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Study
finds holes in carbon sink theory
|
Wet
tropics carbon sink?
Are
the wet tropics really a sink for carbon?
Feature
by
Kerie Hull
June 16, 2003
After years of believing the continental shelves in wet
tropical areas around the world help soak up fossil fuel
carbon dioxide out of the atmosphere, an international
consortium of scientists have found holes in the theory.
It was long believed that continental shelves act as sites
of atmospheric carbon dioxide removal by burying organic
matter in sediments.
|
The latest evidence shows
the continental shelf of the Gulf of Papua is a source
of carbon dioxide to the atmosphere, which is contrary
to most current climate change models depicting the
coastal ocean as an important site of removal of CO2
from the atmosphere.
It is an important
finding because of the implications to the global carbon
budget. Dr Gregg Brunskill of the Australian Institute
of Marine Science (AIMS) said, "Wet tropical parts
of the world contribute over 50% of global continental
runoff and sediment, but these regions do not appear to
be sites of large carbon sequestration (burial, removal
from the atmosphere). It could force those predicting
the fate of fossil fuel carbon dioxide and climate
change to rework their models."
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Sediment traps being deployed.
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Dr Gregg Brunskill is a biogeochemist, and is one of the
leaders of the international research project called TROPICS
– Tropical River-Ocean Processes in Coastal Settings –
which has been trying to identify the role continental shelves
play in the carbon cycle (amongst other things). The research
team uses data collected over a decade by an informal,
international consortium of marine scientists.
More than 20 international research institutions are
involved in the project which has reached its final phase.
Project coordinators are in full swing collating and writing
up their findings for publication.
One task of Brunskill’s team was to demonstrate that the
most important burial site for organic matter in the ocean is
on continental shelves. It would mean that continental shelves
are a major sink, a place where carbon is removed from the
atmosphere and buried in the sediment. The team set out to
measure how much carbon is buried as organic matter and how
fast this process occurs in the river deltas, the steamy
mangrove swamps, and out across the continental shelf to 1000
metre water depths.
Background
The emission of carbon dioxide from the combustion of
fossil fuels and land use change is the leading cause of the
build up of greenhouse gases in the atmosphere, which many
people believe is the main culprit behind an increase in the
earth’s temperatures.
For a long time scientists have known that forests, crops,
soils, and other organic matter soak up some of that carbon,
thereby slowing down the rate of global warming. The
calculations of how much carbon is absorbed by the land and
ocean is the basis of much debate.
Scientists have been given the hard task of trying to
measure the exchange rates of organic matter and CO2
between the ocean, atmosphere, and land, and to create an
inventory of natural & technological sources of CO2,
and natural and technological CO2 removal rates
(sinks) in terrestrial soils and oceanic sediments.
Oceanographers have found that different zones of the ocean
absorb CO2 from the atmosphere (sink), or release
CO2 to the atmosphere (source) at different times
of the year. Because wet tropical regions of the world deliver
large amounts of organic matter-rich soils to the coastal
ocean, it was thought that burial of this terrestrial and
associated marine organic matter on continental shelves would
be an important site of removal of fossil fuel CO2
in the global inventory.
| Oceanographers
who are helping to create this global inventory of
organic carbon and CO2 in the ocean have
found that carbon burial rates on continental shelves of
the temperate and polar regions are very slow. It was
expected that carbon burial rates in wet tropical parts
of the world would be very fast. Scientists are now
trying to determine if this is true. |
Map
of Papua New Guinea
( Click here
for a larger view )
|
The Gulf of Papua was one of many sites studied. The Great
Barrier Reef, the north West shelf of Australia, South China
Sea and the coastal zone of the Amazon River are just some of
the areas contributing to the global inventory, and the Gulf
of Papua is considered to be most representative of the wet
tropics. "Everything happens very quickly and on a large
scale in this region," Brunskill said.
"Due to high rainfall, high mountain ranges, and
tectonically active land mass, the island of New Guinea
contributes more water, solutes and sediment to the coastal
ocean than the Amazon River."
