Magnetic and sedimentological properties of Holocene sequences from the St. Lawrence Estuary and Saguenay Fjord, Canada

Three long sedimentary sequences were recovered from the St. Lawrence Estuary (MD99-2220 and MD99-2221) and Saguenay Fjord (MD99-2222) as part of the International Marine Past Global Change Study (IMAGES) program in 1999 in order to (1) reconstruct changes of Earth's magnetic field intensity and direction during the Holocene, (2) set a Holocene chronostratigraphy for Eastern Canadian sedimentary sequences, (3) determine the frequency of catastrophic events such as floods and earthquakes in the Saguenay Fjord during the Holocene and (4) develop sedimentological and/or magnetic proxies of Holocene decadal to millennial climatic variations in the St. Lawrence Estuary.; The magnetic, sedimentological and physical properties of core MD99-2222 allowed the identification of at least 14 rapidly deposited layers (RDL) that were accumulated in the Saguenay Fjord during the last 7200 years. The presented data along with the geological and sedimentological setting of the Saguenay Fjord suggest that these RDL were triggered by strong earthquakes. This thesis also illustrates that earthquake frequency was dramatically reduced since ∼4000 years.; Finally, high-resolution grain size analyses from core MD99-2220 and a box core sampled at the same site in the St. Lawrence Estuary were used to derive a paleodischarge proxy for the St. Lawrence River. The results illustrate that an increase in the spring freshet discharge of the St. Lawrence River increases the transport and deposition of silts in the St. Lawrence Estuary. Therefore, high frequency decadal-scale fluctuations of the spring freshet discharge along with a few exceptional events were reconstructed for the past 800 cal BP using the percent-silt record. Decadal-scale variations are also observed in the reconstructed February sea-surface temperatures (SST) of the St. Lawrence Estuary and indicate an important variability in winter conditions over Eastern Canada during the last centuries. A relationship between the percent silt and a NAO index from 1865 to 2000 AD is also observed, suggesting that multidecadal-scale precipitation variations in the Great Lakes/St. Lawrence River watershed may have been influenced by the NAO during the last centuries. Finally, the percent silt record indicates that the frequency of large spring freshets drastically increased since 1975 AD. (Abstract shortened by UMI.)...