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HORIC RANGE OF VARIABILITY AS A CONTEXT FOR RESTOPION 219 <br /> Aggradation Rates in Lulu City Wetland <br /> sD.o <br /> ...sw xwe lest.3tw pot romwowpl <br /> 45.0!! <br /> r X53 @ 158(est.3tM6 peat co CM ) <br /> 400 - `tons w@I s0 leit.3016cwnpxtion) <br /> I I�r1t54 @I12.5 <br /> :i <br /> 35.0 <br /> o :I <br /> N <br /> v <br /> 25.0 <br /> o <br /> 0 <br /> V20.0 f••■ ',.. <br /> 15.0 I ................ --•--_-. <br /> loo i i I #................................................ <br /> � <br /> lk................. ..-..-.-.-. ........................ <br /> so <br /> o0 <br /> 0 500 l000 1500 200D 2500 3000 3500 4000 4500 <br /> Years Before Present(2009) <br /> Figure 8. Aggradation rates were calculated based on the mean radiocarbon age for each sample and assuming constant rates between each age <br /> control point.Each age control point is indicated by a symbol.A correction for compaction was applied to all peat deeper than 1 m below the ground <br /> surface.An estimate of 30%compaction was assumed.Locations of radiocarbon-dated cores are presented in Figure 2. <br /> Ground Surface Depths in Lulu City wetland <br /> 0-SW Hole <br /> 50 <br /> -W-Long W @150 f <br /> r r <br /> E <br /> u 100 ..A..XS3@158 r`----� <br /> ,� --- ••X•XS4 @112.5 -- <br /> r r <br /> r <br /> v150 ------------------------------------...--_-_-____�_l-_ _ _ <br /> o ' NI' <br /> 200 -- -- __ <br /> d � <br /> d <br /> 250 — - <br /> c <br /> o300 � <br /> low, <br /> ` <br /> 350 ___---_-----------------------------__. __— <br /> I <br /> 400 - ----------------------------- ----------- ----------- <br /> 4500 4000 3500 3000 2500 2000 1500 1000 500 0 <br /> Years Before Present <br /> Figure 9. The ground surface over time at dated profile locations is generally consistent throughout the Lulu City wetland, assuming no loss of <br /> elevation(e.g.deposition and subsequent erosion,or compaction)between dated samples.Steeper slopes indicate faster aggradation rates. Each <br /> age control point is indicated by a symbol.Locations of radiocarbon-dated cores are presented in Figure 2. <br /> suggest that evaluations based on short-term sedimentation would have been substantially higher than the millennia-scale <br /> tend to underestimate long-term average sediment yields by averages distinguished by the 11 radiocarbon dates(Table 1)and <br /> missing infrequent, large magnitude events that may be may have been comparable with recent rates.The one short- <br /> responsible for a significant portion of the long-term yield duration, pre-disturbance rate quantified by the 11 samples <br /> (Kirchner et al.,2001). Because our sampling was conducted is the 160 years between 4089 and 4249 yr BP that aggraded <br /> specifically to target known deposition of the past 1-2 centu- at an average rate of 20 cm/100 yr,or half the rate of the last <br /> ries, the Sadler example is a better description of bias than 1-2 centuries. <br /> the case exemplified by Kirchner et al. We therefore suggest There is also the question of validity in comparing the <br /> that the measured aggradation rates of the past 1-2 centuries currently deposited material with what will ultimately be <br /> actually represent a maximum for the time scale investigated preserved. The Lulu City wetland has been aggrading during <br /> in this study. Our recent measured rates may be somewhat the past four millennia, but it is reasonable to expect some <br /> exaggerated because we sampled immediately following a large fluvial reworking and transport out of the wetland following <br /> debris flow. Similarly, we expect that some pre-settlement rates sediment inputs.Indeed,the question of transient versus persis- <br /> of aggradation over periods of equivalent duration(1-2 centuries) tent features is central to this assessment of the impacts from <br /> Copyright©201.1 John Wiley&Sons,Ltd. Earth Surf,Process.Landforms,Vol.37,209-222(2012) <br />