Abstract

Neutron-activation analysis, loss on ignition (LOI), and X-ray diffraction of eight cores collected in Second and Gignac lakes are correlated with historic and palynologic records to identify elements linked to erosion from deforestation and farming.Forest disturbance and farming are identified in cores of organic detritus sediment (gyttja) by decreased LOI and increased Na, Mg, Ba, Al, Ti, and Dy. LOI is not suitable for identifying forest disturbances in carbonate sediments. From neutron-activation analysis of carbonate mud only Na, Al, and Dy indicate erosion.Elements linked to the quantity of organic matter in sediments include U, V, and Cl, whereas Mn and I in surface sediments presumably correspond with variations in the oxidation potential and plant productivity, respectively.Sedimentation patterns relate to basin morphology. In Gignac Lake the basin is steep sided and relatively deep. Clastic detritus entering the lake is carried over the shallow, nearshore carbonate bank into deeper water. In Second Lake the basin is shallow and gently sloping. Minerals eroded from onshore are more equally distributed in this basin. In Second Lake the most rapid sedimentation is nearshore where submerged aquatic macrophytes produce and trap carbonate mud. In Gignac Lake the top of the shallow carbonate bank has few aquatic macrophytes; lime mud formed on the bank is washed into deep water offshore, where it dissolves.

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