Using noble gases and tritium to identify trace metal sources and scavenging in the oceans
Trace metals include elements such as zinc, lead, nickel, manganese, iron, copper, cobalt, chromium and aluminium. They are also known as micronutrients as they are required in extremely small amounts in a wide range of physiological processes that are essential to life on earth.
The distribution and concentration of most trace metals in the oceans is not well constrained. At present there are ~60 published measurements for iron (Fe) and ~30 published measurements of zinc (Zn) in oceanic waters deeper than 2000 metres. A clearer picture of sources, sinks and the cycling of trace metals in the oceans is required to understand their role in biological productivity and biogeochemical cycles in the oceans. Although the distribution and cycling of macronutrients (chemical elements required in large amounts by organism), nitrogen, phosphorus and silicon, is well understood this information has proven insufficient in determining their role in biogeochemical processes, as it is micronutrients that are the limiting factor in biological productivity in the oceans.
Up until recently it was not possible to measure the tiny amounts of trace metals that exist in the oceans. Now there are standardized ultra-clean measurement procedures that allow not only the detection of trace metals but the intercomparison of these observations across different research labs, regions and time. Fe is one such trace metal and it is extremely difficult to sample its concentration (nmol/kg) especially as most sampling at sea takes place from a steel ship collecting water samples using bottles attached to metal wires.
So what is the connection between detecting trace metals in the ocean and noble gases? Noble gases are are not affected by biological or chemical processes, only by physical processes. Noble gases, in particular 3He and 20Ne allow us to identify source inputs of trace metals. Trace metals are known to be released along mid ocean ridges.
The concentration of helium in the atmosphere is ~5 ppm. It has very low solubility in seawater (2 nmol/kg). Helium in the atmosphere is a mixture of two stable isotopes 3He and 4He. The isotopic ratio of 3He/4He is 1.4×10-6 in air with 4He being one million times more prolific than 3He. Helium in the surface waters of the world ocean is in solubility equilibrium with the atmosphere. Hydrothermal activity on the seafloor are a source of helium to intermediate depth waters in the ocean. The ratio of 3He/4He in helium that originates from mantle out gassing is between ten and thirty times greater than the atmospheric 3He/4He ratio. 3He/4He ratios are therefore a useful indicator of the origin of intermediate depth waters. Alongside hydrothermal inputs of 3He there are also atmospheric inputs of 3He. To separate this signal from 3He released from the hydrothermal sources 20Ne, which comes only from the atmosphere also needs to be measured.
Fe and manganese (Mn) are two trace metals thought to be released at mid ocean ridges. It was thought at one point that the amount of dissolved Fe supplied to the ocean from mid ocean ridges was limited as iron from hydrothermal sources precipitates out of solution very quickly and therefore is not available to biology. More recent work suggests that there is an increase in dissolved Fe concentration around mid ocean ridges as iron binding ligands prevent iron from precipitating out of solution. In the oceans, ligands usually take the form of organic compounds produced by micro-organisms, but little is really known about their composition and origin. They hold Fe in solution making it available for biological processes. This means that hydrothermal sources of traces metals especially Fe are important for primary production in the ocean.
With so few observations of trace metals from intermediate and deep waters, those >1500 metres in depth, it is difficult to assess sources of micronutrients. Some recent work has successfully linked Mn and 3He concentrations in the northern Drake Passage to identify water with a Pacific hydrothermal vent signature thus identifying the Mn source at intermediate depths (1500 to 2500m). Noble gases (3He/4He ratio and 20Ne) along with tritium (which I will talk about another time) allow us to identify source inputs of trace metals from hydrothermal sources and allow us to trace the scavenging of trace metals away from the hydrothermal source. Identifing the source inputs of micronutrients and means a greater understanding of their biogeochemical cycles and their importance in biological productivity in the oceans.