Carbon Cycling in the World’s Deepest Blue Hole


In the South China Sea, the Yongle blue hole was recently found to be the deepest blue hole on Earth, reaching 300 meters down—deep enough to submerge the Eiffel Tower. A blue hole, essentially a marine sinkhole with an opening to the sea surface, forms a natural laboratory where scientists can research patterns that are harder to detect in the unsheltered waters of the open ocean.

Blue holes are usually circular pits with steep walls. Tides wash over their tops, but their unique structure keeps the environment deep inside relatively isolated. As a result, blue holes can have unique microbial communities, sediment archives, and chemical gradients. Yao et al. tested the waters of the Yongle blue hole to learn more about its carbon cycling processes, which are not often studied in blue holes.

The researchers found the lowest concentrations of dissolved organic carbon ever recorded in coastal waters. At the same time, they found some of the highest concentrations of dissolved inorganic carbon in similar conditions. Both the organic and inorganic carbon molecules were much older than carbon found at parallel depths in the open ocean.

The scientists concluded that the carbon cycling process in this blue hole depends strongly on the natural gradients found in its depths; between about 80 and 100 meters, dissolved oxygen disappears, and salinity, temperature, and pH all change sharply. They attribute the bulk of carbon cycling in this blue hole to the processes of the microbes that live there, including sulfur cycling and methane production. The role of carbonate dissolution from the walls of the blue hole in affecting the ages of carbon in this system remains uncertain, yet there appears to be no evidence of inflow of subterranean fresh water into the bottom waters of the blue hole.

Blue holes allow scientists to study chemical gradients with a precision impossible in the open ocean. As marine environments around the globe experience growing patches of low oxygen, understanding these gradients and cycles is more important than ever. Further, the amount of dissolved inorganic carbon in the hole seems to be increasing, so blue holes may become a carbon dioxide source that must be factored into wider climate change and blue carbon predictions. (Journal of Geophysical Research: Biogeosciences, 2020)

—Elizabeth Thompson, Science Writer

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Earth’s deepest ‘blue hole’ holds a cache of ancient carbon

earth's deepest blue hole

Carbon more than 8,000 years old lies deep inside a yawning sinkhole in the South China Sea.

Blue holes are underwater caverns carved into dissolving rock. The deepest known marine cavern is the Yongle blue hole, which measures roughly 300 metres from top to bottom. Its waters are mostly isolated from the surrounding ocean and receive little fresh water from rainfall, making it a rare spot to study the chemistry of oxygen-deprived marine ecosystems.

Peng Yao at the Ocean University of China in Qingdao and his team collected bottles of water at various intervals, from the Yongle’s mouth to a depth of 270 metres. The researchers found low levels of dissolved organic carbon and high levels of dissolved inorganic carbon, both of which are unusual for coastal waters. Radiocarbon dating showed that the deep dissolved carbon was thousands of years old, meaning that water in the depths of the hole had probably not mixed with the open ocean for a very long time.

The blue hole offers a rare glimpse of unusual ocean conditions, which may help scientists to understand the extreme chemistry of past and future oceans.

Bianchi receives International Award from the Shandong Province of China

From October 29 to 30, the 2018 Ceremony of the Qilu Friendship Award was held in the city of Weihai, at which Prof. Thomas Bianchi, a Foreign Expert at Ocean University of China (OUC) and Professor of University of Florida, was presented with the award.


Prof. Thomas Bianchi is an outstanding overseas scholar and leading foreign expert at OUC’s Marine Chemistry. For the past decade, he has visited Qingdao every year to conduct cooperative research. With his efforts, fruitful collaborations have been achieved through joint programs with the Key Laboratory of Marine Chemistry under the Chinese Ministry of Education, joint education programs for doctoral candidates, as well as cooperative research. All of this has made great contributions to the development of the marine chemistry discipline and to the cultivation of young talent.


The Qilu Friendship Award, established in 1993, is the top award given by Shandong People’s Government to commend foreign experts for their outstanding contributions to the province’s socio-economic development and international exchanges, and is conferred on 20 experts each year. To date, eight foreign experts associated with OUC have won the award.


News release from Ocean University of China.

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Engineered river diversions sequester carbon in deltas

Re-posted article from UF News by Rachel Wayne, original page here.

Researchers from the University of Florida have found that a delta of a distributary on the Mississippi River created by coastal engineering efforts may have the potential to build long-term sinks of greenhouse gases.

