17:17 09-12-2025

Black Sea's hidden dead zone: hydrogen sulfide rising

Discover why 90% of the Black Sea lacks oxygen, how hydrogen sulfide builds in its depths, and what rising dead zones mean for coasts, fisheries, and climate.

Each summer the Black Sea coast slips into its familiar holiday rhythm: beaches fill with sunbathers, promenades hum with voices, vendors pass by with corn and sweets. Standing on a lively seafront, it’s easy to forget what lies beyond the horizon.

Behind that tranquil scene hides a system with no real counterpart anywhere in the world’s oceans. An enormous share of the sea—about 90% of its volume—contains no oxygen. Down in the depths, where life cannot survive, hydrogen sulfide builds up. We’re talking about quantities measured in billions of tons.

Scientists describe this zone as one of the region’s most unusual—and most troubling—natural features, a reminder that beauty on the surface can mask a far more complicated reality.

When the sea was a lake

Roughly 7,000 years ago, the Black Sea was a freshwater lake. Water levels were much lower, and ancient communities lived along its shores. Then glaciers melted and global sea levels rose. The Mediterranean burst through a natural barrier, and salty water poured into the lake through the Bosporus.

The rise was rapid—geologists estimate rates of up to 15 centimeters a day. Freshwater organisms died and sank to the bottom. Decomposing in a world without oxygen, they laid the groundwork for the hydrogen sulfide zone that exists today. Some historians link that dramatic transformation to the biblical story of the flood.

Two seas in one

The Black Sea’s structure is like nowhere else. The upper layer—down to roughly 150–200 meters—is oxygen-rich, home to fish, jellyfish, and plankton. Below it begins a different realm: saltier, colder, and devoid of oxygen. Only bacteria live there, breaking down organic matter and producing hydrogen sulfide. The boundary between these layers, the chemocline, is so abrupt it feels like a line drawn between the living and the dead.

Why the layers don’t mix

Density is the key. Heavier, saltier bottom water stays put, while the fresher surface layer remains above. Geography cements the divide. The sea connects to the ocean through the shallow Bosporus, where the depth is only about 27 meters. For a basin this large, that narrow neck does not allow a full exchange of water masses. According to oceanographers, it would take several centuries for surface water to reach the seafloor.

Bacteria building a dangerous layer

Only sulfate-reducing bacteria can survive in the depths. They draw energy from sulfates and release hydrogen sulfide—the gas known for its sharp, rotten-egg odor. Their numbers are immense—up to a million cells per milliliter of water. The process never stops, so the hydrogen sulfide zone keeps expanding.

Why hydrogen sulfide is dangerous

Hydrogen sulfide is highly toxic and acts quickly. In high concentrations it can cause respiratory paralysis. Even small doses irritate, trigger headaches, and dull the sense of smell—an insidious effect that makes the gas particularly treacherous. History records tragic releases of such gases, including those of natural origin.

When the sea caught fire

The starkest example came in 1927. After the Crimean earthquake, people along the coast reported fiery columns above the water. Scientists explain the spectacle as a burst of hydrogen sulfide that reached the surface and reacted with oxygen; methane likely rose with it, amplifying the effect. The episode was local, but it showed how fragile the sea’s balance can be.

The hydrogen sulfide boundary is rising

Research in recent decades points to a worrying pattern: the dead zone is creeping upward. The upper, oxygenated layer is shrinking as rivers carry pollution into the sea. Excess nutrients fuel algal blooms. When the algae die, more organic matter sinks, feeding bacteria and increasing hydrogen sulfide production. Climate shifts play a role too, making the layers more stable and vertical mixing weaker. That slow rise at the boundary feels less like an oddity and more like a warning.

Life only at the top

Because of this structure, the sea’s fauna is poorer than in other marine basins. The deep zone is entirely uninhabited. Yet the anoxic depths preserve shipwrecks that sank centuries ago: without oxygen, wood and metal decay far more slowly.

What lies ahead for the sea

Oceanographers consider several scenarios. The most likely is a continued thinning of the oxygenated layer. That would affect fisheries and the coastal economy. Catastrophic releases are possible but considered highly unlikely. Still, local disruptions of stratification during earthquakes cannot be ruled out. The trend can be eased by cutting pollution, improving wastewater treatment, and restoring natural filters such as wetlands and limans.

A sea with a long memory and a complex design, the Black Sea reacts to everything happening around it—and today it is in a particularly sensitive phase. Its condition is central to the region’s ecological stability. How carefully sources of pollution are controlled and natural processes supported will shape the future of coastal ecosystems. The question is not whether the sea will respond, but whether we choose to notice in time.