[meteorite-list] Meteorite Minerals Hint at Earth Extinctions, Climate Change

[Ron Baalke]

http://www.astrobio.net/exclusive/5678/meteorite-minerals-hint-at-earth-extinctions-climate-change

Meteorite Minerals Hint at Earth Extinctions, Climate Change
Jeremy Hsu
Astrobiology Magazine
September 12, 2013

Summary: Tiny minerals leftover from many small meteorites could provide 
the geological evidence needed to show how rocks falling from the sky 
often changed the course of life's evolution on Earth.

A huge asteroid that wiped out the dinosaurs may not have been the only 
cosmic event to cause mass extinctions or change Earth's climate. Tiny 
minerals leftover from many smaller meteorites could provide the geological 
evidence needed to show how rocks falling from the sky changed the course 
of life's evolution on our planet more than just once.

The tiny minerals called spinels - about the size of a sand grain - can 
survive the harshest weather and chemical changes on Earth's surface. 
Swedish researchers hope to collect enough of the spinels in different 
parts of the world to connect the dots between the breakup of huge asteroids 
in space and certain extinction or climate events during Earth's history.

"I think it would be very interesting if our spinel approach in the long 
run could provide empirical evidence for how asteroid showers in the inner 
solar system correlate with the onset of ice ages," said Birger Schmitz, 
a marine geologist at Lund University in Sweden.

The hunt for spinels has led Schmitz and his colleagues to dissolve tons 
of rocks in acid over the past decade or so - many of the rocks originating 
from a commercial limestone quarry in Thorsberg, Sweden. Such a tactic 
could reveal convincing patterns in the geological records that scientists 
cannot piece together from the occasional fossilized meteorite or rare 
impact crater.

Without a trace

Most meteorites that have fallen to Earth end up disappearing without 
becoming fossilized or leaving an impact crater for scientists to find. 
That has made it virtually impossible for scientists to recover enough 
evidence to back theories about how astronomical events have tied into 
Earth's history.

"We know that in desert areas on Earth, meteorites typically decompose 
within 20 to 30 thousand years," Schmitz explained. "In wetter areas, 
decomposition goes faster."

But the vanishing meteorites do leave behind different types of spinels, 
including extremely tough chromium or alumina oxides. The chemical and 
isotopic fingerprints of such spinels reveal what type of meteorites they 
originate from. Scientists now know that extraterrestrial spinel minerals 
can be found in the layers of built-up sediment of almost any age over 
the past 3.5 billion years.

Such spinels can reveal differences in the rate and types of meteorites 
falling to Earth at different times in the planet's history, Schmitz said 
in the June issue of the journal Chemie der Erde. They could also reveal 
more about the chemical makeup of asteroids, or help scientists understand 
if any asteroid breakups affected life on Earth in the past.

Beyond dinosaur extinction

The spinel record could reveal more extinction events with extraterrestrial 
links than just the mass dinosaur extinction. An earlier mass extinction 
during the Frasnian-Fammenian period about 372 million years ago coincides 
with at least three large impact craters.

"There are many large craters on Earth associated with this event, but 
no close connection between one large impact and the extinction event 
has yet been shown," Schmitz said.

But falling space rocks may bring more than just destruction to Earth. 
Perhaps the greatest explosion of new ocean life in Earth's history took 
place during the Great Ordovician Biodiversification Event about 470 million 
years ago - a period coinciding with the largest known asteroid breakup 
in the solar system's asteroid belt over the last 3 billion years. Schmitz 
and colleagues discovered a rapid increase in the number of spinels found 
in the limestone rock layers marking the start of that diversification 
period at sites in Sweden, western Russia and central China. They speculate 
that the asteroid breakup led to frequent impacts on Earth by kilometer-sized 
asteroids that could have spawned the resulting changes in the diversity 
of life.

Another theory links asteroid showers to Earth's three most recent major 
ice ages that occurred about every 250 to 300 million years. No definitive 
proof exists yet, but the ice age periods roughly coincide with the sun's 
orbit around the galaxy every 225 to 250 million years - an event that 
could expose Earth to more frequent meteorite falls at certain periods. 
The study of spinels could help prove such theories right or wrong in 
the coming years.

Lab science on acid

Building a record of Earth's history through extraterrestrial spinels 
still represents a fairly new idea, despite scientists having known about 
spinels for several decades. But Schmitz is looking forward to continuing 
the hunt for spinels with a specially designed lab at Lund University. 
The new lab will use acid to dissolve about 5 to 10 tons of sedimentary 
limestone per year in search of spinels - a big step up from how Schmitz's 
group handled just over one tenth of a ton per year about 15 years ago. 
Industrial-grade lab equipment includes acid-resistant pumps for injecting 
acid into large plastic barrels holding different rock samples.

Schmitz's approach has slowly won over skeptics in the meteorite research 
community, said Philipp Heck, associate curator of meteoritics and polar 
studies at the Field Museum of Natural History in Chicago. He added that 
the spinel approach would prove most effective when sediment layers representing 
past ages of the Earth are highly condensed and the rate of meteorites 
hitting the Earth was much higher than it is today.

"This is certainly a very useful approach to study the extraterrestrial 
record of ordinary chondrites in sediments," Heck said. "This approach 
needs now to be applied to different types of meteorites."

But geologists won't need an entirely new lab to start getting in on the 
action by tackling smaller amounts of rocks that could hold hidden 
extraterrestrial treasure.

"The best thing is that all you need for the identification of the extraterrestrial 
spinels is a regular scanning electron microscope with an attached standard-type 
element identification system (EDS)," Schmitz said. "Most geology departments 
have this equipment."