sweden analyzed by Becher et al., cryoturba-

tion creates nonsorted circles: bare soil

surrounded by trees or shrubs. The churning

soil is inhospitable for the plants’ roots, and

if anything, only a light dusting of moss or

lichen covers the centers of the circles.

When long periods of cold give way to

warming, frost spreading from the circles’

centers can bury the nearby vegetation,

producing a distinct organic layer in soil

samples. The burial of organic carbon in this

way is important for the long-term sequestra-

tion of carbon in seasonally frozen soils.

Using radioactive carbon and lead dating,

the authors found that for their study site

there were three phases during which such

buried organic layers were produced: from

0 to 100 CE, from 900 to 1250 CE, and from

1650 to 1950 CE. The authors found that

these time periods align with regional

deglaciation.

Using aerial photographs from 1959 to

2008, the authors found that over this period,

the plants—normally ringing the circles—

have steadily moved in on the uncovered

mineral circles in their study site, a sign that

cryoturbation is slowing. The authors’

findings suggest that the burial of organic

layers that drives long-term carbon storage,

normally assumed to occur at a steady rate,

depends instead on climate conditions and

that this rate is likely decreasing with the

declining rate of cryoturbation. (Journal of

Geophysical Research- Biogeosciences,

doi:10.1002/ jgrg.20016, 2013) —CS

Are element ratios in coral coresa good proxy for past temperature?

Strontium- to- calcium ratios (Sr/Ca) in cores

from coral skeletons are widely used as a

proxy for past temperature (Sr/Ca has a

negative relationship with temperature—this

ratio typically declines as temperatures rise).

As corals grow, they build aragonite (CaCO3)

skeletons, and minor chemical elements are

incorporated into those skeletons at different

proportions depending on conditions such as

temperature. However, those elemental ratios

can also be affected by biological factors in

the individual coral, including an individual

coral’s growth rate. This makes the use of

coral Sr/Ca ratios as a paleothermometer

potentially problematic.

Taking a detailed look at how Sr/Ca varies

with temperature and with biological factors,

Grove et al. compared cores taken near

northeast Madagascar from two corals of the

same species located just 0.72 kilometer

apart. They examined the Sr/Ca ratios and

growth rates of the corals over a 43-year

period, along with historical sea surface

temperature data for the region, looking at

seasonal and interannual variability as well as

longer-term trends.

Both cores showed similar seasonal

variations in growth rate and Sr/Ca ratios, but

on interannual time scales, the two corals’

growth rates had opposite responses to

temperature: In one core, increasing sea

surface temperature was accompanied by

increasing growth rate and increasing Sr/Ca;

in the other, the rising temperatures led to

declining growth rate and declining Sr/Ca

over the same period.

The authors show that most of the

variation in Sr/Ca ratios is accounted for by

the individual coral’s growth response to

temperature; in fact, this effect overwhelms

the direct effect of temperature on Sr/Ca

ratios. They conclude that scientists need to

take this into account and should consider

analyzing multiple coral cores when

attempting to use Sr/Ca ratios as a proxy for

past temperature. (Geochemistry, Geophysics,

Geosystems, doi:10.1002/ ggge.20095, 2013)

—EB

Nonnative salmon alter nitrifi cationin Great Lakes tributaries

Nonnative species can affect the biogeo-

chemistry of an ecosystem. For instance,

Pacific salmon have been introduced for

sport fishing in many streams and lakes

beyond their native range, and their introduc-

tion may be altering nitrogen cycling in

those ecosystems.

Salmon excrete ammonium (NH4

+), which

can be transformed into nitrate (NO3

–) by

bacteria in a process known as nitrification.

Nitrate can be used by plants as an inorganic

nitrogen source, but in excess it can also

cause potentially harmful algal blooms to

grow and, at high concentrations, is consid-

ered a pollutant in drinking water.

When scientists excavated at the transition between the inner and outer portions of a non-sorted circle, they found a buried organic layer.

Mar

ina

Bec

her

Several species of Pacific salmon have been introduced to the Great Lakes and migrate up tributaries annually to spawn and die. Salmon and their carcasses, such as the (top) male and (bottom) female Chinook salmon (Oncorhynchus tshawytscha) pictured here, may have unintended consequences for ecosystem- scale processes within these Great Lakes tributaries.

Pet

er L

evi

Eos, Vol. 94, No. 22, 28 May 2013

© 2013. American Geophysical Union. All Rights Reserved.

Levi and Tank measured sediment

nitrification rates before, during, and after the

salmon run in 2009 to study the effects of

nonnative Pacific salmon in five tributaries to

the Great Lakes in Michigan and Ontario.

Though the variation in nitrification rates was

habitat specific, the researchers did observe

increases in sediment nitrification rates in

these streams. These changes in the form of

dissolved inorganic nitrogen can affect

nutrient dynamics not only where the salmon

are but also in ecosystems located down-

stream. Fisheries managers may need to

monitor the quantity and type of inorganic

nitrogen export to avoid possible unintended

consequences for ecosystems associated with

introduced salmon populations. (Journal of

Geophysical Research- Biogeosciences,

doi:10.1002/ jgrg.20044, 2013) —EB

Plasma injection eventsin Saturn’s magnetosphere

Periodic radio emissions, including the

strong radio emissions known as the Saturn

kilometric radio (SKR) emissions, originate

from the northern and southern polar regions

and are observed throughout Saturn’s

magnetosphere. Scientists have been studying

these phenomena but have not been able to

pin down the physical mechanism linking

periodicities between the inner and outer

magnetospheres.

