Seawater is a cocktail of elements. Some – like the sodium and chloride that make it salty – are abundant. Others exist in vanishingly small quantities but pack a powerful punch. Iron controls where life can thrive; mercury has the power to snuff it out; and a delicate balance in selenium levels can drive bursts in biodiversity and mass extinctions.
Like unseen puppet masters, these trace elements control all the living things in the oceans, yet stubbornly resist our best efforts to detect them. Now, for the first time, elaborate studies are revealing where they come from and the grip they exert on ocean ecosystems.
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Iron
Even in iron-rich regions of the oceans there is just 30 nanograms of it per kilogram of seawater. It is so critical that it is the main factor limiting life in one-third of the ocean – all living things need iron to survive, and it is essential to photosynthesis. This has led to suggestions that we should fertilise the oceans with iron to promote plankton growth and stem climate change.
Since the industrial era, oceans have sucked up roughly 40 per cent of the carbon dioxide we have emitted, and . Some CO2 simply dissolves into the water, but the rest is taken up by phytoplankton during photosynthesis.
To figure out whether sprinkling the waves with iron would boost CO2 uptake, we need to know how iron cycles through the oceans. The basics have been known for decades, but recent studies have thrown up a few surprises. For instance, we used to think iron was primarily eroded from rocks on land and carried to the oceans by winds or, to a lesser extent, rivers. We now know that is only part of the story.
GEOTRACES is a global collaboration of oceanographers aiming to map the movement of trace elements in the oceans. In August, it released the results of the first 10 years of its efforts. That data includes nearly 2000 samples from below 2000 metres – a massive improvement on the eight readings that had previously been collected in 10 years of deep ocean iron measurements.
Cruises in the Atlantic, Pacific, Southern and Arctic oceans revealed that deep-sea hydrothermal vents . Iron doesn’t hang around in the water for a long time – no more than a few years – before sinking to the sea floor. This makes its distribution patchy, which suggests iron fertilisation would be more effective in some regions than others.
Nitrogen
The importance of nitrogen wasn’t lost on the architects of the 20th century’s green revolution, which saw a huge rise in the use of nitrogen fertilisers and the dawn of intensive farming. Without it, organisms can’t build proteins, and in many land-based ecosystems, the amount of nitrogen available to plants is the primary factor setting a limit to how much life can thrive. Agriculturalists of the 20th century realised this meant that by adding more nitrogen to the soil, they could boost crop yields.
The price we pay is that large quantities run off farmland and into rivers and the oceans. Yet despite the extent to which we are supercharging the nitrogen cycle, we know surprisingly little about what happens once the element enters our oceans. One recent study suggests it is influenced by the Coriolis effect, where ocean currents are deflected by Earth’s rotation. Beyond that, how it moves around the seas is a bit of a mystery.
What we do know is that too much nitrogen in the oceans is a problem. Excesses trigger explosions in algal growth that are large enough to suffocate everything else, creating what’s known as a dead zone. These are set to worsen: a study published in July shows that predicted increases in rainfall due to climate change will push more nitrogen into the oceans.
It’s a vicious cycle, too. As well as being influenced by climate, ocean nitrogen levels affect the climate. Waters that are rich in nitrogen, for example due to agricultural run off, help the formation of nitrous oxide. Some of that ends up being emitted into the air above, which is a problem because nitrous oxide is a greenhouse gas and it depletes ozone. A third of atmospheric nitrous oxide comes from the oceans – which just goes to show how the hidden trace elements of the oceans can impact life across the globe.
Phosphorus
Phosphorus is part of the structure of DNA, cell membranes and the energy molecule ATP. As a result, its availability sets a limit to how much life there can be in the oceans, together with nitrogen and iron.
Nitrogen has historically been the big one, setting a cap on life in 60 to 70 per cent of the oceans. But phosphorus is creeping up in the stakes, as intensive fertiliser use on land releases more nitrogen into the oceans. As a result, some oceans, like the tropical North Pacific, now have plenty of nitrogen.
Elsewhere, things are a bit more complicated. In some ecosystems, two nutrients put an upper bound on how much life can exist in one location at the same time, perhaps because two or more organisms in that environment have different requirements. Current estimates suggest that 5 to 10 per cent of the surface of the ocean is co-limited by nitrogen and phosphorus.
Increasingly, researchers are finding that these interactions between two trace nutrients are in control. “The more experiments we do, the more systems we find that are co-limited, so it’s clearly more prevalent than many of us thought it was,” says Mark Moore at the University of Southampton. His team is investigating co-limitation by lifting tanks of phytoplankton out of the sea and adding nutrients to see what happens. “It’s kind of bucket biology, it’s just that you need a very clean bucket because a lot of the work we do is with trace metals.”
Selenium
There is a narrow window where levels of this micronutrient are just right for life – call it the selenium Goldilocks zone. Selenium is essential for most organisms, including us. We get it from various foods – fish, chief among them – and build it into the proteins we need for making DNA, for reproduction and to regulate our hormone levels, among other things. It even helped drive the Cambrian explosion some 500 million years ago, when most animal groups first appeared on Earth.
But in higher concentrations it can be toxic. “This is the case in the San Francisco Bay and some lakes and reservoirs in the US and elsewhere,” says Greg Cutter at Old Dominion University in Virginia. In the mountains upstream of San Francisco Bay, selenium is eroded off the rocks and carried into the bay – a journey that has been amplified by irrigation for agriculture. Nearby oil refineries also discharge effluents rich in selenium into the ecosystem.
