A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Carbon Accumulation in Amazon VárzeasMoreira-Turcq, P. 1* , Turcq, B. 1 , Seyler, P. 2 , Jouanneau, J.M. 3 and Guyot, J.L. 21I.R.D. - Institut de Recherche pour le Developpement, 32 avenue Henri Varagnat, 93143Bondy cedex, France.2 IRD - UMR LMTG, Université Paul Sabatier,39 allées Jules Guesde 31000 – Toulouse,France3UMR-CNRS 5805EPOC, DGO, Université de Bordeaux I, av. des Facultes, 33405 Talencecedex, France.* Corresponding author: Patricia.Turcq@bondy.ird.frFloodplains of large rivers or “Várzeas” (Amazon river floodplains) are dynamic andcomplex wetland systems which periodically oscillate between terrestrial and aquatic phases.Sediments are constantly exchanged between river channels and floodplains. The rates atwhich sediment is transferred to and from “várzeas”, and the residence time of “várzea”storage, are few known. They affect mineral erosion, transport and sedimentation fluxes in theriver-várzea system and have a special importance for the carbon cycle.Amazon “várzeas” are an important source of organic carbon to Amazon river.Organic production in Amazon “várzea” systems represent about 8.4 10 6 ton C yr -1 (Junk,1997), which are composed by : macrophytes (c.a. 5 10 6 ton C y -1 ), tree and grasses (c.a. 2.410 6 ton C y -1 ) and plankton (c.a. 1 10 6 ton C y -1 ). A significant portion of this carbon isexported by “várzeas” to atmosphere such as methane and CO 2 . A great part of the plankton(Richey, 1982) and the bulk of carbon (Junk, 1997) are probably degraded or is burned in situ.But the major part of the carbon produced by Amazon “várzea” seems to remain in situ andcan be subsequently exported to the river. Today there are very few data of organicsedimentation and carbon exportation by várzea lakes. The aim of this study is to evaluate thesedimentation and carbon accumulation rates in Amazon várzea systems and its importance inthe Amazon total carbon budget.Sediment traps and cores were used to better understand seasonal variation of organicand mineral matter and the recent organic carbon sedimentation in the Amazon “várzeas”.One core was sampled in the “Várzea do Lago Grande de Curuai” during low water stage(October 2000). Accumulation rates were obtained by 210 Pb radioisotope geochronology.Subsamples were analysed for granulometry, mineralogy, and organic matter determination.We have observed a high seasonal and spatial variation in the sediment and carbonsettling. Located phytoplankton blooms can be responsible for a significant carbon settling inthe traps. The highest fluxes of settling particles were observed during the falling water periodand varied between 300 and 2000 g m -2 day -1 and the lowest were found during the risingwater period and varied between 4 and 60 g m -2 day -1 . The greatest flux of organic carbon andnitrogen were observed in the same period. These flux ranged between 5 and 50 g C m -2 day -1and 0.34 and 6 g N m -2 day -1 during the falling water stage and between 0.041 and 4 g C m -2day -1 and 0.009 and 0.209 g N m -2 day -1 during the rising water stage. A part of this material isdegraded in the water column and another part is incorporated in the sediment. Sedimentationrates, ranged between 1.0 and 1.3 cm yr -1 . These high sedimentation rates associated with arelatively high organic carbon content in sediments show a very high capacity of Amazonvárzeas to accumulate organic carbon.