According to a new study in the journal Nature Geoscience, fjords absorb approximately 18 million tones of organic carbon each year, equivalent to 11 percent of annual marine carbon burial globally.
Tracy Arm Fjord, Alaska. Image credit: Maria Michelle.
Fjords are found in locations where current or past glaciations extended below current sea level.
Scientists had known for years that they were coastal environments with high carbon storage, but because they make up just 0.1% of the world’s oceans, no one realized just how important they were.
“As deep and often low oxygen marine environments, fjords provide stable sites for carbon-rich sediments to accumulate,” said co-author Dr Candida Savage of the University of Otago in New Zealand and the University of Cape Town in South Africa.
Dr Savage and her colleagues from the United States conducted fieldwork in Fiordland, a vast tract of mountainous terrain that occupies the south-west corner of the South Island of New Zealand.
The team also analyzed data from 573 surface sediment samples and 124 sediment cores from nearly all fjord systems globally.
They found that per unit area, fjord organic carbon burial rates are twice as large as the ocean average.
Carbon burial is an important natural process that provides the largest carbon sink on the planet and influences atmospheric carbon dioxide levels at multi-thousand-year time scales. Dr Savage and co-authors suggest that fjords may play an especially important role as a driver of atmospheric carbon dioxide levels during times when ice sheets are advancing or retreating.
“The Earth is currently in an interglacial period after ice sheets receded around 11,700 years ago. During glacial retreats, fjords would trap and prevent large volumes of organic carbon flowing out to the continental shelf, where chemical processes would have caused carbon dioxide to be produced,” Dr Savage said.
“Once glaciers started advancing again this material would likely then be pushed out onto the shelf and carbon dioxide production would increase.”
“In essence, fjords appear to act as a major temporary storage site for organic carbon in between glacial periods. This finding has important implications for improving our understanding of global carbon cycling and climate change,” she said.
“Carbon sequestration is the big buzzword, but we’re still getting a handle on how it works. In order to make informed land-use decisions and accurate climate predictions, finding and understanding these hot spots is critical,” added co-author Dr Thomas Bianchi of the University of Florida.
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Richard W. Smith et al. High rates of organic carbon burial in fjord sediments globally. Nature Geoscience, published online May 04, 2015; doi: 10.1038/ngeo2421
