River transport accounts for up to 90% of the flux of material delivered to the oceans, with the suspended particulate phase being the overwhelming vector compared to the dissolved load. This highlights the importance of river-suspended sediments when studying the biogeochemical cycles of elements at the Earth surface. However, the previous studies focus mainly on river systems that drain pristine basins or on elements having mainly natural origins, little is known about the element geochemistry in the solid phases of smaller draining intensely populated and industrialized regions, where river chemistry also integrates the impact of human activities.

“Anthropophile elements” was proposed for the first time by Professor CHEN Jiubin from the State Key Laboratory of Environmental Geochemistry (SKLEG), Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) and his collaborators in their systematic study of the elements in the Seine River (France).

Their new approach of the study was obtained based on the most extensive sediment sample sets ever collected on the Seine River (maybe even on any river): a 3 year-long time series of suspended particulate matter (SPM) in Paris, a basin transect of SPM from source to estuary, coarse fresh riverbank deposits (RBD), two depth-profiles of SPM collected in Paris, a set of samples reflecting typical anthropogenic inputs (such as plant-treated wastewater), and representative bedrocks. The researchers measured concentrations for a large array of chemical elements (major, minor and trace), thereby producing an unprecedented dataset for sediment chemistry over a fairly large river. Importantly, this dataset accounts for sediment chemistry variability with grain size and discharge variations along the hydrological cycle.

The study reveals that important enrichments are due to heavy mineral hydrodynamic fractionation between SPM and RBD and within SPM itself (as a function of depth or along the hydrological cycle). This results in enrichment / depletion patterns of elements (e.g. Ca, Na, Zr) that are greatly depending on the hydrodynamics of the river. Moreover, significant additional enrichments, most prominent at low water stage, in a series of “anthropophile” elements such as Cd, Cu, Mo, Pb, or Zn, are due to anthropogenic input. This additional input at low water stage can represent more than 10-20% of the total flux of these elements over a year.

By systematic comparison with the Seine SPM, the study indicates that the Amazon River, displaying similar geochemical characteristics as UCC (upper continental crust) the finer natural end-member of the Seine SPM, may serve as a pristine reference for elements in human-impacted rivers. The study also points out that the RBD have different geochemistry from both low and high water stage SPM, the coarse floodplain deposits cannot thus be used as a surrogate for fine suspended sediment composition.

Given that elements such as Zn、Cu、Cd、Pb are strongly impacted by human activities and are characterized with contrasting geochemical signatures, and based on the generally accepted conceptions (lithophile and siderophile), the researchers brought up “anthropophile elements”. Their findings emphasize the need for systematic studies of these anthropophile elements in other human-impacted rivers and environmental geochemical studies using geochemical normalization techniques, and stress the importance of studying the chemical variability associated with hydrodynamic conditions when characterizing riverine element geochemistry and assessing their flux to the ocean.

The above results entitled “Anthropophile elements in river sediments: Overview from the Seine River, France” was published in Geochemistry，Geophysics and Geosystems.

Contact:
CHEN Jiubin
Institute of Geochemistry, Chinese Academy of Sciences
E-mail: chenjiubin@vip.gyig.ac.cn

(By WANG Zhuhong)