SILICA, IRON AND ALUMINUM MOBILIZATION AND TRANSPORT FROM POSTGLACIALSEDIMENTS 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 w as studied in the laboratory by keeping samples of these clays in susp ension for 1,000 hours in distilled water and in salt water of various salinities. The results show that SiO2, Al and Fe are released prefer entially 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, thes e amounts are much less. For all three elements, there is a sharp decr ease in amounts released with increasing salinity. However, the SiO2 r eleased from the SPM above a salinity of 7% are higher than for the se diments and remain constant, presumably because of the dissolution of diatom remains found to be present in the material. The release of SiO 2 from the labile (amorphous) fraction, suggests that this process cou ld 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 a s colloids of hydrated oxides inherited from the original environment of deposition of the sediments.