Multi-proxy paleoenvironmental study of Lake Ontario coastal lagoons: Frenchman’s Bay and Lynde Creek, Ontario

Abstract

European settlement (between 1770 and 1850 CE) and urbanization in the mid-20th century resulted in the rapid degradation and disruption of Lake Ontario watersheds and coastal wetland environments. This study assessed the impacts of historic land use changes and urbanization on sediment and water quality in two coastal lagoons in the eastern Greater Toronto Area: Lynde Creek (LC) and Frenchman’s Bay (FB). Twelve short sediment cores (0.5-2 m) were collected in both lagoons to determine the thickness and distribution of contaminated sediments, using a range of environmental proxies (ITRAX micro-XRF elemental geochemistry, magnetic susceptibility, testate amoebae, loss-on-ignition, AMS 14C, 210Pb chronology). Lithochemofacies were identified by principal component analysis and hierarchical clustering of elemental data. The lagoon stratigraphy consists of a lowermost package of Late Holocene interbedded sand and peat (2 500- 600 14C YBP) overlain by a distinctive ~1-1.5 m thick, organic muddy silt unit (ca. ~1850), representing the European Settlement Layer (ESL). PCA revealed 10 distinctive lithochemofacies representing ESL and underlying natural wetland (NW) deposits. The ESL lower boundary is defined by a rapid increase in magnetic susceptibility and terrigenous elements (Si, K, Ti, Fe, Zr), and lower organic content. The ESL is a basin-wide marker in Frenchman’s Bay and Lynde Creek, signalling the onset of European land clearance, soil erosion and enhanced delivery of terrigenous elements to coastal environments. The transition is also tracked by an increase in Cucurbitella tricuspis, recording increasing basin eutrophication. The onset of urbanization in the mid-20th century is indicated in FB by an upward increase in heavy metals (Zn, Pb), C. tricuspis and elevated calcium (Ca), chlorine (Cl). LC lagoon shows similar elemental trends but with a lower relative abundance of heavy metals. The widespread extent and thickness of the contaminant-impacted layer highlight the need for further assessment and monitoring of lagoon sediment and water quality.