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.
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 are absorbing more and more each year. 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