Tracing Metals

Tracing metals

I love the ocean, studying it, and before joining the University of Southampton as a Research fellow, I had put much thought into the particular role of shelf seas in the global marine system.

In previous years I have put my focus on the deep ocean. I have been analysing trace metals in seawater to look at the big picture – how water masses with billions of liters per second are distributed along the ocean conveyor belt. I have looked at different tracers to understand where water masses come from and how they mix with each other. One particular tracer, neodymium, has been my focus for more than six years now – a study that involves collecting and processing thousands of liters of seawater from several expeditions.

Bad weather during a cruise to Antarctica.

Neodymium is a lithogenic element, which means it comes from land into the ocean via various weathering sources and it is only present in a few billionth of a gram per liter seawater. The cool thing about neodymium is that its composition in water masses gives direct information about their formation regions. For example, North Atlantic Deep Water has a distinct isotopic composition because its surrounding landmasses mix their isotope signal into the source region where this water mass forms. We can also reconstruct past ocean circulation to a certain degree with neodymium isotopes archived in marine sediments. The problem with this isotope system is that the observed values not always meet the expected ones. In other words: water mass mixing is not the only process that governs trace metal isotopic composition of seawater. Even though we have quite a good understanding on how water masses move and how they mix thanks to the help of reliable proxies, such as salinity, temperature and nutrients, there are processes involved, which we haven’t quite understood about neodymium, particular when it comes to sources and sinks of this element.

How do we analyse trace elements?

Their name already indicates the problem we have – they’re only present in traces in seawater. Most of our sample composition in seawater is dominated by major elements, like sodium, potassium or calcium. When analysing trace elements these vastly present elements need to be separated from the trace elements, we are interested in or they will disturb the analysis. This is particularly important when we look at the isotopic composition, which is an analysis sensitive to atomic mass differences of one particular element. Some elements share a very similar atomic masses, which the mass spectrometer (the instrument that measures these isotopic differences, e.g. a MC-ICPMS) can’t discriminate. So the presence of these interfering elements would cause an erroneous measurement and this is why we need to isolate the trace elements as good as possible.

 

Plasma torch of a Multi-Collector Inductively Mass Spectrometer (MC-ICPMS). Image: Torben Stichel

Why is that important for us?

I’m looking at ocean boundaries to better understand source and sink mechanisms that imprint the neodymium isotope signal on the water masses we are tracing. The shelf seas are potentially significant sources of neodymium and many other trace elements into the ocean. So connecting shelf seas’ processes with the global ocean conveyor belt will help us to better understand the cycle of trace metals in general in the ocean.

The climate of our planet has been changing on large (glacial to inter-glacial) and smaller scales (modern climate change). Much of these changes are closely linked with ocean circulation. Understanding proxies that trace water masses are therefore vital to reconstruct past, assess present, and predict future ocean conditions.