Our view of Earth's history sharpens (posted on Portside)

(1) A New Leap Forward for Radiocarbon Dating
(2) Reports Put New Spin on Story of Moon's Creation

(1)
A New Leap Forward for Radiocarbon Dating
By Joseph Stromberg
Smithsonian Magazine
October 18, 2012
https://blogs.smithsonianmag.com/sci...carbon-dating/

Until 1949, when archaeologists dug up prehistoric
bones, stone points, charcoal remnants or other
artifacts from early human history, they had no way of
knowing exactly how old these objects were. Chemist
Willard Libby changed that, devising an ingenious method
for dating ancient objects based on the types of carbon
atoms contained within them.

Libby and his colleagues based their idea on the fact
that living things incorporate tiny amounts of a certain
isotope of carbon (C-14) from the atmosphere into their
structure; when they die, they stop adding new C-14, and
the quantity left inside slowly degrades into a
different element, nitrogen-14. By figuring out that the
half-life of C-14 (the amount of time it takes for half
of a given quantity of C-14 to decay into N-14) is 5,730
years, they could chemically analyze the ratio of C-14
to N-14 inside a piece of wood or bone and determine
just how long it had been dead.

This technique has revolutionized archaeology,
anthropology and other fields, allowing us to determine
the absolute age of objects up to around 60,000 years
old. All along, though, the precision of this technique
has been limited by the fact that the amount of C-14 in
the atmosphere has varied over time-and there has never
been a great record of just how much it has fluctuated
over the years.

With this in mind, a team of scientists from the
University of Oxford and elsewhere was particularly
excited when they excavated fossilized leaves and cores
of sediment layers from beneath Japan's Lake Suigetsu.
These samples might not look like much, but because of
the sediment's unique layering and pristine condition,
the find constitutes an unprecedented comprehensive
record of atmospheric C-14 from roughly 11,200 to 52,800
years ago. The samples of sediment from this one
location on the earth's surface, in other words, will
make our ability to date ancient artifacts found
anywhere on the planet significantly more precise.

"The new results offer an important refinement of the
atmospheric radiocarbon record and place the radiocarbon
timescale on a firmer foundation," said Jesse Smith, an
editor at Science, where the findings were published in
a paper today.

A specific set of processes and conditions that occur in
the lake help to explain why the sediment cores and leaf
samples are so valuable. Each winter, small light-
colored algae called diatoms die and cover the lake
floor; each summer, they are in turn covered by a darker
layer of sediment. Because the lake is extremely still,
is low in oxygen and has not been disturbed by glaciers
or geologic activity anytime in the last 52,800 years,
these microscopic layers comprise a complete, annual
record preserved in sediment cores.

Moreover, because leaves and other organic materials
have been trapped between the layers, the scientists
were able to use the amount of C-14 in each leaf to
construct a complete picture of atmospheric C-14 over
time. Previously atmospheric C-14 records came from
marine samples (which differ from those on land) or tree
rings (which only dated to a little more than 12,000
years ago), so these cores will greatly improve the
precision of radiocarbon dating for older objects. The
researchers "anchored" the new C-14 record to previous
data by matching up the levels found in the more recent
layers of the cores to those already known from the tree
rings.

"Although this record will not result in major revisions
of dates, for example in archaeology, there will be
changes in detail that are of the order of hundreds of
years," said University of Oxford archaeologist Bronk
Ramsey, the lead author of the paper. "Such changes can
be very significant when you are trying to look at human
responses to climate, [which are] often dated by other
methods, for example through the Greenland ice cores. A
more accurate calibrated time-scale will allow us to
answer questions in archaeology, which previously we
have not had the resolution to address."

Researchers suspected that the conditions in Lake
Suigetsu could yield such a crucial C-14 record as early
as 1993, but they had encountered technical difficulties
in extracting and analyzing intact cores until now.
"This is a realization of a 20-year-long Japanese
dream," said co-author Takeshi Nakagawa of the
University of Newcastle upon Tyne in England. Although
it's taken some time to successfully recover the
samples, they will now help researchers to figure out
the ages of much older specimens and artifacts.

(2)
Reports Put New Spin on Story of Moon's Creation
Studies shed new light on the massive collision
that planetary scientists believe led to the moon's
birth. But will we ever know which theory is correct?
By Eryn Brown
Los Angeles Times
October 17, 2012
https://www.latimes.com/news/science...0,316262.story

Scientists may never know exactly how the moon and Earth
were formed some 4.5 billion years ago, but this week
their understanding of the cataclysmic event made a
significant leap forward.

