<p>Frenchman's Bay is a shallow coastal lagoon (0.84 km2) located near the eastern limits of the Toronto urban area. Wholesale land clearance in the 1850's and subsequent industrialization and urbanization of the watershed have had severely impacted wetland habitats and degraded sediment and water quality. Prior to implementation of remediation work, a detailed sedimentologic and magnetic property study was conducted to determine the impacts of post-colonial land use changes in Frenchman's Bay. 11 vibrocores (2-4.5m length) were extracted from the lagoon and 3 5 magnetic susceptibility profiles were collected using a probe driven 1-2 m in the lagoon floor. The core lithofacies were logged in detail and magnetic susceptibility (K, x.) and remanence parameters (NRM, SIRM, Bcr) were measured at 2 cm intervals. Magnetic property and lithofacies data were integrated with geochemical analyses (TOC, C03) and 210Pb dating of core in order to reconstruct the lagoon lithostratigraphy and the thickness of the post-colonial 'anthropogenic layer'. </p> <p>The stratigraphic succession in the lagoon consists of a thick upper sequence of marly gyttja and peat-rich silty marls overlying Holocene laminated marls. The postcolonial layer (Unit 1) is recognized as an uppermost high magnetic susceptibility (x = ~200-300 xl0-8 m3/Kg) gyttja layer that extends to 1-1.5 m depth. The base of the unit has a 210Pb age of 1850 (±55.6), corresponding with the main phase of land clearance and onset of industrialization of the harbour. Titanomagnetite, maghemite and magnetite spherules are the primary magnetic minerals, indicating soil erosion and coal burning as the predominant sources of magnetic particles. The underlying Unit 2 consists of peaty marls with abundant plant fragments recording a more extensive marsh. Unit 3 consists of more carbonate-rich laminated sands (magnetic susceptibility x = 6000 x10-8 m3/Kg) deposited in a low energy oligotrophic lagoon. The basal layer (Unit 4) consists of high magnetic susceptibility massive pebbly muds, which record a pre-lagoon phase of higher water levels in post-glacial Lake Iroquois (ca. 13,500 Ka). </p> <p> Isopach mapping of the magnetostratigraphic units clearly identifies that the anthropogenic layer (Unit I; post-1850) is thickest within a central basin which has acted as trap for sediment carried into the lagoon by several streams. The total volume of impacted anthropogenic sediment is estimated at 4 x105 m3. Isopach maps also identify two thin(< 2 m) wedges of sand (9.6 x104 m3) near the north and southern shores of the bay that records periodic overwash and growth of the beach barrier. </p> <p>The major environmental changes in the lagoon since it inception (ca. 2. 7 Ka include: 1. The formation of a shallow coastal embayment following water level rise from a mid-Holocene low-stand in Lake Ontario (Unit 4); 2. Development of a spit and beach barrier by eastward longshore transport (Unit 3); 3. Closure of the lagoon and development of a stabilized marsh habitat with low sedimentation levels (Unit 2); 4. Destruction of marsh habitats and eutrophication of the lagoon coinciding with land clearance (post-1850's) and an increased in the influx of sediments eroded from the catchment area. </p> / Thesis / Master of Science (MS)