A GEOTRACES process study in the Amazon estuary

Interactions of trace metals with dissolved organic matter and colloids in the Amazon and Pará River estuaries and associated plumes as key processes for trace metal fluxes into the Atlantic

The Amazon and Pará River systems discharge the earth’s largest volume of freshwater into the ocean and form an important interface for trace metal and dissolved organic matter (DOM) input from land into the ocean. These huge river-derived nutrient fluxes have a large influence on the biological productivity within the coastal and shelf region and beyond. The main goal of the project is to understand the role of chemical speciation and physico-chemical size fractionation of trace metals in the mixing continuum of the Amazon River and the neighboring Pará River to the Atlantic Ocean. We study the interactions of trace metals with DOM and colloids in the water column and surface sediments of the Amazon and Pará estuaries and the associated mixing plume as well as the mangrove belt with groundwater discharge southeast of the Pará River. This will help to better understand the contribution of large DOM-rich tropical rivers to the trace metal budget in the ocean.

Images from the M147 expedition highlighting the different colors of the Amazon river in different locations

Based on surface water and depth profile samples as well as sediment and pore water samples taken during RV Meteor research cruise M147 in the high-discharge period of 2018, we investigate changes of trace metal distributions and speciation in the larger Amazon outflow region along the salinity gradients. To assess what controls the chemical and physical speciation and transport of trace metals in the estuary and plume we focus on three different processes:

  • Trace metal size fractionation, sorption and removal:  sorption of trace metals on riverine particulate matter and removal due to coagulation of colloids, and size fractionation; how do trace metal association with different soluble, colloidal and particulate fractions change along the salinity gradient, and what does this mean for the source-sink balance?
  • Solution complexation: formation of soluble metal-organic complexes; how does this process enhance metal transport by competition with the sorption on colloids and removal in a DOM-rich estuarine system? 
  • Accumulation of trace metals in sediments: how do sediments incl. pore waters act as a source and sink of trace metals and contribute to the trace metal fluxes in the region? 

Sketch of the larger Amazon and Pará estuaries and plume as well as mangrove belt with trace metal processes to be studied in this project (Figure by A. Hollister)

In addition to voltammetric and ICP-MS analyses of the M147 samples, we complete a systematic investigation of the mixing behavior of diverse groups of elements (conservative, particle-reactive, and complexed with DOM) by conducting laboratory mixing experiments with seawater and river water endmembers collected during the cruise M174 in the Amazon region. In conjunction with the data from our second approved GEOTRACES Process study cruise during the low-discharge period (anticipated for 2024), we expect to obtain a holistic picture of the complex processes of trace metal biogeochemistry and elemental fluxes in this largest (by water flux) estuarine system of the world. This knowledge will also be important to foresee potential effects in this area due to the ongoing anthropogenic impact in this region and drastically changing climatic conditions.

This project is carried out in collaboration of:

  • Jacobs University Bremen (Project coordination)
  • GEOMAR Kiel
  • ICBM, Univ. Oldenburg
  • University of Kiel
  • Universidade Federal de Santa Maria, Brazil
  • Universidade Federal do Rio Grande do Sul, Brazil
  • Universidade Estadual do Norte Fluminense, Brazil
  • Universidade Federal do Rio de Janeiro, Brazil
  • And other partners

Related publications of our team:

  • Schneider, A.B., Koschinsky, A., Krause, C.H., Gledhill, M., and de Carvalho, L. (2022). Dynamic behavior of dissolved and soluble titanium along the salinity gradients in the Pará and Amazon estuarine system and associated plume. Marine Chemistry 238: 104067.
  • Hollister, A. P.; Whitby, H.; Seidel, M.; Lodeiro, P.; Gledhill, M.; Koschinsky, A. (2021) Dissolved concentrations and organic speciation of copper in the Amazone estuary and mixing plume. Marine Chemistry 234: 104005. doi: 10.1016/j.marchem.2021.104005
  • de Carvalho, L. M.; Hollister, A.P., Trindade, C.; Gledhill, M. and Koschinsky, A. (2021) Distribution and size fractionation of nickel and cobalt species along the Amazon estuary and mixing plume. Marine Chemistry 236: 104019.
  • Schneider, A.B., Velasquez, I., Paul, S.A.L., Krause, C.H. and Koschinsky, A. (in review). Mixing and partially non-conservative behavior of molybdenum, uranium and vanadium along the salinity gradients of the Amazon and Pará estuaries and associated plume. Submitted to Marine Chemistry

Cruise M147 and the associated project are funded by the German Science Foundation DFG.