Carbon more than 8,000 years old has been found inside the world’s deepest blue hole — the Yongle Blue Hole (YBH) — which was recently discovered in the South China Sea, according to journal JGR Biogeosciences.
Blue holes are marine caverns filled with water and are formed following dissolution of carbonate rocks, usually under the influence of global sea level rise or fall. What distinguishes them from other aquatic caverns is that they are isolated from the ocean and don’t receive fresh rainwater.
They are generally circular, steep-walled and open to surface.
According to JGR Biogeosciences, researchers found low levels of dissolved organic carbon and high levels of dissolved inorganic carbon in YBH, both with radiocarbon ages of more than 6,000 years.
Such concentrations of carbon, usually found in deep marine holes like YBH, provide a natural laboratory to study carbon cycling and potential mechanisms controlling it in the marine ecosystem.
YBH has a depth of 300 metres, far deeper than the previously recorded deepest blue hole, Dean’s Blue Hole in Bahamas, which had a depth of 202 metres.
Though a largely enclosed geomorphology, YBH is influenced with some oceanic exchange in the surface water. However, like most blue holes, it is anoxic i.e. depleted of dissolved oxygen below a certain depth. This anaerobic environment is unfavorable for most sea life.
Low oxygen environments are an area of wide research in the aquatic ecosystem. According to the journal, such anoxic ecosystems are considered a critical environmental and ecological issue as they have led to several mass extinctions.
The transition from aerobic to anaerobic environment adversely affects the biogeo-chemistry of the ocean. However, in blue holes, the transition happens within several hundred metres depth with lesser influences.
According to a related article published in Nature, this makes blue hole ecosystem a more accessible habitat to examine the physical and biological processes affecting carbon cycling and unusual ocean conditions.