Sulfur promotes carbon storage in the Black Sea

For the past 7,000 years the Black Sea, which has a surface area nearly the size of France, has been characterised by conditions that can be found in only a handful of other water bodies around the globe: stable layering largely prevents any mixing between the surface water and deep water. The upper 150 metres are home to low-salinity, oxygen-rich water that chiefly stems from rivers like the Danube. Below it gathers heavy, high-salinity water, which flows from the Mediterranean and over the Bosporus to the Black Sea. Yet on the surface there is no indication that the Black Sea is a stagnant body of water in which bacteria, due to the lack of oxygen, produce foul-smelling hydrogen sulfide.

This highly reactive molecule, the new study shows, can form compounds with a diverse group of substances containing carbon that can be found in every litre of seawater: the dissolved organic matter (DOM). In turn, the products of the reaction are longer-lived than the reactants, and accordingly accumulate in the water.

The team compared water samples from various sites within and beyond the Black Sea, and found that nearly a fifth of all organic molecules in the anoxic areas of the Black Sea contained sulfur – far more than in other seas. In addition, the team confirmed that a large percentage of compounds were unique to the Black Sea. Their conclusion: the sulfurous substances are created there, via chemical reactions in the hydrogen-sulfide-rich water.

Since dissolved organic matter represents an enormous carbon sink – taken together, the world’s seas and oceans contain roughly as much dissolved carbon as there is CO2 in the atmosphere – the outcomes of the study are also climate-relevant: “From 1960 to 2010, the volume of anoxic ocean regions quadrupled. Consequently, in the future this sulfur-based mechanism for carbon storage could have an influence on ocean chemistry,” explains first author Gomez-Saez, while conceding that, under current conditions, this feedback effect is too weak to appreciably affect climate change. However, in the Earth’s geologic past, there were several periods in which broad expanses of ocean were anoxic. Back then, the effect may have helped to gradually remove carbon dioxide from the atmosphere.

The water samples from the Black Sea were gathered on a cruise with the research vessel Maria S. Merian. In addition to the AWI / ICBM team, experts from the Max Planck Institute for Marine Microbiology in Bremen took part in the study.