Ocean trace element cycles.

The oceans are salty. Each litre contains about 35 g of dissolved salts, resulting from the balance between inputs and outputs to the ocean over geological time. The constituents of common salt—Na and Cl—make up more than 90% of this dissolved mass, and together with Mg, S and Ca, represent 99%. Add only three more elements— K, Br and C—and you have more than 999 parts in a 1000 of the dissolved mass in seawater. Yet there are 82 other naturally occurring elements, and all are found dissolved in seawater, sometimes at extremely low concentration. It is tempting to think these other elements are at such low concentration that they simply do not matter. It is particularly tempting when trying to measure them. It can be a phenomenal analytical challenge to measure at such low concentrations, in a matrix with high ionic strength, on samples recovered in hostile ocean environments from kilometres below the ocean surface, and on ships that represent a huge potential source of contamination. But these low concentration elements— the trace elements—are fundamental to the functioning of the ocean and its ecosystems. Iron, for instance, makes up only one-billionth of the dissolved mass in seawater, but is an essential nutrient, required for all life in the ocean, and is often the nutrient that limits life when Fe concentrations fall close to zero in surface waters. Without knowledge of the cycle of Fe, we cannot understand the controls on ecosystems in the ocean, nor their role in the global cycling of carbon and other elements. Another example, mercury, makes up one-trillionth of the dissolved mass in seawater, only one part in 1012. Hg is highly toxic, naturally bioaccumulated so that it enters the human food chain in seafood, and prone to significant anthropogenic contamination. Assessing safe diets and setting policy on future use and release of Hg rely on accurate understanding of the oceanic Hg cycle. A further three orders of magnitude lower in concentration, radium makes up just one-quadrillionth of the dissolved mass in seawater; one part in 1015. The four radioactive isotopes of Ra, however, provide us powerful tracers of ocean processes, including the rates of mixing and of groundwater input.