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36
SCIENTIFIC A MERIC A N
CREDIT
RING OF FIRE: Mountain-size vents
exploding around the outer edge of
an active supervolcano smother the
landscape in clouds of hot gas and ash.
COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC.
JUNE 2006
The Secrets of
upervolcanoes
By Ilya N. Bindeman
CREDIT
S
Microscopic crystals of volcanic ash are revealing
surprising clues about the world’s most devastating eruptions
w w w. s c ia m . c o m
COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC.
SCIENTIFIC A MERIC A N
37
L
A supervolcano eruption packs the
devastating force of a small asteroid colliding with the earth and occurs 10
times more often — making such an explosion one of the most dramatic natural catastrophes humanity should expect to undergo. Beyond causing immediate destruction from scalding ash
flows, active supervolcanoes spew gases
that severely disrupt global climate for
years afterward.
Needless to say, researchers are eager
to understand what causes these giants
to erupt, how to predict when they might
wreak havoc again, and exactly what
challenges their aftermath might entail.
Recent analysis of the microscopic crystals in ash deposits from old eruptions
has pointed to some answers. These insights, along with improved technologies for monitoring potential disaster
sites, are making scientists more confident that it will be possible to spot warning signs well before the next big one
blows. Ongoing work hints, however,
that supervolcano emissions could trigger alarming chemical reactions in the
atmosphere, making the months following such an event more hazardous than
previously suspected.
Almost all volcano experts agree
that those of us living on the earth today
are exceedingly unlikely to experience
an active supervolcano. Catastrophic
eruptions tend to occur only once every
few hundred thousand years. Yet the
sheer size and global effects of such episodes have commanded scientific attention since the 1950s.
Early Awe
on e of g e ol o g i s t s’ first discoveries was the existence of enormous circular valleys — some 30 to 60 kilometers
across and several kilometers deep — that
looked remarkably similar to the bowlshaped calderas located atop many of
the planet’s most well-known volcanoes.
Calderas typically form when the chamber of molten rock, or magma, lying under a volcanic vent empties out, causing
the ground above it to collapse. Noting
that these calderalike valleys sit close to
some of the earth’s largest deposits of
volcanic rocks laid down during a single
event, those early investigators realized
they were seeing the remnants of volcanoes hundreds or even thousands of
times larger than the familiar Mount St.
Helens in Washington State. From the
Overview/Mighty Eruptions
■
■
■
38
Recent analysis of the composition of tiny crystals inside ash deposits
from prehistoric eruptions is overturning old beliefs about supervolcano
behavior— and revealing new surprises about the aftermath.
The inner workings of the magma chambers that fuel supervolcanoes can
evolve in ways that strongly influence the style of future eruptions.
The volcanic winter that grips the planet in the wake of a supereruption is
probably shorter than once suspected, although chemical reactions in the
atmosphere may be much more dangerous.
SCIENTIFIC A MERIC A N
extreme scale of the calderas and the estimated volume of erupted material, researchers knew that the magma chambers below them had to be similarly
monstrous.
Because the thick continental crust
and heat sources needed to create such
massive magma chambers are rare, supervolcanoes themselves are also uncommon. In the past two million years, a
minimum of 750 cubic kilometers of
magma has exploded all at once in only
four regions: Yellowstone National Park
in Wyoming, Long Valley in California,
Toba in Sumatra and Taupo in New Zealand. The search for similarly large eruptions continues in other areas of thick
continental crust, including in western
South America and far eastern Russia.
By the mid-1970s, investigations of
past events revealed some ways that the
chambers can form and become dangerous. Under the surface of Yellowstone,
the North American tectonic plate is
moving over a buoyant plume of warm,
viscous rock rising through the mantle,
the 2,900-kilometer-thick layer of the
earth’s interior that is sandwiched between the molten core and the relatively
thin veneer of outer crust. Functioning
like a colossal Bunsen burner, this socalled hot spot has melted enough overlying crust to fuel catastrophic eruptions
for the past 16 million years. In Toba,
the source of the chamber is different.
That region lies above a subduction zone,
an area where one tectonic plate is slipping under another; the convergence
produces widespread heating, mainly
through partial melting of the mantle
above the sinking plate.
No matter the heat source, pressure
COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC.
