A set of 196 14C dates was used to reconstruct Late Holocene aeolian activity in the Hudson Bay area along a south-north transect crossing the northern boreal forest, the forest tundra and the shrub tundra zones. The record indicates a minor period of aeolian activity between 4650 and 4050 BP, and three major periods at 3650-2750 BP and after 700 BP. In the northern boreal forest, there was little temporal variation in dune activity, except around 1300-950 BP where a major peak of activity is recorded. In the forest tundra, two major peaks of activity are recorded between 1650-950 BP and after 700 BP. Only minor activity occurred in the shrub tundra during the last 4000 14C-yr and some aeolian events appear to have been slightly out-of-phase compared to the two other zones. Most dates were obtained from non-charred material in the shrub tundra where, unlike in the Subarctic, the aeolian activity responded to a triggering process unconnected with fire. The dune record is compared with that of gelifluction activity in snowpatch environments and with a detailed fire chronology derived from radiocarbon-dated conifer charcoal sampled in present-day treeless sites of the forest tundra. Post-fire gelifluction reached a maximum around 1500-1100 BP and 750 BP. Fire activity was also at a maximum after 2000 BP in the forest tundra. The similarity of the chronologies suggests that fires occurring during cold periods of the late Holocene were catastrophic, having a significant impact on soil erosion and deforestation.
This study examines the spatial distribution of soil organic carbon (SOC) and other properties in soils in active floodplains using a high precision digital elevation model (DEM) and geographic information system. Floodplain microtopography may influence wetland hydrology and the physical and chemical properties of soil, thus affecting the balance of plant nutrients in this riverine environment. The selected sites were composed of three floodplains dominated by a forest consisting of silver maple (Acer saccharinum L.) with some green ash (Fraxinus pennsylvanica Marsh.). These floodplains were frequently subject to floods during spring (and also in summer and fall) that create dynamic processes in the formation of alluvial soils and in the structure and composition of floodplain vegetation. The maximum and minimum SOC rates that were obtained ranged from 0.05% to 3.16%, with median values of 0.87%. In addition, no preferential pattern was detected on SOC spatial distribution in floodplain soils even when a high precision digital elevation model was used to define floodplain microtopography. Frequent floods and vertical aggradation maintain the soil in an immature state and cause a depletion of the soil's organic carbon content.
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