Prairie wetland sediment : a record of the recent past

Abstract

The introduction of agriculture to the Canadian prairie by European settlers brought about many changes to the natural landscape. The changes, resulting from removing the natural vegetation and cultivation, accelerated geomorphic processes such as erosion and deposition. The general design of this study is based on the lakecatchment system approach and the wetland and its drainage basin are therefore considered one ecological system. The objective of this research project is to relate changes in the sedimentary record of a wetland drainage basin to documented land use changes that have occurred since the onset of settlement. This thesis project is part of the PECOS study. The objective of the PECOS project is to evaluate land use and communities in Census Region 3BN from the perspective ofsustainability.

A 52-cm sediment core from a wetland slough in the northeast portion of Census Region 3BN was analyzed to determine physical and chemical characteristics. The sediment was separated into allogenic and authigenic fractions to help identify the source of the sediment. Close interval 210Pb and 137Cs dating allowed calculation ofthe influx of sediment and individual elements to the wetland. There were several episodes of intense influx of sediment to the core. Pre-settlement episodes of increased influx are beyond the time period of documented land use and therefore most likely due to naturally occurring, high magnitude, low frequency events. The post-settlement sediment characteristics, however, are consistent with human disturbance that occurred near the wetland. A postsettlement period of increased sedimentation rates occurred from 1941 to 1943, the years following the drought and depression of the 1930s. This period also corresponded to a peak in the number of acres under summmerfallow.

Sedimentation rates and chemical influx to the wetland began to increase following the damming of the slough in 195 5. Sedimentation rates continued to increase from 1965 to 1968, at a time when agriculture intensified around wetland. The second highest peak in sedimentation rates occurred from 1989 to 1991. This period occurred at the time when the island in the wetland was seeded following the drought of 1988.

Two conclusions were drawn after evaluating the impact of anthropogenic activity on wetland sedimentation. First, the onset of agriculture was associated with an influx of geochemical elements associated with topsoil erosion as shown by increases in allogenic A1, and fertilizer application as shown by increases in trace elements such as U. The second conclusion is that increasing sedimentation rates are associated with the onset of agriculture and accelerated by human disturbance such as damming of the wetland and road construction.