JUNE 2006
J U L I A G R E E N ( p r e c e d i n g p a g e s)
urking deep below the surface in California and Wyoming are two
hibernating volcanoes of almost unimaginable fury. Were they to
go critical, they would blanket the western U.S. with many centimeters
of ash in a matter of hours. Between them, they have done so at least four
times in the past two million years. Similar supervolcanoes smolder
underneath Indonesia and New Zealand.
BIG, BIGGER, BIGGEST
Supervolcanoes (orange and blue) spread ash much farther
than even large versions of what most people think of as “normal”
volcanoes (yellow and purple), because the behemoths, with
their massive magma chambers, eject so much more material.
Yellowstone National Park:
Lava Creek tuff eruption
1,000 cubic kilometers of debris
640,000 years ago
Mount St. Helens:
1980 eruption
< 0.5 cubic kilometer of debris
Crater Lake National Park:
Mount Mazama eruption
50 cubic kilometers of debris
7,600 years ago
in the magma chambers builds over time Why magma sometimes oozes slowly patterns of magma evolution, but they
were insufficient for determining the age
as more magma collects under the enor- to the surface is still uncertain.
A look at the composition of tiny of the ejected magma and the depth at
mous weight of overlying rock. A supereruption occurs after the pressurized crystals trapped inside erupted lava and which it formed.
Every chunk of rock is actually made
magma raises overlying crust enough to ash at Yellowstone has suggested a parcreate vertical fractures that extend to tial answer, by providing new insight up of thousands of tiny crystals, each
the planet’s surface. Magma surges up- into how magma forms. For decades, ge- with its own unique age, composition
ward along these new cracks one by one, ologists assumed that magma sits as a and history. So when technological adeventually forming a ring of erupting pool of liquefied rock for millions of vances made it possible in the late 1980s
vents. When the vents merge with one years at a time and that each time some to analyze individual crystals with good
another, the massive cylinder of land in- of it pours out onto the earth’s surface, precision, it was like being able to read
side the ring has nothing to support it. new liquid rises up from below to refill individual chapters in a book rather than
This “roof” of solid rock plunges down— the chamber immediately. If that con- relying on the jacket blurb to explain the
either as a single piston or as piecemeal ception were correct, one would expect story. Investigators began to see that
blocks — into the remaining magma be- many more catastrophic, voluminous some crystals— and thus the magmas in
low, like the roof of a house falling down eruptions, because it is mechanically which they originally formed — arose
when the walls give way. This collapse and thermally infeasible to keep monster much earlier than others, for instance,
forces additional lava and gas out vio- magma bodies in the crust without emp- and that some formed deep underlently around the edges of the ring [see tying them frequently.
ground, whereas others formed near the
The old idea was based largely on so- earth’s surface.
box on next two pages].
called whole-rock analyses in which reDuring the past 10 years, geochemsearchers would obtain a single set of ists have been paying particular attenFingerprinting Eruptions
y e t m y s t e r i e s r e m a i n e d. Nota- chemical measurements for each fist-size tion to an especially durable type of volbly, as researchers soon realized, not piece of volcanic rock they collected. canic crystal called zircon. Knowing that
every large magma chamber will neces- Those data provided important general zircons can withstand extreme changes
sarily erupt catastrophically. Yellowstone, for example, is home to three of
ILYA N. BINDEMAN is a geochemist and assistant professor in the department of geothe world’s youngest supervolcano callogical sciences at the University of Oregon. Born in Moscow, Bindeman first became
deras — they formed 2.1 million, 1.3
interested in volcanology while studying the remote volcanoes of Kamchatka in far eastmillion and 640,000 years ago, one
ern Russia. After completing his Ph.D. at the University of Chicago in 1998, he began
nearly on top of the other— but in the
investigating nearly microscopic crystals of ash for clues about the origin and effects
gaps between these explosive events,
of the world’s largest eruptions. He worked at the University of Wisconsin–Madison and
the underlying chamber released similar
at the California Institute of Technology before joining the Oregon faculty in December
volumes of magma slowly and quietly.
2004 and setting up his own geochemistry laboratory.
THE AUTHOR
JEN CHRIS TIANSEN; SOURCE: U.S. GEOLOGIC AL SURVE Y
Long Valley:
Bishop tuff eruption
750 cubic kilometers of debris
760,000 years ago
w w w. s c ia m . c o m
COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC.
