Overlooked riverine inputs of dissolved neodymium and hafnium to the ocean.
Overlooked riverine inputs of dissolved neodymium and hafnium to the ocean.
Rivers carry vital nutrients and trace metals into the ocean. These elements carry characteristic isotopic signatures that geologists can use to identify where the water in the ocean comes from. As part of the international GEOTRACES project, which aims to map trace metals in the global ocean using such signatures, researchers from GEOMAR Helmholtz Centre for Ocean Research Kiel studied the waters of the Amazon estuary. In doing so, they have discovered that the hitherto ignored Pará river system has a major influence on the composition of the water masses there. Their study has now been published in the journal Nature Communications.
The Amazon River is the largest river in the world. It discharges about one fifth of global freshwater runoff, resulting in a freshwater plume rich in nutrients and trace elements entering the Atlantic Ocean. Until now, it was assumed that the suspended solids partially dissolve in the water plume of the estuary and thus represent an important source of trace metals, but latest results refute this theory. Isotopes of the elements neodymium (Nd) and hafnium (Hf) were examined. These can serve as tracers or origin, i.e. their analysis can be used to determine where water masses come from. Each river has its own isotopic signature that represents the source rock in the hinterland.
"A previous study had found an increase in the dissolved concentration and variability of neodymium isotopes in the Amazon estuary and concluded that these are dissolved from particles carried by the river on its way to the open ocean," says the study's first author Antao Xu. He is a PhD student in the Chemical Paleoceanography group headed by Professor Dr Martin Frank at GEOMAR Helmholtz Centre for Ocean Research Kiel, who was co-chief scientist of the METEOR expedition M147 (official GEOTRACES process study GApr11) in the Amazon estuary (chief scientist was Prof. Dr Andrea Koschinsky, Constructor University Bremen). "We have now disproved this conclusion," says Martin Frank. "We can show that the changes in isotope composition are a result of the admixture of freshwater from the nearby Pará River."
Read more at Helmholtz Centre for Ocean Research Kiel (GEOMAR)