The carbon sequestration potential and guidelines for future engineering and restoration shown in the Wax Lake Delta (WLD) of Louisiana’s Atchafalaya River are described in a paper published today in Nature Geoscience by co-lead authors Michael Shields, postdoc in the Department of Geological Sciences, Thomas Bianchi, Jon and Beverly Thompson Endowed Chair of Geological Sciences, and David Mohrig at the University of Texas at Austin.

The WLD formed naturally after the initial river diversion was engineered, according to the study.

“We discovered that in a system losing land at a rate equivalent to one football field every hour, an engineered river diversion not only built land, but also buried carbon at rates comparable to, or greater than, that of the most efficient terrestrial carbon sinks of similar area,” Shields said.

The land-building accomplished by deltas can reduce atmospheric carbon, and therefore the greenhouse effect, by trapping the carbon in sediment. Careful engineering can divert sediment deposition in the context of other factors, such as storms, runoff and avulsion (when a river abandons its channel). Louisiana’s Coastal Master Plan aims to divert Mississippi River sedimentation into proper receiving basins.

The paper focuses on WLD, a subdelta, that has potential to create a blue carbon habitat (carbon stored in marine and coastal ecosystems). An effect of a diversion built in 1941 to reduce the Atchafalaya’s flooding in a nearby city, it has built about 35 square kilometers of new land. “Engineered river diversions that return sediment to wetlands and bays utilize natural processes to build land and bury carbon in new subdeltas,” explained Shields.

Delta studies must now accommodate a variety of anthropogenic factors, including reservoirs, levees, and subsurface fluid extraction. Delta restoration combines engineering and geological science to encourage continued sedimentation, which “buries” organic carbon, preventing it from returning to the atmosphere.

The researchers sought to measure total carbon storage within the entire delta deposit to account for carbon buried while the delta was still subaqueous (i.e. underwater). Many deltas are threatened by greater subsidence (subterranean sinking and caving) and relative sea-level rise compared to coastlines without deltas. Thus engineering efforts to expand carbon-sequestering habitats must accommodate total carbon sequestration in order to reduce atmospheric carbon.

“When considering the current problems we face with global warming and sea level rise, a greater understanding of how we can stabilize our coastlines and help preserve coastal wetlands is vital for our future,” Bianchi said.

The research was conducted with generous support from Bianchi’s endowed chair by Jon and Beverly Thompson, in collaboration with William F. Kenney of the Land Use and Environmental Change Institute, as well as the Louisiana Universities Marine Consortium.


Link to original article here.

Photo of Tom Bianchi kneeling next to Lake Alice.

Delta Blues

“Burn and burial,” offers Thomas S. Bianchi, the Jon L. and Beverly A. Thompson Endowed Chair of Geological Sciences, as a central theme of his research. He’s referring to carbon cycling, especially the release of carbon into the atmosphere or its sequestration in flora in “blue carbon” areas, such as wetlands and rivers. Bianchi, sitting in front of a whiteboard with an impressive list of pending publications, talks about his slate of projects, which, like their subject matter, flow into diverse outlets. He’s working on multiple fronts to study “burn and burial” in the face of pollution, dams, and sea level rise.

“Deltas are going to be the first to be inundated by sea level rise.”

“My original focus was not in climate change,” Bianchi says. “Sometimes I wish I had more projects that didn’t connect to it in some way.” It’s a distressingly politicized topic of research (and funding, or lack thereof), although Bianchi is pleased that it’s been “an integrative force for multiple disciplines.” As a biogeochemist, he’s certainly representative of the academic portmanteaus. His passion, however evolved, is palpable as he discusses threats to the cradle of civilization: the fertile delta. “Deltas are going to be the first to be inundated by sea level rise,” says Bianchi. “Some areas are sinking due to natural subsidence and from extraction of oil and natural gas. The Mississippi Delta is experiencing this as sea levels rise while oil and gas reserves are drained.” The loss of deltas is a key topic of Bianchi’s latest book, Deltas and HumansIt’s his first publication for a lay audience and his personal contribution toward expanding the audience for climate science.

Thomas Bianchi stands on a fallen tree by Lake Alice

Reaching that audience is crucial to bridging the gap between the general public and scientific community, especially on a hot-button topic. Scientists can easily lose debates to those who use “grandstanding and trickery to overwhelm their opponent,” he says. He encourages his peers to train themselves to better express, to the public and the press alike, the dire problem of climate change. There’s plenty of brainpower among the many disciplines already united under the “climate change umbrella,” as he calls it. “Scientists inherited this problem from the Industrial Revolution. We’ve known it longer than the public has,” asserts Bianchi. “Now, scientists must do a better job of getting the word out.”