SCIENTIFIC A MERIC A N
39
(left), are not obvious cone-shaped peaks like Washington State’s Mount
St. Helens (above). Instead they are marked by enormous calderas,
depressions in the earth’s surface that formed when the land collapsed
into the magma chambers that fed the most recent supereruptions.
in heat and pressure without compro- once near the earth’s surface. The zirmising their original composition, a few cons we studied were depleted in oxygen
researchers — among them John W. Val- 18 relative to the mantle, and such deley of the University of Wisconsin–Mad- pletion occurs only if the crystals formed
ison— have been using them to study the from rocks that interacted with rain or
early evolution of the earth’s crust [see snow. We thus suspected that the col“A Cool Early Earth?” by John W. Val- lapsed roof rock from one of the two
ley; Scientific American, October oldest Yellowstone supereruptions must
2005]. When I joined Valley’s team as a have melted to form the bulk of the
postdoctoral fellow in 1998, we used magma that was ejected during the
Yellowstone zircons to trace the history younger Lava Creek catastrophe and
of their parent magma— which in turn smaller eruptions since. This hypothesis
revealed important clues about how the gained strength when we learned that
the ages of the zircons from post–Lava
volcano may behave in the future.
The first step was to measure the ra- Creek eruptions span the entire twotios of different forms of oxygen in zir- million-year duration of Yellowstone
cons from the youngest Yellowstone su- volcanism. Such old zircons could exist
pereruption — which after exploding in the youngest ash only if they origi640,000 years ago gave rise to the Lava nated in material that was ejected durCreek tuff, a fossilized ash deposit 400 ing the oldest eruptions and if that mameters thick in some places — as well as terial later collapsed back into the magyounger deposits that were expelled ma chamber and remelted to help fuel
during milder eruptions since then. the youngest eruptions.
Our fi ndings mean that scientists
When I fi nished my initial analyses, Valley and I were both surprised to see that can now expect to make certain predicoxygen composition of those zircons tions about how the Yellowstone superdid not match that of deep, hot mantle, volcano, and possibly those elsewhere,
as would be expected if drained cham- will behave in the future. If a new round
bers always fi lled from below. Zircons of small, precursor eruptions begins in
born of mantle-derived magmas would Yellowstone — and they usually do so
have had a distinctive signature: as ele- weeks to hundreds of years before a catments that are dissolved in magmas astrophic explosion— testing the oxygen
come together to form a zircon, that fingerprint of those lavas and the ages of
crystal takes on a notably high propor- their zircons should reveal what type of
tion of oxygen 18, a heavy isotope of magma is abundant in the chamber beoxygen that has 10 neutrons in its nucle- low. If the next eruption is depleted in
us instead of the usual eight.
oxygen 18, then it is most likely still beValley and I saw immediately that ing fed by stagnant remnants of the
the magma must have originated in rock original magma, which by now is prob-
40
SCIENTIFIC A MERIC A N
ably more of a thick crystal mush than
an explosive liquid. On the other hand,
if the new lava carries the fi ngerprint of
fresh magma from the mantle and does
not contain old zircons, then it very likely came from a large volume of new
magma that has filled the chamber from
below. Such findings would imply that a
new cycle of volcanism had commenced — and that the newly engorged
COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC.
C OUR T E S Y OF J. S . L A CK E Y College of Wooster;
D A T A F R O M U . S . G E O L O G I C A L S U R V E Y ( l e f t) ;
T O D D C U L L I N G S N a t i o n a l P a r k S e r v i c e ( r i g h t)
SLEEPING SUPERVOLCANOES, such as that in Long Valley in California
SUPERCYCLES
The vast chambers of molten magma that feed
supervolcanoes form above hot spots
(buoyant plumes of rock rising from deep
Upper magma
chamber
Continental
crust
Rising
magma
Sinking plate
Lower magma
chamber
1
Partial melting of the mantle rock above the
sinking plate of oceanic crust produces
magma that works its way up toward the base
of the continental crust and pools there. This
lower magma chamber acts as a colossal
Bunsen burner that eventually melts parts of
the continental crust, which has a lower melting
point than the rock below. Some magma also
rises via small vertical conduits between
the two chambers.
JUNE 2006
magma chamber had more potential to
explode catastrophically.
