'Chemical atlas' provides unique understanding of ocean geochemistry

The atlas, which includes 3D maps and rotating images, provides a wealth of information including the distribution of micronutrients, such as iron, zinc and cadmium. They are important for the growth of marine phytoplankton (microscopic floating plants) and determine how much planet-warming carbon dioxide the ocean can soak up through biology, plus they can indicate areas of lead contamination from cars burning petrol laced with the toxic metal.

The $300m GEOTRACES project has involved researchers from 30 labs in 10 countries gathering data on almost 30 cruises since 2010. They have collected nearly 30,000 water samples at various depths from 787 locations from around the world and developed strict sampling and analytical methods.

The researchers measured more than 200 substances, both manmade and natural, and have produced incredibly detailed digital maps of important elements such as iron, aluminium, lead and cadmium. Iron can fuel plankton blooms and influence how the ocean responds to climate change, while the lead images show the impact of past pollution on the ocean and continuing contamination in some parts of the world and aluminium is used as a tracer of desert dust inputs to the ocean.

Ocean and Earth scientists from the University of Southampton have been at the forefront of the UK GEOTRACES activities. Professor Eric Achterberg led the GEOTRACES cruise in 2011 in the tropical Atlantic and his group undertook the dissolved trace element work on the cruise in the South Atlantic.

Professor Achterberg, who is based at the National Oceanography Centre, Southampton, says: “There is an urgent need to understand micronutrient, trace element and isotope cycles to advance our understanding of ocean biogeochemistry in the present day ocean and in the past.

“A fundamental principle of GEOTRACES is that measurements of a range of chemical species along sections crossing chemical gradients provide complementary information that cannot be derived from isolated studies of small subsets of such species. The wide geographical and analytical efforts required to measure many chemical species and isotopes in all oceans is a major undertaking, beyond the scope of a single nation and requires international co-operation.”

Dr Christian Schlosser, a chemical oceanographer formerly at the University of Southampton, has recently published his latest work that analysed the distribution of iron in the surface ocean of the Atlantic and its biogeochemical consequences, as part of the GEOTRACES project, in the journal Proceedings of the National Academy of Sciences (PNAS). The paper ‘Seasonal ITCZ migration dynamically controls the location of the (sub) tropical Atlantic biogeochemical divide’ highlights how dust blowing off Africa’s deserts amplify surface levels of iron in the Atlantic that in turn control the spatial activity of nitrogen gas fixing phytoplankton and phosphate content in the Tropical Atlantic.

Dr Schlosser says: “This study is unique and makes a pivotal contribution to our understanding of the biogeochemical cycles of trace elements in the world’s oceans.”

Other University of Southampton authors on the paper are Jessica Klar, Dr Bronwyn Wake, Joe Snow, David Honey, Professor Eric Achterberg and Dr Mark Moore.