Global Carbon Cycle Regulated by Rivers

As greenhouse gas levels hit record highs and summer temperatures reach their warmest ever, scientists are frantically working to find ways of reducing the amount of carbon dioxide that enters the atmosphere. But now, new research shows that we may be able to rely - at least in part - on nature alone, which has its own methods for removing atmospheric carbon. This includes rivers, which reportedly are crucial in regulating the global carbon cycle.

So far, humans concerned about climate change have investigated various ways of capturing carbon dioxide (CO2) from the air. For example, they have created carbon-trapping sponges, developed artificial photosynthesis, and even turned to baking soda.

But Mother Nature is going to work as well, with plants and various vegetation also absorbing CO2 - even more than we originally thought. But rivers have also long been known to play a role, though until now, their significance was largely a mystery.

According to new findings published in the journal Nature, the world's river systems offer long-term storage of carbon by transporting decaying organic material and eroded rock from land to the ocean.

However, scientists from Woods Hole Oceanographic Institution (WHOI), who led the study, are quick to point out that river transport of carbon to the ocean is not on a scale that will bail us out of our CO2 problem. But, this study is the first to provide the first direct estimate of how much and in what form organic carbon is exported to the ocean by rivers. Their measurements will help humanity predict how the carbon export from global rivers may shift as Earth's climate changes.

"The world's rivers act as Earth's circulatory system, flushing carbon from land to the ocean and helping reduce the amount that returns to the atmosphere in the form of heat-trapping carbon dioxide," lead author and geochemist Valier Galy said in a news release. "Some of that carbon - 'new' carbon - is from decomposed plant and soil material that is washed into the river and then out to sea. But some of it comes from carbon that has long been stored in the environment in the form of rocks - 'old' carbon - that have been eroded by weather and the force of the river."

They collected sediment data from 43 river systems all over the world - which together account for 20 percent of the total sediments discharged by rivers. These systems represent a broad range of climates, vegetation and geological conditions, and also take into account human influences. The WHOI team found that the world's rivers annually transport 200 megatons (200 million tons) of carbon to the ocean - that's about .02 percent of the total mass of carbon in the atmosphere.

Although that number doesn't seem like a lot, if you add it up over 1,000 to 10,000 years, it makes a significant dent (20 and 200 percent extracted from the atmosphere).

Generally, plants convert CO2 from the atmosphere into organic carbon via photosynthesis. But most of this carbon eventually returns to the atmosphere when plant material (or animals that eat plants) decompose - or via deforestation. A small fraction of this material, however, ends up in rivers. They carry it out to sea, where, rather than being released into the air, some settles to the seafloor and is buried for millions of years - and eventually makes its way back to the surface in the form of rocks.

Meanwhile, rivers also erode carbon-containing rocks into particles carried downstream. The process exposes carbon to air, oxidizing the previously locked-up carbon into CO2 that can leak back out to the atmosphere. Until now, scientists had no way to distinguish how much of the carbon whisked away by rivers came from either the biospheric or petrogenic (rock) sources. Without this information, scientists could not accurately predict carbon sequestration under different scenarios.

But using a novel approach, WHOI researchers solved this dilemma for the first time ever. By analyzing the amount of carbon-14, a radioactive isotope, in the river particles, they could distinguish between carbon from eroded rocks or from decomposed plant and soil material.

It turns out that about 80 percent of released carbon from rivers came from decomposed material. However, despite this fact, that doesn't mean rivers surrounded by lots of plant life transport more carbon to the ocean. Rather, export is "primarily controlled by the capacity of rivers to mobilize and transport" particles, researchers say. Erosion is the key factor - the more erosion occurs along the river, the more carbon it transfers to sea and sequesters from the air.

"The atmosphere is a small reservoir of carbon compared to rocks, soils, the biosphere, and the ocean," the scientists wrote. "As such, its size is sensitive to small imbalances in the exchange with and between these larger reservoirs."

Hopefully these new findings can help scientists better measure how rivers contribute to the global carbon cycle, and how their role might change as Earth's climate changes.

"This study will provide geochemical modelers with new insights on an important link between the global carbon and water cycles," concluded Don Rice, program director in the National Science Foundation's Division of Ocean Sciences, which funded the research.

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