JEN CHRIS TIANSEN
Immediate Aftermath
t i n y c ry s ta l s and their isotopic signatures have also revealed surprises —
good and bad— about the aftermath of
supereruptions. One of the best-studied
examples of supervolcano aftermath is
the Bishop tuff, a volcanic layer tens to
hundreds of meters thick that is exposed
at the earth’s surface as the Volcanic Tablelands in eastern California. This massive deposit represents what is left of the
estimated 750 cubic kilometers of magma ejected during the formation of the
Long Valley supervolcano caldera some
760,000 years ago.
For decades, many geologists assumed that a series of distinct eruptions
over millions of years must have occurred to produce the extensive Bishop
tuff. But careful studies of microscopic,
magma-filled bubbles trapped inside
tiny crystals of quartz tell a different
story. The rate at which magma leaves a
chamber depends primarily on two fac- the sides of Kilauea Volcano in Hawaii,
tors: the magma’s viscosity, or ability to these eruptions feature supersonic blasts
flow, and the pressure difference be- of superheated, foamlike gas and ash
tween the chamber and the earth’s sur- that rise buoyantly all the way into the
face. Because the pressure inside a bub- earth’s stratosphere, 50 kilometers high.
ble matches that of the chamber where As the land above the magma chamber
the magma formed, the bubble acts like collapses, immense gray clouds called
a mini version of the chamber itself.
pyroclastic flows burst out horizontally
Aware of this correspondence, Alfred all around the caldera. These flows are
Anderson of the University of Chicago an intermediate stage between lava and
and his colleagues studied the size of the ash, so they move extremely rapidly— up
bubbles under a microscope to estimate to 400 kilometers an hour, some sources
how long it took the magma to leak out. say; cars and even small airplanes would
Based on these and other experiments have no chance of outrunning them.
and field observations from the 1990s, These flows are also intensely hot— 600
geologists now think that the Bishop to 700 degrees Celsius — so they burn
tuff— and probably most other super- and bury everything for tens of kilomeerupted debris — was expelled in a single ters in every direction.
As bad as the pyroclastic flows are,
event lasting a mere 10 to 100 hours.
Since that discovery, investigators the ash injected into the atmosphere can
have had to modify their reconstructions have even more far-reaching conseof supervolcano eruptions. Here is what quences. For hundreds of kilometers
they now generally expect from an event around the eruption and for perhaps
the scale of those that struck Long Valley days or weeks, pale-gray ash would fall
and Yellowstone: Instead of a slow leak like clumps of snow. Within 200 kilomeof red-hot lava as is seen creeping down ters of the caldera, most sunlight would
within the earth) and subduction zones (regions where one
tectonic plate is slipping underneath another). In both cases, the
giant volcanoes tend to follow an eruption cycle that is now far
Explosive vent
New fracture
As the upper magma chamber grows, the
land above bulges and cracks. The silicarich composition and low temperature of this
magma relative to those that form in the
mantle make it particularly flow-resistant, so
water and gas have trouble rising through it.
Thus, when a plug of the sticky magma
suddenly works its way to the surface along a
vertical crack, the high-pressure material
underneath tends to explode violently rather
than oozing slowly.
w w w. s c ia m . c o m
Dome
Lava flow
New ash layer
Pyroclastic flows
High-pressure
magma
2
better understood than it once was. Here are the four basic steps,
beginning with initial formation of the magma chamber, depicted
for a subduction zone:
Caldera rim
Collapsing rock
3
The earth’s strained surface eventually
shatters as new explosive vents form a
ring as wide across as the magma chamber.
The fractured pieces of rock plunge down into
the chamber, forcing additional magma up the
outside edges of the ring. The sudden release
of this magma transforms it into vast,
scalding clouds of ash, gas and rock known
as pyroclastic flows, which destroy
the landscape for tens of kilometers
in all directions.
4
After the volcano's eruption, a craterlike
depression known as a caldera sits above
the partially drained magma chamber. Over
time, the collapsed land that is within the
chamber begins to melt from the inside,
thereby creating a smaller batch of magma that,
along with other forces, forms a dome in the
center of the caldera. Slow-moving lava may
leak from this region many times before
enough magma accumulates to fuel a
new supereruption.
COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC.
SCIENTIFIC A MERIC A N
41
Nevada. They are the remains of scalding ash flows resulting from
supereruptions that struck nearby approximately 12.8 million (lower layer)
and 12.7 (upper layer) million years ago.
