The world’s so-called dead zones, where nothing or very little can live, sheds light on life on this planet – but also what to expect on worlds outside our own.
Many are created when a surfeit of nutrients in water causes a rapid increase in algal growth which then dies and in turn produces an over-abundance of bacteria that deplete the oxygen supply.
Too much salinity or simple aridity are other causes. They are fascinating, often alien places, and frequently dangerous to visit. Here are 10 of the most extraordinary of them all.
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10 dead zones
The Gulf of Oman
The world’s largest dead zones is the Gulf of Oman just to the east of the Strait of Hormuz – the narrow stretch of water that separates Iran from the United Arab Emirates that has become the focus of global attention since the latest conflict with Iran began – and scientists believe it’s getting bigger.
The gulf spans a gigantic 180,000km2, an area roughly four times the size of the Irish Sea, and it’s thought it’s being fuelled by warming sea temperatures and nitrogen and phosphorus run-off from the use of fertilisers. The almost complete absence of oxygen in the area means marine plants, fish and other marine creatures cannot survive there.
The Atacama Desert
One of the driest places on the planet, the Atacama Desert, which stretches for some 1,600km north to south in northern Chile, was used as a testing ground for equipment NASA sent on its Mars 2020 rover mission.
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The extremely hot and salty environment makes life almost impossible, though some very tough bacteria were detected while the rover underwent thorough field trials. The scientists who carried out this work say where they did find signs of life could help colleagues do the same on Mars – though, to date, there has been no significant breakthrough there.
The Danakil Depression
Known as the Gateway to Hell, the Danakil Depression is a 10,000km2 area of northern Ethiopia that looks like a scene from a science fiction film. Lakes of lurid-coloured acids and copper and cobalt deposits create an eerie, alien world. It lies 100m below sea level and is intensely volcanic and incredibly hot – temperatures regularly reach 45˚C and can get above 50˚C, near the limit at which humans can survive. There’s little in the way of complex life here, but polyextremeophiles – bacteria that are adapted to high acidity, temperatures and salinity – have been detected.
Mount Everest
Not surprisingly, little survives at 8,849m at the top of the world’s highest mountain on the border between China and Nepal. Indeed, there are few signs of life above 6,000-6,500m. That said, scientists have collected soil samples from Everest’s South Col at 7,900m and detected highly specialised bacteria from theModestobacter genus and fungi from the Naganishia genus.
Their research describes how they grew cultures of these micro-organisms, but they believe at these extreme heights, the organisms would not be active. Indeed, they speculate they have been deposited there from the atmosphere or by some of the many climbers on the way to the summit of this most sacred of peaks.
Antarctica’s Dry Valleys
When a research team visited the McMurdo Dry Valleys of Antarctica’s Victoria Land, they collected soil samples to see what was living there – even in the most extreme locations on Earth, there’s usually something. But there was nothing. "We couldn't even detect DNA in these samples," said Professor Byron Adams. "No life there, period. At least not in the last 20-30,000 years or so.”
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The strange thing about these dry valleys is that, unlike most of the rest of Antarctica, they aren’t covered in snow – it’s bare ground. The lakes, which are covered in a layer of ice 3-5m thick throughout the year, are home to microbial mats of cyanobacteria.
The Dead Sea
Given its name, you wouldn’t expect there to be much life in the Dead Sea – and there isn’t. Though there are freshwater springs that support lush vegetation around its 600km2 expanse, the Dead Sea has a salt density of about one third, roughly 10 times higher than our oceans and seas. No complex life forms – fish or other animals, or plants – survive in its extreme environment, but scientists have found salt-loving microbes called halobacteria which can give the water a reddish tinge, as well as occasional blooms of a green micro alga called Dunaliella salina.
Mississippi River Delta
Every summer, a dead zone develops where nutrient-laden, freshwater from the Mississippi spills into the Gulf of Mexico, and it can reach a total area of 18,000km2. While mobile fish and marine mammals and some other organisms can easily move away from this dead zone, everything else dies because of the lack of oxygen in the water, leaving a vast, barren expanse. One of North America’s longest rivers, the Mississippi drains an area of 2.5m km2, bringing potentially deadly nutrients from fertilisers used in farming to the Gulf.
The Black Sea
There’s plenty of life at the surface of the Black Sea, but below 150-200m (and it has a maximum depth of more than 2,000m), nothing at all. At these depths, there’s a permanent hydrogen sulphide zone where oxygen, and therefore respiring organisms, are absent. For a variety of reasons to do with its geography and hydrology, there are two distinct layers of water in the Black Sea which almost never mix, thus creating the conditions in which an anoxic dead zone can exist. At the surface, however, a wide range of fish, marine mammals including dolphins and otters and other life forms flourish.
Rub’ al-Khali
Covering an area larger than France, Rub al-Khali – an Arabic name meaning the empty quarter – receives just 3cm of rain a year, half that of the renowned Death Valley in the USA. Explorers in the early part of the twentieth century heard rumours of a lost city, leading TE Lawrence – Lawrence of Arabia – to talk about it as the Atlantis of the Sands. But nothing has ever been found.
Despite the vast stretches of seemingly lifeless sand dunes, there is some scarce wildlife here, including Arabian oryx, Arabian gazelles and ostriches which have all been reintroduced to the Shaybah Wildlife Sanctuary.
Brine pools
Brine pools are so strange they sound like the creation of someone’s fertile imagination – underwater lakes that are so salty and dense, they literally have their own shorelines, surfaces and even waves.
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These bizarre natural phenomena have only ever been found in the Gulf of Mexico and the Mediterranean and Red Seas. And while the lack of oxygen within these pools makes them lethal to wildlife, creatures, including mussels, tube worms, amphipods and sharks, amass at their boundaries. Shrimps gather at the very edge of the dead zone and wait for a hapless creature to get caught and killed by its lack of oxygen. Hagfish have evolved to be able to temporarily enter a brine pool to retrieve animals that have died within them.
