Improving our knowledge of the fluxes and of chemical elements and tracers transported from the continent to the ocean together with their impact on the chemistry of the ocean is of a prime interest since: i) on geological timescales, progress in weathering laws allowed estimating rates of CO2 consumption resulting from silicate alteration which, together with volcanic fluxes of CO2 allowed modeling the earth climate on scales larger than one million year; ii) on holocene timescales, transfer of material and elements from the continent to the ocean is a key factor driving the chemical state of the ocean and its variation during glacial/interglacial periods; iii) on present-day and future timescales, anthropization and global change are modifying these fluxes and one has to understand the impact of these modifications for the ocean health.
However, one of the main unknown for quantifying these fluxes concerns the particle natures and their transformations at the land/ocean interface, a major objective of GEOTRACES. Estuarine and margin systems are the favored locations where these transformations occur.
To this extent, studying the Amazon river mouth is of particular interest: the discharge of the Amazon river represents 20% of the freshwater discharge to the ocean and 6% of the global riverine sediment discharge, causing river/ocean mixing to take place out on the continental shelf, which waters are submitted to estuarine-like processes with freshwater lens extending 120 km offshore. The tidal influence is strong on the shelf, generating internal waves and recirculations; it can be seen up to ~100 km inland. Strong physical reworking and redox processes make that the Amazon shield appears as one of the largest source of dissolved elements to the Atlantic Ocean. The Amazon River was identified as the most probable source of low salinity zone with elevated concentrations of nutrients, particle organic carbon, Cu and Cd in mid Atlantic Ocean encountered far as off the African coast.
Two major results & prospective
Tidal modelling Important progress was made on the ocean circulation model for the Amazon plateau by the PhD work of Y. Le Bars (financed by ANR). Model-observation correlation standard deviation for the M2 wavelength was improved by a factor 2 (Le Bars et al, 2009) and a tidal atlas is currently under construction. As part of her Ph.D (2012), Christine Lion added the fresh water input to this model, thanks to the radar interferometry data. The final phase of this work will include an improved simulation of wave impact on sediment resuspension in order to properly account for sediment transport.
|Simulation of circulation on the Amazon shelf (left: high flood, may; right : low flood, nov; Lion, 2012)|
Geochemitsry (T. Rousseau thesis, 2013, french-brasil cooperation)
Futur…improving the wave impact modelling allowing us to take in account particle dynamic and chemical exchanges in these simulations; 2 on going publications of Rousseau et al; on going publication coupling Ra data and the circulation model (vanBeek et al, in prep.)
Some indicators…Today: 25 peer reviewed papers, 26 international communications, 4 thesis, 3 masters were based on AMANDES. Data are stored at SISMER, CYBER data bank and will be transferred to the Geotraces Data Center in 2014-2015.
Participants: Catherine Jeandel (PI), Pieter van Beek, Marc Souhaut, Tristan Rousseau, Florent Lyard, Yoan Le Bars, Christine Lion, Jeroen Sonke (GET)
Partnership and funding. LEGOS, GET, CEREGE (Fr); CNPq (Br) ; Universidade Federal de Pernambuco (Br); Universitade de Brasilia (Br)
Funding : INSU/LEFE, INSU/DT, IRD, ANR, CNPq