Changing sedimentation patterns due to historical land-use change in Frenchman’s Bay,Pickering, Canada: evidence from high-resolution textural analysis |
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| Authors: | P J van Hengstum E G Reinhardt J I Boyce C Clark |
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| Institution: | (1) School of Geography and Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada, L8S 4K1 |
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| Abstract: | This study examines the anthropogenic alteration of sedimentation in Frenchman’s Bay in Lake Ontario, using high-resolution
particle size analysis in two 200 cm cores. Lithofacies were determined using the particle size data of both the terrigenous
sediment and terrigenous sediment + diatom fractions. Terrigenous particle size data from the centre of the lagoon provided
the most representative record of anthropogenic impacts. Three distinctive lithofacies were recognized: (1) a Natural Wetland
(NW) lithofacies (106–200 cm) had an average mean particle size of 49.4 μm, a mode of 29.4 μm and an average standard deviation
of 119.1 μm; (2) an Agricultural and Deforestation (AD) lithofacies (40–105 cm) had a statistically significant lower average
mean (30.8 μm), mode (13.5 μm), and standard deviation (48.5) μm; (3) an Urbanized (U) lithofacies (0–40 cm) showed a continued
trend towards smaller particle sizes with an average mean of 21.2 μm, a mode of 9.4 μm, and an average standard deviation
of 32.7 μm. The lithofacies correlated with previously identified trends in thecamoebian biofacies and magnetic susceptibility
data showing post-colonial lagoon eutrophication and increased overland soil erosion. The up-core trend towards finer and
less variable particle sizes is attributed to erosion of fine-grained watershed sediments (glacial Lake Iroquois silts and
clay) during land-clearance and modification of natural drainage patterns. The influx of silts and clays into the lagoon is
also recorded by increased sediment accumulation rates and a reduction in seasonal sediment variability in the wetland. Based
on the 210Pb dates, sedimentation rates increased at 1850 ±56 AD (AD lithofacies) and suggest an exponentially increasing trend in accumulation
rates. Increasing sedimentation rates can be attributed to the progressive loss of native vegetation and intensified erosion
of Lake Iroquois deposits via stream and hillslope erosion. Ecologically, the increased input of fine-grained sediments into
the wetland has resulted in reduced water clarity and has altered the wetland substrate contributing to wetland loss in Frenchman’s
Bay. |
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| Keywords: | Land-use change Wetlands Particle size distributions Deforestation |
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