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218 • Z.RUBIN ETAL. • <br /> Sediment descriptions from auger core at XS3 @ 158 m <br /> Depth(m) Clay Silt Sand Gravel <br /> f m c <br /> Sand and silt with carex roots <br /> 0.5 m Gray fine sand and silt,no organics <br /> Rounded gravel and sand <br /> 1.0 m <br /> ---== Gray silt,no organics <br /> Gray silt with organics(beaver pond) <br /> —- Age at 1.2 m=679 BP. <br /> Gray gravel/coarse sand,no organics,1.5 m <br /> : angular and poorly sorted(debris flow) <br /> Brown peat;water at 1.65 m;Age at 1.6 m=1459 BP <br /> Gravels <br /> 2.0 m <br /> Brown peat <br /> 2.5 m <br /> �. a.' �' •0' ^'d •a'.-'a'°'.'.a Coarse sand/fine angular gravel;no organics <br /> 3.0 m Sand with organics in patches <br /> Brown peat;Age at 3.25 m=3389 BP <br /> 3.3 m <br /> Figure 7. XS3 @ 158 m is in the northeast portion of the wetland.The most recent radiocarbon date is from a historic beaver pond at—1.2 m <br /> depth and is dated as —679yr BP. Previously, peat was the dominant aggradational process,although four coarse deposits are present from <br /> —679 to 2289 yr BP. <br /> I <br /> Table ll. Approximate relative contributions of aggradational processes rates are therefore expected to have been higher between 4000 <br /> and 2500yr BP.Supporting these expected trends,the profile at <br /> Debris Overbank or sorted Gyttja SW Hole(Figure 6)demonstrates a regime from—4100-2300yr <br /> flow M) debris flow M Peat(%) (beaver pond)(%) BP dominated by sediment deposition,with peat persisting from <br /> —2300 yr BP until modern sediment deposits of the past century. <br /> West 50 35 10 5 Conversely, the profile at XS3 @158m is peat-dominated from <br /> East 10 30 50 10 —3400-1500 yr BP and sediment-dominated after 1500 yr BP. <br /> The processes of aggradation are spatially varied and the <br /> influence of climate on vegetative and erosive processes is <br /> 1990). Peat production is therefore expected to have been complex. With a limited number of samples, spatial and <br /> slower between 4000 and 2500yr BP. However, variation in temporal correlation is uncertain. Results depict significant <br /> peat accumulation rates is substantially lower than variability variations in Holocene aggradation rate associated with <br /> in sedimentary processes. Long-term variability in peat accu- climatic variability, and a substantial increase in the post- <br /> mulation rates is typically minimal (Blaauw and Christen, Grand Ditch aggradation rate. <br /> 2005), while sediment deposition can vary over orders of Uncertainty emerges when comparing the modern(100-200 yr) <br /> magnitude depending on the time scale and magnitude of aggradation rate with longer (1000yr) time periods because <br /> climatic change(Sadler, 1981).Variability in aggradation rates the episodic and spatially varied processes of sediment depo- <br /> is therefore more likely to be controlled by sediment inputs sition make extrapolation difficult. Some studies have shown <br /> than by peat accumulation. In forested watersheds, precipita- that mean accumulation rates decrease with longer sample <br /> tion is negatively correlated with increased sediment yields periods because longer sample periods include periods of <br /> because precipitation induces vegetative growth and soil stability no deposition, but shorter periods, in order to be sampled, <br /> (Langbein and Schumm, 1958; Knighton, 1998). Sedimentation must have had some deposition (Sadler, 1981).Other studies <br /> Copyright @ 2011 John Wiley&Sons,Ltd. Earth Surf.Process.Landforms,Vol.37,209-222(2012) <br />