Seasonal and spatial variations of silicon isotopes in large tropical 

rivers

Hughes H., Cardinal D., Sondag F., Cocquyt Ch.,Bouillon S., Borges A.V., Pandi A. & André L.

Third scientific meeting of the Hybam Observatory on Amazonian Rivers, Tabatinga (Brazil) & Leticia (Colombia), 20‐23th October 2009 

Silicon

• Si = 28.8 wt % in the Earth’s crust

• Continental fluxes are still poorly constrained

• Si isotopes should help to provide a constraint on the origine and mass budget of riverine dissolved Si

Continental Si cycle

Modified from Meunier et al., 2001

Silicate rocks + CO2 + water  Clay minerals + dissolved Si

2 KAlSi3O8 + 2 CO2 + 11 H2O => Al2Si2O5(OH)4 + 4 H4SiO4 + 2 K+ + 2 HCO3-

Alkali feldspar Kaolinite

Weathering (e.g.)

δ30Si in natural environments

Crustal meanChondrites

Rivers and lakesSeawater

SoilsClaysSilcretes

PhytolithsDiatomsSponges

Rocks formation :low fractionation

Biologic processes:medium to high fractionation

Water‐rock interaction :medium fractionation

δ30Si vs. NBS28 (‰)

28Si 92.23%

29Si 4.67%

30Si 3.10%

Available data (rivers and lakes)

To be studiedseasonal variationsestuarine processinflow to the oceananthropic impact(agriculture, dams, climate change)

De La Rocha et al. 2000

Georg et al. 2006, 2007….Ding et al. 2004

Alleman et al. 2005

Ziegler et Al., 2005

Available data (this study)

Congo, Amazon, Tana, Mekong 

Congo River

R2= 0,68

10.000

100.000

10 100 1.000 10.000BSi (µg/L)

Flow

rate

(m3 /s

)

R2= 0,82

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0 200 400 600 800 1000 1200BSi (µg/L)

δ30 S

i

- Diatoms are clearly the driving parameter of δ30Si variations in the Congo

... but variations induced by biological activity are mainly below 0.4 ‰

- Out of diatom activity, the Congo River seems to have a rather constant δ30Si background around 0.72 ‰

Globaly stable over a 2 years time-scaleMean δ30Si export = 0.98 ± ?.?? ‰

Guyana shield

Brasilian shield

Amazon trough

Amazon River & tributaries

Samples supplied byORE HYBAM

Mean value : δ30Si = 0.89 ± 0.21 ‰ (1σSD)

Global scale

e.g. : 2 KAlSi3O8 + 2 CO2 + 11 H2O => Al2Si2O5(OH)4 + 4 H4SiO4 + 2 K+ + 2 HCO3-

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

4 5 6 7 8 9 10pH

δ30 S

i vs

NB

S 28

(‰)

Congo(Cardinal et al., in prep.+ mean value this study)

Congo trib.(Cardinal et al., in prep.)

Tanganyika Lake trib.(Allemand et al., 2005)

Tana & trib.

Amazone & tributaries

Mekong

R2 = 0.79

Conclusions

• Except during isolated diatoms blooms and when induced by anthropic effects (dams) :

In tropical rivers with silicate rocks environment :

→ main tendency seems to be given by clay formation

→ the influence of biological activity is clearly demonstrated…… but these biologically induced seasonal variations are superimposed on

a relatively constant background.

At global scale, Si isotopes may be useful as a proxy for clay formation

Obrigado

Gracias

Merci

Thank you

Tana River & tributaries

Output to ocean :δ30Si = 1.93 ± 0.06 (1σASD) 2σSD for δ30Si ≈ 0.12 ‰

Masinga dam