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August 14, 2003
Nitrous Oxide Record Sheds Light on Glacial Carbon Dioxide
University Park, PA ---- A 106,000-year-long record
of nitrous oxide concentrations and a shorter record
of nitrogen and oxygen isotopes show that both marine
and terrestrial nitrous oxide production increased in
unison and effectively by the same proportional amount
during the end of the last glacial period, according
to Penn State researchers.
Equal terrestrial and marine production of nitrous
oxide also suggest that increased storage of carbon
in the oceans was not the cause of low atmospheric carbon
dioxide during ancient glacial periods, the researchers
report in the Aug. 15 issue of Science.
"Nitrous oxide is a greenhouse gas, but there is so
little of it in the atmosphere, that it hardly contributes
to climate change through changes in the radiation budget,"
says Dr. Todd Sowers, research associate in geosciences.
"Changes in nitrous oxide loading can, however, provide
clues about systems that control carbon dioxide in the
atmosphere."
Sowers, working with Dr. Richard B. Alley, the Evan
Pugh professor of geosciences, and Jennifer Jubenville,
former graduate student, looked at nitrous oxide from
the Greenland Ice Core Project II ice core to catalog
atmospheric nitrous oxide concentrations through time.
"This is a new record of concentration variations back
this far, only a small portion had been done before"
says Sowers. "We found a 40 percent increase in the
concentration of nitrous oxide in the atmosphere as
the Earth warmed at the end of the last glacial period."
The concentration data alone shows how much nitrous
oxide was in the atmosphere at any particular time.
It cannot, however, suggest how much of that gas came
from the oceans or land. The researchers also looked
at an ice core from the Taylor Dome, Antarctica, to
create a 30,000-year history of the isotopic composition
of the nitrogen and oxygen in the nitrous oxide.
Bacteria on land and in the oceans produce nitrous
oxide in one of two ways. Ocean bacteria tend to create
nitrous oxide that has more of the heavier isotopes
of nitrogen and oxygen, while terrestrial bacteria tend
to produce nitrous oxide with the lighter atoms. By
looking at proportions of isotopes in the trapped gases,
the researchers could determine how much was made on
land and how much in the oceans.
"Before we had the isotope records, common wisdom suggested
changes in terrestrial emission were probably the major
player responsible for the observed concentration changes,"
says Sowers. "Our isotope data, however, show that both
oceanic and terrestrial emissions changed in roughly
the same proportion throughout the last 30,000 years."
Carbon dioxide in the atmosphere hits lows during glacial
periods and some researchers have suggested that increased
productivity in the glacial oceans could have removed
carbon dioxide from the atmosphere. If the oceans behaved
as they do today, then increased oceanic productivity
during the glacial period would have produced elevated
oceanic nitrous oxide production. However, if the relationship
between terrestrial and marine nitrous oxide did not
change, then this cannot be an explanation for the low
levels of carbon dioxide in the atmosphere during glacial
periods.
"When we thought terrestrial emissions were the dominant
control on atmospheric nitrous oxide concentrations,
then this hypothesis could have been true," says Sowers.
"Now that we know that the land and oceans contributed
equally, we have to look for another explanation for
the low carbon dioxide levels."
Contact: A'ndrea Elyse Messer aem1@psu.edu
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