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Kinetics: Geochemical reactions between lithogenic matter and seawater

by Webmaster Legos last modified Nov 20, 2020 10:41 AM



Knowing that the weathered particulate element flux to the ocean is up to two orders of magnitude larger than the dissolved one, the objective is to improve the quantification of the amount of chemical species released by dissolution or desorption once these particles are entering in seawater. As part of the KINETICS project (LEFE, EC2CO, 2015-2017), we focused on the fate of two nutrients and their isotopes (Si and Ni) under agitated conditions, in order to simulate what is observed under sudden floods or under the pressure of the internal waves as in the surf zone


KINETICS addresses scientific questions related to processes occurring at the land-ocean interface, coupling concentration and isotopic composition measurements of dedicated tracers. Isotopic ratios of micronutrients (made possible by considerable improvement in mass spectrometry) are showing that most of their oceanic budgets are imbalanced: these imbalanced are critical because the missing sources (or sinks) are significant. In other words processes controlling the chemical state of the ocean are still badly constrained, which has consequences on the biological carbon pump estimate and therefore on climate simulations.


Strategy & organization

Experimental devices allowed simulating these turbulent conditions and following the kinetics of release of these elements.  


Results & Perspective

Quartz dissolution: Using pure quartz grains (sand of Fontainebleau) we demonstrated that the quartz dissolution kinetic (and DSi release) considerably increases with an agitation that simulates waves of 1meter. We fit the data with an original “shrinking sphere model”. On a global scale, the resulting additional DSi flux due to the wave effect could double the present flux of DSi to the ocean. (Fabre et al, 2018).
Mineral dissolution: Same experiments were done with other pure minerals (anorthosite, pyroxene, olivine, Figure). DSi is more released in the order pyroxene, olivine and anorthosite, according to their respective solubilities. DNi concentrations show a rapid increase and then a marked decrease, due to the formation of secondary phases (DRX analyses). Regarding the isotopic compositions, the most striking features are i) for DSi (left), the rapid dissolution is releasing light Si, but almost simultaneously the secondary phase formation is making the solution heavier while the concentrations are still indicating release or reaching a plateau; for DNi (right), the dissolution is also releasing light DNi and the precipitation of secondary phases enriching the solutions in heavy Ni. We are currently modeling this simultaneous dissolution/precipitation using a Monte Carlo approach. This work was presented at the Goldschmidt conferences of Yokohama (2016) and Paris (2017) and the article is in preparation.
Iron concentrations and isotopes are currently analyzed on the same aliquots of the second experiment (F. Lacan, post doc of J. Klar)

In addition, we would like to test the hypothesis of a “missing DSi flux due to the wave surf” on a specific adapted field (sand beaches, waves, …). Discussions with D. Cardinal (LOCEAN), E. Machu (IRD Dakar) and T. Gorgues (modeling) during the LEFE days (Clermont Ferrand) could yield a LEFE/EC2CO new application to develop these ideas off Senegal coasts in a near future.



Post doc RTRA: N Estrade; M2: Katia Yefsah
Fabre, S., Jeandel, C., Zambardi, T., Roustan, M. and  Almar, R. Clastic coasts: an overlooked term of the geochemical Si cycle? Frontiers in Marine Sciences, 2018.
Estrade N., S. Fabre, G. Quitte, T. Zambardi, N. Yin, C. Cloquet, A. Probst, and C. Jeandel Experimental dissolution of lithogenic material into freshwater and seawater  Intl Goldschmidt Conf. Yokohama, 2016 POSTER
Estrade N., S. Fabre, C. Cloquet, A. Probst and C. Jeandel Isotopic and modelling constraints on experimental dissolution of lithogenic material Intl Goldschmidt Conf. Paris, 2017 POSTER
Fabre, S., Zambardi, T., Roustan, M., Almar, R. and Jeandel, C. (2017) Clastic coasts: an overlooked term of the geochemical Si cycle? Intl Goldschmidt Conf. Paris, 2017 POSTER



Catherine Jeandel (CNRS, TIM), Nicolas Estrade (post-doc RTRA, TIM), Sebastien Fabre (PRAG, IRAP), Anne Probst (CNRS, ECOLAB), Katia Yefsah (Master 2), François Lacan (CNRS, TIM) et Jessie Klar (post-doc, TIM), Geraldine Sarthou (CNRS, LEMAR), Christophe Cloquet (CRPG)

Collaborations out of OMP: Geraldine Sarthou (CNRS, LEMAR), Christophe Cloquet (CNRS, CRPG); Derek Vance (ETH)



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