With big expectations of the Gulf of Papua as a major
carbon sink, the measurements coming from the region had
researchers, including Brunskill, scratching their heads.
|
"Global
models of oceans assign large carbon burial rates to
continental shelves. While it’s true, we found the
burial rates to be large, the decomposition rate of
terrestrial and marine organic matter was even greater,
"said Dr Gregg Brunskill.
"The
measurements suggest that most of the organic carbon
being delivered to the continental shelf of the Gulf of
Papua by rivers and by estuarine productivity is being
oxidised and returned to the atmosphere," he said.
|
Preparing
to take a core from the mud bank.
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This is the thrust of Dr Brunskill’s own work. He and his
colleagues Irena Zagorskis and John Pfitzner at AIMS are
charged with the task of studying the burial rate of organic
carbon in the rapidly accumulating sediment of the Gulf of
Papua. Dr Brunskill measures the burial rate using sediment
cores. He measures the organic matter content of the mud, and
measures how fast the mud and organic matter is being buried
using natural radioactive elements.
|
"Our
research is carried out on big ships where a tonne of
lead is lowered on top of a big pipe into the mud. I get
a sample of mud about two, three, or four metres long
and slice it up into layers like pages in a history book
for the last 100 to 1000 years.
"I can determine how organic matter concentration changes
with depth of the mud, and figure out the accumulation rate of
the organic carbon and sediment. I can then tell you the
removal rate of carbon for 20 or 30 locations in the Gulf of
Papua, and then estimate the total organic carbon burial per
year on the whole shelf. We then compare this burial rate to
marine plant annual growth rates and the annual input of
organic matter from rivers.
|
Taking
cores from the rear work area
on the RV Cape Ferguson.
|
We also try to measure how fast this river and marine plant
organic matter is being decomposed."
The research team has found that the decomposition rate of
organic matter is equal to or greater than the measured supply
rate of organic matter from rivers and plant growth. To
balance the budget, the team has to import more organic matter
from the Coral Sea to feed this voracious incinerator of
organic matter. This finding suggests that more CO2
is being released from the sea to the atmosphere, than is
being buried in the mud. The region is thus a net source of CO2
to the atmosphere, even if large amounts of organic matter is
being buried in the mud today.
|
"Wet tropical continental shelves are probably good
incinerators of organic matter because they are warm, have a
high and steady nutrient supply, and have many kinds of
microorganisms that decompose organic matter in the water and
deep in the mud," said Brunskill.
The credentials, expertise and experience on this scientific
team add weight to the findings that are sure to send carbon
accountants scuffling to consult and even re-jig their
models.
|
The
RV Cape Ferguson
in the Gulf of Papua.
|
Dr. Brunskill is confident his research that shows that
tropical continental shelves aren’t siphoning off as much
fossil fuel CO2 as expected, and could be a source of CO2,
will stand the test of time.
Project TROPICS is a joint Australian, Indonesian, Papua
New Guinean and American Marine science team focused on wet
tropical inputs of riverine matter to the world ocean.
While the project is winding up, the research is not. It
has evolved into a new research programme called Margins.
MARGINS is a USA funded research consortium which is taking
the TROPICS results on to the next stage of investigation.
Scientists are studying the cycles and transport of water,
sediment, and chemical elements from the PNG mountain tops,
down the rivers to the floodplain, into the deltas and
mangrove swamps, the continental shelf and slope, and into the
Coral Sea. Larger ships are being used, which allow detailed
maps of the seafloor, seismic soundings of the thickness of
New Guinea terrestrial sediments on the shelf, and longer
histories of burial rates of the chemical elements. Scientists
and students from USA, PNG, and Australia are participating in
research cruises in the Gulf of Papua in 2003-2005.
 |
 |
Locals
visiting the coring area.
|
Sediment
plume from the Fly river
in the Gulf of Papua.
|
For more information
contact
Dr Gregg Brunskill,
(AIMS) researcher
Telephone: 07 4753 4218 or 0417231429
Email: g.brunskill@aims.gov.au
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2430
20030616 20060410