[meteorite-list] Freshwater Creatures Less Affected by K-Pg

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 23 Jul 2013 10:56:28 -0700 (PDT)
Message-ID: <201307231756.r6NHuSxf016989_at_zagami.jpl.nasa.gov>

>
> Freshwater Creatures Less Affected by Dino-Killing
> Asteroid, National Geographic, July 23, 2013
> http://news.nationalgeographic.com/news/2013/07/130723-chicxulub-asteroid-dinosaurs-freshwater-survival-extinction/

 
Freshwater Creatures Less Affected by Dino-Killing Asteroid

Organisms living in rivers and lakes had certain advantages over
marine counterparts.

Ker Than
National Geographic
July 23, 2013

The large asteroid that crashed into the Earth 65.5 million years ago
is best known for killing off the dinosaurs, but it also triggered a
mass extinction in the oceans. Curiously, though, organisms living in
inland rivers and lakes showed much lower rates of extinction.

Some scientists now think they can begin to explain this discrepancy: A
new study proposes that the biological adaptations some organisms
evolved to deal with the challenges of living in freshwater environments
also helped shield them from the months of frigid darkness that followed
the asteroid impact.

"[Freshwater] organisms are adapted to physical and chemical changes
that go well beyond what marine organisms need to be adapted for," said
study co-author William Lewis, a freshwater scientist at the University
of Colorado.

For example, many freshwater creatures are adapted to annual freeze-thaw
cycles and periods of low oxygen. Therefore, many of them can go dormant
by burying themselves or their eggs in the mud, and thus would have been
able to wait out the worst effects of the impact.

"We do see some dormancy in the marine environment," Lewis explained,
"but it's unusual because it's not necessary for most organisms."

Details, Please

The findings, published online
<http://onlinelibrary.wiley.com/doi/10.1002/jgrg.20086/abstract> in the
Journal of Geophysical Research - Biogeosciences, brings much-needed
specificity to an area of research that has long been plagued by
generalities, experts say.

"I think before that most people had concentrated on the collapse of the
food chain to explain why certain groups went extinct," said Alison
Murray, a paleontologist at the University of Alberta in Canada.

"In this current paper, the authors are developing that food chain
collapse, but in a more detailed manner, examining different groups and
determining which might survive a prolonged period without light and the
corresponding loss of the photosynthetic organisms," Murray, who did not
participate in the research, said in an email.

David Fastovsky, a paleontologist at the University of Rhode Island,
agreed. "I've never seen a coherent statement of how we think [that mass
extinctions] actually happen," Fastovsky said.

"But here it is, in living color on NBC, a model of how it actually
might have worked."

Extinction in Two Parts

In earlier work, study first author Douglas Robertson, a geophysic
ist who is at the University of Colorado, showed that the asteroid that
slammed into present-day Chicxulub, Mexico, likely triggered a global
firestorm and hurled huge amounts of vaporized rock high above the
atmosphere.

When the ejecta fell back to Earth a few hours later, it would have then
reentered the atmosphere so fast that the heat of its descent would have
caused the sky to glow red and tinder on the ground to burst into flames.

"The radiation and fires would have been fatal within hours to
everything that was not sheltered underground or underwater," Robertson
said. "Dinosaurs all died within a few hours of the impact."

Next, the mixture of dust and ash still in the air would have darkened
the sky and plunged the planet into an "impact winter" lasting months to
years. Plants and other organisms that relied on the sun's light for
energy quickly died.

According to this model, the oceans were largely shielded from the
initial burst of heat and fire, but soon after, entire groups of
organisms, including the giant marine reptiles known as plesiosaurs
and shelled, squid-like creatures called ammonites,
became extinct when marine food chains collapsed.

About 20 years ago, however, scientists noticed that the extinction
levels among freshwater creatures were more subdued: Whereas marine
environments lost as much as half of their groups of creatures, the
freshwater extinction rate was only about 10 to 20 percent.

At the time, some scientists, including Fastovsky and Peter Sheehan, a
paleontologist at the Milwaukee Public Museum and a co-author on the new
study, explained this curious pattern by pointing out that freshwater
organisms are more accustomed to feeding off detritus, or dead organic
matter.

During the impact winter, freshwater environments would have received a
steady influx of dissolved organic matter that was regularly washed into
rivers and streams from dead plants and animals on land. Those same
flowing water sources would have also kept freshwater ecosystems well
oxygenated.

Robertson and his team agree that detrital feeding was a factor in
explaining why freshwater organisms better survived the dark winter that
followed the impact. However, the ability to enter "dormancy and
[greater availability of refuges] in freshwater are probably more
important," Robertson said.

Testable?

The challenge now for paleontologists will be to figure out ways to test
the hypotheses outlined in the paper. "That's the bugaboo," Fastovsky said.

For example, according to the model developed by Robertson and his team,
the collapse of the ocean food chain would have happened more quickly
than in freshwater environments.

"That's something that logically follows from this model, but that's not
a testable statement," in part because current scientific techniques are
not yet capable of resolving time differences of months or a few years
in the fossil record, Fastovsky said.

"I don't know that we have the resolution to do that," he said. "We
might at some point."

Study co-author Robertson disagreed that the food chain collapse was
faster in the oceans - he thinks the collapse occurred on similar
timescales in both freshwater and marine environments, but that lakes
and rivers recovered faster - but agreed that testing his team's model
would not be easy.

For one thing, "the evidence for the survival of freshwater ecosystems
comes entirely from the fossil record in a small region in Montana,"
Robertson explained. "It will be both important and difficult to find
similar evidence elsewhere on the planet."
Received on Tue 23 Jul 2013 01:56:28 PM PDT