MASSIVE WALL of gray rock in western Nebraska originated as
a suffocating pile of ash left by a supereruption at an unknown site
28 million years ago. Elements in the ash imply that such explosions
can alter the chemistry of the stratosphere.
from the great volumes of problematic
gas expelled into the upper atmosphere,
would also transpire and could persist
for many years. New work suggests that
some of these outcomes may not be as
bad as once feared but that others may
be worse. Once again, looking at the
composition of small by-products from
past eruptions has been illuminating.
Of the varied gases that make up any
volcanic eruption, sulfur dioxide (SO2)
causes the strongest effect on the environment; it reacts with oxygen and water
to produce tiny droplets of sulfuric acid
The Long Haul
i n v e s t ig at or s h av e reason to be- (H 2SO4). These droplets are the main
lieve that other consequences, arising sun-blocking source of the dramatic climatic cooling that would grip the planet
in the wake of a supereruption. Knowing
that the planet’s hydrological cycle takes
months or years to fully wash away the
acid droplets, many researchers made
apocalyptic estimates of “volcanic winters” lasting decades, if not centuries.
But in recent years other investigators
have uncovered evidence that drastically
reduces that calculation.
Almost always, traces of the sulfuric
acid produced after large volcanic eruptions are trapped in snow and ice as the
acid precipitates out of the contaminated atmosphere. In 1996 investigators
studying ice cores from Greenland and
Antarctica found the sulfuric acid peak
that followed the supereruption of Toba
74,000 years ago. That eruption ejected
SKELETONS OF ANIMALS buried in ash from a catastrophic eruption in Idaho 12 million years ago are
2,800 cubic kilometers of lava and ash
now exposed in northeastern Nebraska’s Ashfall Fossil Beds State Historical Park. Most of the aniand reduced average global temperamals probably died slowly as the falling ash — which is essentially powdered glass — filled their lungs
tures by five to 15 degrees C. The conseand abraded their teeth. Toxic chemicals in the ash may have poisoned their drinking water as well.
be blocked out, so the sky at noon would
look like that at dusk. Homes, people
and animals would be buried, sometimes crushed. Even 300 kilometers
away, the ash could be half a meter thick;
mixed with rain, the weight would be
plenty sufficient to collapse roofs. Less
ash than that would knock out electrical
power and relay stations. As little as a
millimeter, which could well dust the
ground halfway around the globe, would
shut down airports and dramatically reduce agricultural production.
Only gradually would rain (made
acidic by volcanic gases) wash away the
thick blanket of ash. And because volca-
42
SCIENTIFIC A MERIC A N
nic rock and ash float, it would clog major waterways. Transportation along big
rivers could grind to a halt. Indeed, recent oil drilling in the Gulf of Mexico
struck a surprisingly thick layer of supervolcanic debris near the Mississippi
Delta— more than 1,000 miles from its
source in Yellowstone. Only by floating
downriver and then sticking to sediment
that sank to the ocean bottom could that
amount of debris have accumulated
from a volcano so far away.
COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC.
JUNE 2006
U . S . D E P A R T M E N T O F E N E R G Y ( t o p l e f t) ; I LYA N . B I N D E M A N ( t o p r i g h t) ; R I C K O T T O U n i v e r s i t y o f N e b r a s k a S t a t e M u s e u m ( b o t t o m)
EX TENSIVE VOLCANIC DEPOSITS make up a steep slope of Yucca Mountain in
N A S A J O H N S O N S P A C E C E N T E R ( a t m o s p h e r e s p e c t r u m) ; G R A P H S O U R C E : I LYA N . B I N D E M A N
w w w. s c ia m . c o m
OZONE DESTRUCTION
Dangerous gases spewed from Mount Pinatubo in the Philippines in 1991 appeared
as colors in satellite images of the earth’s upper atmosphere (background). New
evidence suggests that such gases emitted from
ANALYSES OF SULFATE SAMPLES
future supervolcanoes may significantly deplete
the planet’s protective ozone layer before falling
Unidentified
supereruption
as acid rain and mixing with ash to form sulfate.
in western U.S.
Sulfate samples from four supervolcano deposits
carry an unusual excess of oxygen 17 (irregular
colored areas in graph represent collections of
measurements); such abundance occurs only in
compounds that have acquired the rare atoms
during reactions with special gases, very likely
Long Valley Yellowstone
ozone, in the earth’s upper atmosphere.