“We need selenium to make DNA but in high levels it can be toxic”
Federal limits were put in place to stem the arrival of selenium after deformed ducks and other animals were found in wildlife refuges downstream in the 1980s. Recent studies suggest that these limits are still too high, triggering calls for more stringent regulations.
The delicate balance of selenium has caused havoc at the other end of the scale, too. Low selenium levels have been linked to no fewer than . At the end of the Ordovician, Devonian and Triassic periods – when three unexplained mass extinctions occurred – selenium levels in the ocean dropped two orders of magnitude lower than their current levels, well below what’s needed to support animal life.
Mercury
Mercury isn’t a force for good. It has no function within cells and is toxic to nervous systems. It has been found in concentrations approaching toxicity levels in oceans and lakes, accumulates as methyl mercury in aquatic organisms, and causes brain damage, birth deformities and reduces reproduction rates. Yet how much there is, and where it all ends up, has been unclear.
Because of mercury’s hazardous nature, 128 countries have signed up to the Minamata Convention in order to curb its release. In April, between 1850 and 2010 was published, showing that 1.5 million tonnes were released due to human activities alone. That means 78 times more mercury has been released by things like burning fossil fuels than by natural sources – far more than had previously been thought.
“Mercury from the industrial revolution still has the potential to affect us today”
“Once mercury has been released into the [environment], it bounces around for a long time, on the order of hundreds of years to millennia,” says Elsie Sunderland at Harvard University, who contributed to the inventory. So all that mercury from the industrial revolution still has the potential to affect us today.
It’s not just the overall volume of mercury from human activities that has been a surprise. The latest data also sheds light on where it comes from. According to the 1850-2010 inventory, coal burning has accounted for 38,000 tonnes of the stuff. This was previously thought to be the largest human source, but it turns out there’s a far bigger culprit: gold production has chucked out 221,000 tonnes and silver production 365,000 tonnes.
“Most people think human activity has dramatically perturbed the carbon cycle,” says Sunderland. “That same realisation has not taken place for many of the other cycles of the elements that we have been using in human activities. I wouldn’t say there’s too much of a focus on carbon, but our focus needs to be on more than just that.”
Lead
Like mercury, lead accumulates in the food chain. It is absorbed by plankton, and moves into herbivores and the animals that eat them. It can be toxic to all of these, and ultimately to the animal right at the top of the seafood chain: us. Because levels get higher as it moves up the chain, tiny amounts in seawater can have large consequences for big predators. It meddles with enzymes and how our cells function, is harmful to our hearts and kidneys, and can cause irreversible brain damage.
Almost all the lead in the oceans has come from human activities. At the same time, it is tricky to measure. For a long time, ships were coated in lead paint; water sampling devices were made of plastic that contained lead; there is even lead in the air around a ship, from the fumes it belches. “It’s basically everywhere in the human environment,” says Ed Boyle at the Massachusetts Institute of Technology.
To get accurate measurements of lead in seawater, researchers have had to come up with complicated protocols. “We generally don’t sample in the upper 20 metres or so of the water, as that’s basically bathing in ship juices,” says Phoebe Lam at the University of California, Santa Cruz. Sampling tubes are stored in containers filled with filtered, pressurised air. The container doors, made of metal, have to be opened by someone who won’t touch the tubes. Metal frames that hold the tubes are powder coated and steel cables that hold the frames are wrapped in Kevlar. Later, there is the challenge of detecting trace atoms in seawater samples. “You’ve got 3 per cent salt in the water,” says Boyle. Compare that with the tiny amount of lead (see Graph), and “you are looking for a needle in a haystack if you want to see a lead atom in all that sodium chloride”, he says.
Attempts are finally bearing fruit. They have revealed that since the US, Europe, Canada and Mexico banned lead in petrol in the 1990s and 2000s, there has been a 10-fold reduction in lead levels in the North Atlantic. Elsewhere, it’s not such good news. In the North Pacific, initial reductions have been overwhelmed by recent Chinese emissions, mainly from burning coal.
Neodymium
Some trace elements are neither a nutrient or a toxin. Our interest lies in what they tell us about other trace elements. The rare earth element neodymium, for instance, helps us follow the journey of other elements.
That’s all down to how neodymium ages in rocks. The younger the rock, the higher the ratio of two isotopes, neodymium-143 to neodymium-144. When a rock is eroded, the dust that is blown into the ocean carries a neodymium fingerprint with it, like a time stamp.
“The age of the rock has been mapped around the world,” says Bob Anderson at Columbia University’s Lamont-Doherty Earth Observatory. This means that when researchers find neodymium in the ocean, they can look at its time signature and match it to places on land that have the same age to figure out where it is most likely to have come from. And because trace elements often travel together, it helps them identify probable sources of other elements like iron.
Pinning down key sources on land could help us protect the oceans. For instance, farming new areas or building new roads and houses can hold dust down. If we know which regions produce dust that is a particularly important source of iron to the oceans, then we can avoid starving oceans that are thousands of kilometres away.
This article appeared in print under the headline “Seven elements that rule the waves”
Article amended on 7 December 2017
We have corrected the amount of salt in seawater in the graphic headed “Needles in a wet haystack”.
