University of Jyväskylä

Solar radiation mineralizes terrestrial dissolved organic carbon in the ocean

Organic carbon dissolved in water plays a vital role in the Earth's carbon cycle. Understanding carbon cycling is central to understanding climate change and how aquatic communities are structured and supported. Senior Lecturer Anssi Vähätalo and his research group from Department of Biological and Environmental Science at the University of Jyväskylä has found out that solar radiation mineralizes more terrestrial dissolved organic carbon in the ocean than in the inland waters.

Rivers discharge annually 248 teragrams (248 000 000 000 000 grams) of terrestrial dissolved organic carbon from the continents to the ocean. The majority of terrestrial dissolved organic carbon is recalcitrant against microbial mineralization in the ocean, but solar radiation can photochemically mineralize part of it into carbon dioxide.The dissolved organic carbon from the Amazon Rivers spreads into
 the Atlantic Ocean to be decomposed by solar radiation. The river plume
 containing terrestrial dissolved organic carbon can be seen as dark regions
 in the Atlantic Ocean. The picture taken on 30 September 2006 shows that the
 river plume has extended first about 700 km in the front of Guyana and turned
 there towards the open ocean.

An article published in the Global Biogeochemical Cycles on 20th of February 2018 estimates that solar radiation mineralizes 45 teragrams of terrestrial dissolved organic carbon in the ocean. The amount is larger than the corresponding photochemical mineralization in the lakes and the reservoirs. Thus, solar radiation mineralizes terrestrial dissolved organic carbon more in the ocean than in the inland waters concludes Anssi Vähätalo, the leader of the research group. 

“The export of terrestrial dissolved organic carbon from inland water to the ocean is faster than its photochemical mineralization in the inland waters. When terrestrial dissolved carbon enters to the ocean, it will quickly spread into river plumes. “, says Anssi Vähätalo. “The area of these river plumes, where the photochemical mineralization takes place, is 34 million squarekilometers, for example about three times the area of Europe.”, he continues,

For this project, water samples were collected from the ten largest rivers in the world. Water samples were taken, e.g. from the Amazon, the Mississippi, the Congo and the Yangtze Rivers.

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Caption: The dissolved organic carbon from the Amazon Rivers spreads into the Atlantic Ocean to be decomposed by solar radiation. The river plume containing terrestrial dissolved organic carbon can be seen as dark regions in the Atlantic Ocean. The picture taken on 30 September 2006 shows that the river plume has extended first about 700 km in the front of Guyana and turned there towards the open ocean.

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