Tracing metals

By Torben Stichel, University of Southampton

When Prof. Rachel Mills (Head of Department in Ocean and Earth Science, University of Southampton) asked me if I’m willing to help out on one of the Shelf Sea Biogeochemistry Programme’s benthic cruises and carry out some own research, I didn’t hesitate to say yes. I love the ocean, studying it, and before joining the University of Southampton as a Research fellow, I had already thought about 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 litres 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 five years now – a study that involves collecting and processing thousands of litres of seawater. 

Recovery of our trace metal clean rosette that collected seawater at various depths. It is equipped with a conductivity, temperature and pressure, i.e. depth, sensor (CTD). Credit: Torben Stichel

Neodymium is a lithogenic element, which means it comes from land into the ocean via various weathering sources. 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 isotope 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 isotope 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. 

RRS Discovery. Image: Torben Stichel

For this reason 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 like the Celtic Sea are potentially significant sources of neodymium into the ocean. So connecting shelf seas’ processes with the global ocean conveyor belt will help us to better understand the cycle of neodymium and trace metals in general in the ocean. 

Why is that important for us? 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.