In a slew of studies published Wednesday, planetary
scientists provided new evidence supporting the long-
standing - but imperfect - theory that the Earth and
moon formed after the proto-Earth collided with another
huge planetary body, sometimes referred to as Theia.

Some of that evidence comes from super-precise
measurements of the zinc in lunar rock samples collected
by Apollo astronauts. These findings, reported in the
journal Nature, support the idea that the moon's birth
had to have resulted from "a big event with lots of
energy," strong enough to vaporize rock, said study
leader Frederic Moynier, a geochemist at Washington
University.

Separately, two studies published in the journal Science
detailed two scenarios of what such a powerful crash
might plausibly have looked like.

Both collision-simulation papers may solve an
intractable problem with the classic story scientists
told about the moon's birth. That story goes something
like this: Two planets, one Earth-sized and one Mars-
sized, slammed together. The smaller body, Theia, was
obliterated completely, its materials flung asunder to
form a disk around the Earth that before long coalesced
to form the moon.

The theory explains the distance between the two bodies,
their relative sizes and other physical properties. But
in the last decade or so, a problem arose: The chemistry
didn't match up with the physics.

"What's happening now is an attempt to salvage the
theory," said Erik Asphaug, a planetary scientist at UC
Santa Cruz who was not involved in the new research.

According to computer simulations of the theorized
collision, the moon should have been composed mainly of
materials from Theia. Instead, analysis showed that rock
samples from the moon and Earth appeared to contain the
same amounts of the same types of oxygen, titanium,
silicon and other elements.

The similarity of these distinct chemical isotopes was
taken as a sign that the Earth and moon were actually
made of the same stuff - and meant that planetary
scientists would need to rethink the details of how the
giant impact happened, said Harvard University
researcher Matija Cuk, a coauthor of one of the new
simulations.

The main problem the computer modelers faced was that
any collisions resulting in an Earth and a moon with
shared geochemistry required the ancient Earth to be
spinning too fast to allow for the 24-hour rotation that
exists today.

Cuk and his Harvard colleague Sarah Stewart solved the
conundrum by suggesting that a fast-spinning proto-Earth
could have slowed during a period when the moon and the
sun aligned in such a way that gravity warped Earth's
orbit, putting the brakes on its rotation.

Plugging the appropriate conditions into their computer
simulation, they found that a small body about half the
size of Mars striking the early Earth nearly head-on
would completely obliterate both bodies, with all the
material mixing together.

"Everything is molten," Cuk said.

Most of the heavy iron from both planetary cores would
combine and coalesce to form Earth's core. The blended
lighter rock from both bodies would form the outer
layers of the Earth as well as the moon.

Robin Canup, a planetary scientist at the Southwest
Research Institute in Boulder, Colo., used Cuk's and
Stewart's idea about how the Earth's rotation might have
slowed and developed another scenario for the moon's
creation. Also writing in Science, she showed that two
similarly sized bodies, each about half the mass of the
modern Earth, could have collided at a relatively slow
speed and merged, their contents creating a pool of
material that later split apart into Earth and moon.

By figuring out how Earth's spin might have slowed,
Canup said, scientists have "greatly broadened the class
of impacts that might be viable."

Caltech planetary scientist David Stevenson, who was not
involved with the research, said that the new models
"are a stepping stone toward a more satisfying story"
but that "we're only part of the way."

David Paige, a moon expert at UCLA who was also not part
of either modeling study, said it might not be possible
to know exactly what happened.

"So much of what existed prior to the impact has been
obliterated," he said. "It's a whodunit mystery with
very few clues lying around."

He said, however, that isotopic research might offer
part of the solution.

In the report published in Nature, Moynier and his
colleagues used sophisticated mass spectrometry to show
that the blend of different zinc isotopes on the moon is
not the same as the blend on Earth. Lighter versions of
the metal were slightly depleted on the moon, suggesting
that the lighter zinc must have evaporated during some
kind of impact, the team reported.

That doesn't do much to determine whether either
collision scenario is correct. It may point a way
forward for the planetary scientists who'll try to
figure it out, however, Paige noted.

"It's through more measurements like this zinc one that
we're able to better sort it out," he said.

For his part, Moynier said he planned to examine
rubidium isotopes in lunar rocks next.