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, https://doi.org/10.1029/2019JG005307, 2020)
—Elizabeth Thompson, Science Writer