Late Quaternary variations in the silicon flux from the River Nile to the Mediterranean: silicon-isotope evidence from lacustrine diatoms

Abstract

Silicon isotope compositions of main channel samples of the Yangtze River were systematically investigated along with their chemical compositions. The concentration of suspended matter in the Yangtze River tends to decrease from the upper reaches to the lower reaches, corresponding to settling of the sediments in the lakes and reservoirs due to reduction of the velocity of water flow. The silica contents of suspended matter vary from 52.1% to 56.9% and their δ30Si values vary from 0 to −0.7‰, both similar to those of shales. From the upper to lower reaches, the silica contents of suspended matter tend to increase, whilst their δ30Si values tend to decrease. Both trends reflect the increase of clay minerals and decrease of carbonates in suspended matter.The concentrations of dissolved silicon vary from 97 to 121 μmol/L and their δ30Si values vary over a wide range from 0.7 to 3.4‰. From the upper to lower reaches, dissolved silica concentrations tend to decrease and their δ30Si values tend to increase. These trends mainly reflect the change of chemical and isotopic characteristics of the tributaries from the upper to lower reaches. The major factors responsible for these changes may be the high meteoric precipitation and significant silicon absorption by grass (in wetlands) and rice (in paddy fields) in drainage areas of the middle and lower reaches.There is no correlation between δ30Si of dissolved silicon and that of suspended matter. The Δ30SiDiss-SPM values vary over a wide range of 1.0–3.7‰, indicating that (1) they are out of isotopic equilibrium, (2) dissolved silicon and the associated suspended matter do not belong to one physico-chemical system, and (3) isotopic exchange rate between them is very slow.The δ30Si value of dissolved silicon output from the Yangtze River to the East Sea is estimated to be 3.0‰, much higher than the values reported for the Amazon and Congo rivers. This increases the δ30Si range of dissolved silicon in the world’s rivers from 0.4–1.2%; to 0.4–3.4%.