Ocean dead zones could be caused by dark carbon

Ocean dead zone
© iStock/Mutlu Kurtbas

A new study from the University of Plymouth suggests that the almost oxygen-free dead zones within the world’s oceans could be expanding much quicker than once believed.

The dead zones are created when large amounts of organic material. Produced by algae, this organic material begins to sink to the seafloor, using all the available oxygen from the deep water.

Using computer models, scientists can predict the spread of these zones, hopefully, providing an insight into the impact they might have on the wider marine environment.

A team of researchers recently published their alternative finding in Global Biogeochemical Cycles. Led by Dr Sabine Lengger of the University of Plymouth, UK, the team suggest that dark carbon fixation need to be incorporated into these computer models. Dark carbon fixation is caused by the presence of anaerobic bacteria in the deeper water column.

The research team measured the stable isotopes of organic carbon in sediment cores extracted from the floor of the Arabian Sea, one of the world’s largest natural dead zones. This experiment was intended to give researchers a clear understanding of what is contributing to the organic matter contained within them.

The findings were a mixture of all the distinct signatures from all the organism that produce this carbon, thought to be mostly algae and bacteria living in the oxygen-rich, light, surface ocean where it sinks from.

Using a distinct biomarker produced by anaerobic bacteria, the research team suggested that around one fifth of organic matter on the seafloor could in face stem from bacteria living in or around these dead zones.

Dr Lengger, an organic and isotope biogeochemist at the University of Plymouth, said: “With global warming, and increased nutrients from rivers, oceanic dead zones are forecast to expand. They can draw down carbon and store it in the deep ocean, but as they expand can have devastating effects on marine life, as well as people that are economically reliant on fisheries.”

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