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by Peter Rejcek
Multi-disciplinary project seeks clues to absence
of benthic life in paleological record
The bottom of McMurdo Sound is teeming with life —
from brittle stars to scallops to wildly diverse single-celled
critters called foraminifera, many of which build hard
body parts or shells out of calcium carbonate.
That’s the story today. But what happened in the
ocean millions of years ago? That’s a hard question
to answer. For some reason, there are few signs of these
critters in the fossil records that geologists and other
scientists study in sediment cores taken from below the
seafloor.
“It’s almost like a disconnect from the life
of today with the life of the past,” observed Molly
Miller, a geology professor in the Department of Earth
and Environmental Sciences at Vanderbilt University and
one of the project’s principal investigators for
the National Science Foundation -funded study.
Unraveling the mystery is important for a number of reasons,
including understanding the evolution of the Antarctic
benthic ecosystem through the millennia. Yet the absence
of the seafloor-hugging organisms is also of interest
because many of them are calcifying animals that form
shells and skeletons made of the calcium carbonate materials
calcite or aragonite.
Scientists can use their fossilized remains to reconstruct
things like past climate and ocean circulation. However,
perhaps more importantly, understanding the conditions
under which the remains of calcifying organisms dissolve
will also help climate change scientists predict what
might happen to the modern-day calcifiers, as the oceans
become more acidic from absorbing atmospheric carbon dioxide.
Science divers prepare to enter the water in McMurdo
Sound.In addition, calcium carbonate material naturally
dissolves faster in cold, polar waters — one of the
key factors the scientists believe may have caused disruptions
in the fossil record.
“In tropical zones, calcium carbonate should stick
around for a while,” explained Sally Walker, a paleontologist
from the University of Georgia in Athens who studies the
process of fossilization. “In cold, polar conditions,
you would think that calcium carbonate would rapidly dissolve.
We do have a record of it in some of the cores, so we
know it can fossilize in cooler conditions.”
In effect, while interested in the process of fossilization,
an area of study called taphonomy, Walker, working with
Miller and Sam Bowser from the New York State Department
of Health’s Wadsworth Center in Albany, are also
studying the conditions that destroy the remains of the
marine animals and foraminifera, or forams , a group of
creatures often used by micropaleontologists to study
past climate conditions.
“To understand the fossilization process makes you
realize this connection between the decay of calcium carbonate
and how rapidly it can decay, and then how rapidly it
can get into the chemical carbon cycle,” said Walker,
a principal investigator for the project making her first
trip to the Antarctic.
The scientists believe that in addition to the cold polar
water, the ice-covered marine habitat may play a role
in destroying the animal remains before they can fossilize.
For example, perhaps advancing glaciers ground away shells
and other material. Or as Walker put it, “We’re
looking at the effects of ice on the fossil record of
Antarctica.”
“This project is geobiology, an emerging field of
science looking at the connections between biology and
geology,” said Tom Wagner, program manager of Antarctic
Earth Sciences in the National Science Foundation’s
Office of Polar Programs , which is funding the research.
Unique collaboration
To put their theories to the test, the scientists will
conduct a variety of experiments from a field camp at
Explorers Cove in New Harbor. The camp, located across
McMurdo Sound from the U.S. Antarctic Program’s McMurdo
Station , has been used for a number of seasons by Bowser
and his team of science divers.
A cell biologist, Bowser studies forams and their place
in the benthic food web. Another principal investigator
on the project, Bowser is also interested in forams’
possible applications in area such as nanotechnology and
biomedicine.
The collaboration is an interesting one. Bowser brings
his general expertise in the local benthic ecology to
the table, while Miller is an expert in bioturbation (the
mixing and displacement of sediments by fauna) and Walker
measures the rate of dissolution for the shell-secreting
animals.
“Drill cores taken of sediments found on Antarctica’s
continental shelves provide our most important records
of deep-time climate change,” Wagner explained. “And
they should be teeming with remnants of life from the
sea bed but they aren’t. Why not?
“It’s important information because it would
tell us about past ecosystems while providing another
perspective on climate change,” he added. “It
could be that they aren’t preserved, but it could
also be that we just don't know how to interpret the records
that we have. And that’s what makes this project
so exciting — it could lead to a total reappraisal
of Antarctica’s past.”
Miller’s role will be to understand the relationship
between the benthic fauna and the sediments on which and
in which they live. She wants to know how they churn and
affect the sediments, which is a function of how many
animals are present and how fast the sediment accumulates.
Most of the sediment cores drilled in the region —
such as from the recent ANDRILL program and the older
Cape Roberts project — show little bioturbation,
according to Miller.
“We’re actually doing a direct approach,”
she explained. “We’re collecting animals that
live there, putting them in constrained little aquaria
with sediment, and we’re looking at how they mess
it up.” Divers will also put out sediment traps on
the seafloor to see how much sediment accumulates during
the course of the fieldwork. Some traps will remain out
for two years until the project resumes in 2010.
Molly Miller's Adventures in Antartctic The aquariums,
which are only about 2.5 centimeters wide, include screens
that allow ocean water to move through the chamber. “It’s
basically a little microcosm of the natural world,”
Miller said.
At the end of the season, divers will collect the aquariums,
which the scientists will then freeze and return to the
laboratory, where they’ll X-ray the tanks to detect
any churning of the sediments by the animals. Then they
can compare the results with X-rays taken of sediment
cores.
“We’re just unclear as to the extent by which
that [bioturbation] process is going on and has been going
on,” Miller said. “It looks like it’s not
too extensive in the cores in the ANDRILL and Cape Roberts
projects.”
Walker will use techniques developed for a long-term
experiment in the tropics that she’s involved in
called the Shelf and Slope Experimental Taphonomy Initiative
(SSETI) . For her purposes, it will be important to use
a range of calcifying organisms to determine different
rates of dissolution because the animals create different
skeletal types in various ways. Some, for instance, use
organic materials in their skeletal makeup, which may
slow dissolution by the cold water.
Sally Walker in the field — just across the Golden
Gate Bridge from San Francisco.“Just like a skyscraper
is built differently than a ranch-style home, animals
do that, too,” Walker said. “I would expect
a spectrum of dissolution [rates] in the short term and
the long term.”
For example, forams build complex shells of calcite that
seem to outlast a snail shell of aragonite, according
to Walker. She will also use instrumentation employed
by coral reef researchers, a water-quality-monitoring
sonde, to measure water temperature, acidity, nutrient
levels and other parameters to constrain the conditions
under which the decay occurs.
More generally, the scientists will track the relative
abundance of species in the underwater region. “Even
in a place like Explorers Cover [there are] components
of the fauna that are not very well known at all,”
Miller noted.
“The implications of this project are greater because
of the diversity of people involved,” she added.
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