Silica, Iron and Aluminium Mobilization and Transport from Post-glacial Sediments of the St. Lawrence Lowlands-An Experimental Study
The release of silica, iron and aluminium held in the labile fraction of the St. Lawrence Lowlands post-glacial marine transgressive clays was studied in the laboratory by keeping samples of these clays in suspension for 1,000 hours in distilled water and in salt water of various salinities. The results show that SiO2, Al and Fe are released preferentially in zero salinity waters. For SiO2 , the amounts released from subaerially exposed and submerged samples and from estuarine suspended particulate matter (SPM) are respectively at least 24%, 15%, and 20% of the total silica determined in these sediments. For Al and Fe, these amounts are much less. For all three elements, there is a sharp decrease in amounts released with increasing salinity. However, the SiO2 released from the SPM above a salinity of 7‰ are higher than for the sediments and remain constant, presumably because of the dissolution of diatom remains found to be present in the material. The release of SiO2 from the labile (amorphous) fraction, suggests that this process could provide a minor contribution (perhaps 2.6%) to the dissolved silica load of the St. Lawrence River. The Fe and Al appear to be released as colloids of hydrated oxides inherited from the original environment of deposition of the sediments.