One proposed mechanism is a process

known as the centrifugal interchange

instability, which occurs in rapidly rotating

magnetospheres such as those of Saturn. In

this process, the cold, dense plasma moves

radially outward from the planet and hotter,

less dense plasma is injected inward.

Small-scale and large-scale inward flowing

injections of hot plasma have been observed

in Saturn’s magnetosphere. Adding to the

understanding of these processes, Kennelly

et al. focused on young small-scale injection

events observed using the Radio and Plasma

Wave Science instrument on the Cassini

spacecraft between July 2004 and December

2011. They analyzed the regions where these

injection events are most likely to occur and

concluded that these inward flowing injection

events are strongly ordered and that this

ordering depends on the Saturnian season.

Their ordering is well correlated with the SKR

originating from the hemisphere experiencing

winter (pointing away from the Sun). The study

suggests that injection events could play a role

in communicating the periodicities observed

in the inner and outer magnetosphere and

provides insight into the interchange

instability that could be of interest to others

studying the magnetospheres of outer planets.

(Journal of Geophysical Research- Space

Physics, doi:10.1002/ jgra.50152, 2013) —EB

Uplift of Zagros Mountains slowsplate convergence

Research has indicated that mountain

ranges can slow down the convergence

between two tectonic plates on timescales as

short as a few million years, as the growing

mountains provide enough tectonic force to

impact plate motions. Focusing on the

convergence of the Arabian and Eurasian

plates at the Zagros mountain range, which

runs across Iran and Iraq, Austermann and

Iaffaldano reconstructed the relative motion

of the plates using published paleomagnetic

data covering the past 13 million years, as

well as current geodetic measurements. They

show that the convergence of the two plates

has decreased by about 30% over the past

5 million years. Looking at the geological

record to infer past topography and using a

computer model of the mantle- lithosphere

system, the authors examined whether the

recent uplift across the Zagros Mountains

could have caused the observed slowdown.

They also considered several other geological

events that might have influenced the

convergence rate, but the authors were able

to rule those out as dominant controls. The

authors conclude that the uplift across the

Zagros Mountains in the past 5 million years

did indeed play a key role in slowing down

the convergence between the Eurasian and

Arabian plates. (Tectonics, doi:10.1002/

tect.20027, 2013) —EB

Computer-modeled global censusfi nds thousands of eddies each year

Eddies, the curls and swirls that spin off

swift- moving ocean currents, help disperse

energy and nutrients throughout the world’s

oceans. Floats and other engineered

submersibles can help scientists understand

the evolution and path of specific eddies. But

knowledge of how many eddies exist, as well

as how pervasive, how strong, and how large

these eddies are at any given moment, would

require continuous and detailed monitoring

of the entire ocean at all depths—an

impossible feat.

Instead, Petersen et al. developed a

computer model of the Earth’s oceans that

simulates ocean dynamics on very fine

scales, down to a tenth of a degree. Through

the model’s outputs, they were able to count

all the eddies that lasted for more than

28 days over the course of 7 modeled years.

The authors found that on any given day,

around 2600 such eddies could be found in

the ocean. Over the 7-year period, this

amounted to 152,000 eddies tracked over

their lifetimes, or about 22,000 per year.

Although many of these eddies were small,

thin, short-lived, and slow, a few thousand

each year were enormous—more than

200 kilometers in diameter (about the size of

Massachusetts). In addition, hundreds of

eddies were able to linger for more than

200 days. A full third of all eddies found were

at least a kilometer tall, and many stretched

the full depth of the ocean. The strong

Antarctic Circumpolar Current generated the

most eddies.

If the results of this census of eddies in the

model are representative of what happens on

Earth, the authors note that their techniques

can help scientists uncover any nonlocal and

far- afield effects of eddies. (Journal of

Geophysical Research- Oceans, doi:10.1002/

jgrc.20155, 2013) —MK

Scientists observe near-inertial wavetrapped in Gulf Stream eddy

During a research cruise in September

2009, Joyce et al. detected for the first time the

existence of a near- inertial wave trapped

within most of a 90  kilometer- diameter

warm-core eddy in the Gulf Stream. Just as

oceanic gravitational waves are driven by the

counterbalancing forces of gravity and

buoyancy, inertial waves are driven by the

wind and the inertial Coriolis force. The

downward propagating wave that the authors

detected was bounded by the clockwise-

rotating eddy.

The authors found that within the eddy, the

downward- propagating wave rotated

clockwise with depth. Regions of constant

velocity generally followed lines of constant

density, with these lines being vertically

depressed toward the center of the eddy.

They found that the presence of the near-

inertial wave caused an increase in energy at

depths of 300–700 meters that aligned with

the pycnocline, a region where the density

changes rapidly with depth.

The authors used a shipboard acoustic

Doppler current profiler to observe the wave.

The eddy passed by a moored sensor array,

affording the authors an opportunity to

confirm their observations. After comparing

their data with a computer simulation of

warm-core eddies, the authors suggest that

the trapped near- inertial wave was likely

caused by passing atmospheric storms.

(Journal of Geophysical Research- Oceans,

doi:10.1002/jgrc.20141, 2013) —CS

—ERNIE BALCERAK and MOHI KUMAR, Staff

Writers, and COLIN SCHULTZ, Writer

Eddies around Tasmania, colored turquoise by phytoplankton, are just a few of the many thousands that swirl through the ocean each year, according to a recent model.

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