Most
Materials that originate on the ground and stay
small eruptions
there, such as the products of most small
eruptions, show no such anomaly (blue line).
Oxygen 18 Abundance
Oxygen 17 Excess
quences of such a chill were undoubtedly severe but did not last as long as
once thought: sulfuric acid in the ice record disappeared after only six years;
some researchers suggest that it vanished
even earlier.
That volcanic winters are probably
shorter than expected is the good news.
But a new method developed in the past
five years for studying the composition of
the oxygen atoms in the volcanic acid
rain is revealing an entirely different,
alarming sign about the long-term effects of sulfur dioxide in the atmosphere.
For SO2 to become H 2SO4, it must be
oxidized — in other words, it must acquire two oxygen atoms from other compounds already existing in the atmosphere. Exactly which compounds play
the key role is a hotly debated topic of
current research, so when I started working with John M. Eiler as a staff scientist
at the California Institute of Technology
in 2003, he and I looked for evidence in
my samples of ashes from the prehistoric
Yellowstone and Long Valley eruptions.
We began our analyses with a focus
on a particularly efficient oxidant, ozone.
Ozone is a gas molecule made up of three
oxygen atoms best known for shielding
the earth from the sun’s dangerous ultraviolet rays. Because of rare chemical
transformations that certain gases undergo in the presence of that intense solar radiation, ozone is characterized by
an anomaly in its so-called mass-independent oxygen isotope signature, which
in simple terms can be thought of as an
excess of oxygen 17.
When ozone or any other oxygenrich molecule in the stratosphere interacts with SO2 , it transfers its oxygen isotope signature to the resulting acid— that
is, the oxygen 17 anomaly persists in the
new acid. In 2003 geochemists at the
University of California, San Diego,
found the first evidence that this signature is also preserved in the oxygen atoms of the acid that later falls as rain and
in the sulfate compounds that form as the
acid rain reacts with ash on the ground.
The oxygen 17 excess and other
chemical patterns that we found in sulfate from the Yellowstone and Long Valley ash samples thus implied that signifi-
cant amounts of stratospheric ozone
were used up in reactions with gas from
the supereruptions in those regions.
Other researchers studying the acid layers in Antarctica have demonstrated that
those events, too, probably eroded
stratospheric ozone. It begins to look as
if supervolcano emissions eat holes in
the ozone layer for an even longer period
than they take to cool the climate.
This loss of protective ozone would be
expected to result in an increased amount
of dangerous ultraviolet radiation reaching the earth’s surface and thus in a rise
in genetic damage caused by rays. The
magnitude and length of the potential
ozone destruction are still being debated.
Space observations have revealed a 3 to
8 percent depletion of the ozone layer
following the 1991 eruption of Mount
Pinatubo in the Philippines. But what
would happen after an event 100 times
larger? Simple arithmetic does not solve
the problem, because the details of atmospheric oxidation reactions are extremely complex and not fully understood.
Scientific techniques for studying and
monitoring volcanoes of all sizes are developing with all deliberate speed. But no
matter how much we learn, we cannot
prevent an eruption. And what can be
said about the aftermath of the most catastrophic occurrences is still speculative
at best. The good news, though, is that
researchers now know enough about the
sites of possible eruptions to predict with
reasonable assurance that no such catastrophe will happen anytime soon.
MORE TO EXPLORE
Low-180 Rhyolites from Yellowstone: Magmatic Evolution Based on Analyses of Zircons and
Individual Phenocrysts. Ilya N. Bindeman and John W. Valley in Journal of Petrology, Vol. 42,
pages 1491–1517; 2001.
Sulfate Oxygen-17 Anomaly in an Oligocene Ash Bed in Mid-North America: Was It the Dry
Fogs? Bao Huiming, Mark H. Thiemens, David B. Loope and Xun-Lai Yuan in Geophysical Research
Letters, Vol. 30, pages 1843–1848; 2003.
Rare Sulfur and Triple-Oxygen Isotope Geochemistry of Volcanogenic Sulfate Aerosols.
Ilya N. Bindeman, John M. Eiler, Boswell Wing and James Farquhar in Earth and Planetary Science
Letters (in preparation, 2006).
COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC.
SCIENTIFIC A MERIC A N
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