Our Seas Have Dead Zones Without Oxygen

The estimates of global ocean "dead zones" - where there is almost no oxygen to sustain life - may be seriously sick because of misleading biofeedback in the form of dark carbon, scientists warn in a new study.

Anaerobic bodies dwelling in these dead zones can still ingest dark carbon without sunlight, hence the "dark" in dark carbon. They throw off measurements of where dead zones are and of global carbon results because these organisms are still consuming carbon as fuel even in the absence of sunlight.

The study, led by scientist Sabine Lengger of the University of Plymouth in England and other scientists from the United Kingdom and the Netherlands, measured organic carbons from the floor of the Arabian Sea to get a clear understanding of what is contributing to the organic matter contained within them. The Arabian Sea is one of the world's abundant natural dead zones.

The researchers, using a well-defined biomarker created by anaerobic bacteria, recommended that about one-fifth of the organic matter on the seafloor could stem from bacteria living in or around the dead zones.

As biogeochemical models that work on the premise that all sinking organic matter becomes photosynthetically derived, the authors wrote in their paper that these bodies could significantly underestimate the extent of remineralisation" without adding new carbons.

In the paper, the experts cast doubt on prevailing forecasts around the influence of increasing carbon dioxide concentrations in the atmosphere and consistently rising temperatures.

The dead zones could be growing much faster than previously thought, according to Lenggers and her team. Further calculations, according to the researchers, must take the bacteria into account to precisely predict the full consequences of climate change and human activity on the marine environment.

"Our study [reveals] that organic matter that [settles on] the seafloor is not just coming from the sea surface, but [holds] a major contribution from bacteria that [reside] in the dark ocean and [could fixate the carbon] as well," Lenggers said.

The new study was reported at COP25 by the International Union for the Conservation of Nature (IUCN), where they said that the number of identified hypoxic dead zones has skyrocketed from 45 to 700 sites in the world.

"With global warming and [added] nutrients from rivers, oceanic dead zones are [projected] to [grow]," says lead author Lengger, an organic and isotope biogeochemist at Plymouth.

"The [dead zones could] draw down carbon and [keep] it in the deep ocean but could have devastating effects on marine life, as well as people that are economically reliant on fisheries, as [these areas] expand," she added.

Current models could be missing out on a vital increase as an outcome of which people have miscalculated the amount of the oxygen depletion everyone is expecting in the future, warming world.

"Our findings [clarify that] some of the mismatches in carbon [resources] when [tested] and [compared] modeling estimates should be included in biogeochemical models [forecasting] feedbacks to [global warming]," Lengger said.

The researchers recommended to refine predictions in biogeochemical models as if dead zones will strengthen more than assumed - which was observed in the study - would have severe ecological, economic, and climatic consequences.