Rethink on carbon sinks
Tropical seas are not the carbon sinks they were thought to be, Australian-led international research has discovered, reopening the mystery of where atmospheric carbon is going.
The research was undertaken by coastal oceanographers from the Australian Institute of Marine Science (AIMS) who took core samples from the sediment on the seabed of several tropical sites to determine the burial rate of organic carbon.
They found that the organic matter decomposes and releases carbon back into the atmosphere faster than it is being deposited, which means the areas are net producers of carbon rather than sinks.
"This was a surprise to all of us we thought the wet tropical parts of the world would be carbon sinks. We found we were wrong," said Dr Gregg Brunskill, who headed up the study.
It has long been thought that continental shelves in wet tropical areas around the world act as significant carbon sponges, removing carbon dioxide out of the atmosphere by burying organic matter in sediment. But this finding shows continental shelves to be a source of carbon to the atmosphere.
This new research, part of an international project called Tropical River-Ocean Processes in Coastal Settings (TROPICS), studied the Great Barrier Reef, the North West Shelf off Western Australia, and the coastal zone of the Amazon River in South America.
They also sampled the Gulf of Papua, which is considered to be representative of the wet tropics because everything happens very quickly and on a large scale in the region. The gulf has a huge amount of organic matter buried into it because it is fed by big rivers which naturally erode. But the organic matter also decomposes very quickly.
Working from ships, the researchers dropped steel and plastic pipes into the water which were weighed down with lead. These plunge into the sediment and the resulting cores represent a slice of time in the seabed's history. "If the deposit rate is extremely rapid, a two to three metre core will be a 200-300 year history but if the deposition is slow, such as half a millimetre a year, we will have 1000's of years," Brunskill explained to ABC Science Online.
In the analysis the researchers were trying to determine how fast the organic carbon is piling up on the sea bottom, and therefore the burial rate of organic matter, and how fast it is decomposing. Decomposition of organic matter has the same carbon effect as fire, it releases CO2 back into the atmosphere.
"Our results will help global modellers regig things," said Brunskill. "It doesn't mean a huge change in global models."
The results mean the removal rate of fossil fuel isn't quite as fast as previously thought. Most CO2 is sucked into the ocean at the poles because CO2 is more soluble in cold water, it goes in and sinks to the deep ocean. But in tropical areas this is not happening.
Models of CO2 in the atmosphere are based on calculations of how much coal, oil and gas have been burned in the last century, compared to measurements of CO2 in the atmosphere. Only about half of the expected amounts of CO2 have been found.
"The climate modellers have attributed a third of the remainder to be in land plants and two thirds to be in the ocean," said Brunskill. "We are trying to find out where the missing carbon is going."