Sedimentary Dynamics of an Eroding Tidal Marsh Derived from Stratigraphic Records of 137Cs Fallout, Fraser Delta, British Columbia, Canada

Abstract

The stratigraphic record of ¹³⁷Cs fallout was determined for marsh sediments from the Fraser River Delta near Vancouver, British Columbia, Canada. A series of ¹³⁷Cs profiles was obtained along a transect from the organic-rich muds of the high marsh to fine sands at the marsh's seaward limit. Despite the high biological activity (rootlets, burrowing), the variation in ¹³⁷Cs concentration as a function of depth resembles the global fallout record and regional profiles from the Strait of Georgia. The maximum fallout concentration (1964) and the onset of fallout (1954) are identifiable in five profiles and are used to calculate recent sedimentation rates. Sedimentation in this tidal marsh environment is dynamic during the past 40 years, having variations both laterally and temporally. Mean sedimentation rates vary from 0.26 to 2.05 cm a ^-1. The highest recent sedimentation rate occurs some distance inland from the marsh's seaward edge, possibly reflecting sediment filtering by marsh vegetation, which increases in abundance landward. The lowest recent sedimentation rates occur in the low marsh, near its seaward edge. There is geomorphic evidence of both lateral and vertical erosion in this part of the marsh, suggesting that recent erosion of the low marsh surface may have contributed to a lower net sediment accumulation in the post-1964 period. The ¹³⁷Cs profiles show that sedimentation rates declined by an average of 51% in the period 1964-1991, compared to 1954-1964. It is suggested that this reduction in sedimentation results from the combined effects of erosion and reduced riverine sediment input in recent decades. Sediment budgets and the effects of engineering structures and dredging need to be seriously considered apropos the long term stability of this coastal wetland habitat. This study demonstrates this fallout technique is useful for a variety of sediment types in the intertidal environment of the Fraser River delta and that modem erosion threatens the Sturgeon Bank tidal marsh.