A Lawrence
Livermore National Laboratory (LLNL) scientist, along with
his team, recently used radiocarbon dating to trace
carbon dioxide released to the atmosphere from the deep
ocean at the end of the last ice age.
Radiocarbon
dating employs the use of radioisotope carbon-14 to figure out the
age of ancient and prehistoric carbonaceous materials. This process
can be used on materials as old as 62,000 years old.
Tom
Guilderson, a scientist at the LLNL's Center for Accelerator Mass
Spectrometry and an author of the study, found that an increase
in atmospheric CO2 concentrations corresponded with a
decreased amount of carbon-14 relative to carbon-12 in the
atmosphere.
"This
suggests that there was a release of very 'old' or low 14/12CO2 from
the deep ocean to the atmosphere during the end of the last ice age,"
said Guilderson. He noted that CO2 release may increase the rate at
which ice melts after an ice age.
Ocean
circulation regulates radiocarbon in the atmosphere, and in turn,
this regulates the sequestration of carbon
dioxide in the deep ocean by atmosphere-ocean carbon
exchange. Around 110,000 to 10,000 years ago when the last ice age
occurred, lower atmospheric carbon dioxide levels coincided with
increased atmospheric radiocarbon concentrations, which have been
"credited to great storage of CO2 in a poorly ventilated abyssal
ocean." The circulation of the ocean was drastically different
back then, and Guilderson admits that he and his team do not fully
understand the manner in which carbon was stored in deep ocean at
that time.
The
team dated two sediment cores from the subtropic South Pacific near
New Zealand and the sub-Antarctic to be approximately 13,000 and
19,000 years old. Guilderson was able to determine when the large CO2
release occurred using the carbon-14 in the cores. Also, he was able
to determine the ocean pathway by which it escaped.
"In
this case, the absence of a signal is telling us something
important," said Guilderson. "Deeper waters substantially
depleted in carbon-14 were drawn to the upper layers and this is the
main source of the CO2 during deglaciation. Data suggests that the
upwelling of this water occurred in the Southern Ocean, near
Antarctica. In our cores off New Zealand, which lie in the path of
waters which 'turn over' in the Southern Ocean, we don't find
anomalously low carbon-14/12 ratios.
"This
implies that either water which upwelled in the Southern Ocean, after
16,500 years ago, had a vigorous exchange with the atmosphere,
allowing its 14C-clock to be reset, or the circulation was
significantly different than what the current paradigm is. If the
paradigm is wrong, then during the glacial and deglaciation, the
North Pacific is much more important than we give it credit for."
This
carbon dioxide release sped up the melting, but when asked about
CO2's contribution to
global
warming today, Guilderson said this release of CO2 from the
last ice age "is not relevant." But he did mention that he
has used radiocarbon dating on CO2 in the atmosphere today, and that
isotopic signature shows that use of fossil fuels is what is causing
global warming.
The
study was published in the August 26 edition of Nature.
