mes profiles and inventories in overbank and lake cores sampled in the outer delta showed that erosion processes affected the sedimentary record in study areas (see chapters 3.2.4 and 3.3.4). To get a better understanding of the balance between sedimentation and erosion, erosion rates were estimated in connected lakes and on channel levees. These results will be used for the sediment budget estimate in the delta (see chapter 3.5.3).
deposition li ne • non-eroded cores erosion line 0 eroded cores Figure 3.25: Relationship between 137 Cs inventories and sedimentation by regular erosion in lakes rates in 1ton-eroded cores (deposition fine) and in eroded cores sampled and levees, erosion was in channel levees (A) and in connected lakes (B). Estimates of gross simulated in four cores sedimentation rates and erosion rates (E
=
Rgross·R,.,) in eroded cores. assumed to contain a complete sedimentary record. Lake core 7.1 was first selected as sedimentation rate deduced from the mes profile was consistent with the counting of varves. Ten simulations with increasing erosion rateswere performed by removing more and more samples (table 3.14). For instance, taking one sample out of two would mimic a 50% erosion rate. Simulated erosion rates range from 8 to 80%.
Table 3.14: Simulations of JO different erosion rates by considering only a limited number of samples (N samples) in the cores. For instance: 112 = 1 sample out of 2 is retained for the simulation, 213: two samples out of three is retained etc ..
Simulation
For each simulation, a new sedimentation rate and 137Cs inventory are calculated. The regression line expressing the simulated 137Cs inventories as a function of simulated sedimentation rates connects the zero point to the point representing the real 137Cs inventory and sedimentation rate in lake 7 (point lake 7 on fig. 3.26). Simulation 7 (erosion rate of 50%) always lies half way between the origin and the non-eroded core. Simulation 8 is found at one third of the way from the origin to the non-non-eroded core and simulation 6 at two thirds of the way (table 3.14). The lirnited number of samples in cores makes it difficult to simulate high and low erosion rates. The points representing simulations 1, 2, 3, 4, 5, 9 and 10 can be slightly offset from the line. The line, going through the origin (erosion rate of 100%), the
-71-Chapter 3: Sedimentation in the Mackenzie Delta
calculated from a given erosion rate may be different if the 137Cs peak is eroded or not, particularly when the sedimentation rate is low. However, by using different erosion rates for the simulations (table 3.14), the regression line provides an estimate of all possibilities of erosion rates, with or without the erosion of the 137 Cs peak.
These simulations suggest that the erosion rate of a given core can be estimated graphically by drawing a line from the zero point through the point representing the eroded core (fig. 3.25). The intercept of the erosion line with the deposition line gives the value of the gross sedimentation rate. The difference between this gross sedimentation rate and the net sedimentation rate deduced from the 137Cs profile corresponds to the erosion rate.
Table 3.15: Estimated mean erosion rates (E expressed in g/cm2*y) and proportions of eroded material (E expressed in %) on distributary channel levees (A) and in connected lakes (B). lm= 137Cs inventory
Between 20 and 60% of the annual sediment input to levees is eroded after deposition (table 3.15 A.).
Erosion rates on levees 4 and 5 are lower, probably because they are located far from large channels flowing into the Beaufort Sea. The estimate of gross sedimentation rates on levees ranges from 1.0 to 4.5 g/cm2*y, while it varies between 1.7 and 2.9 g/cm2*y in the middle delta (table 3.15 A). These estimates suggest that the annual sediment input to distributary channel levees in the middle delta is comparable with the one in the outer delta. However erosion is much more active on levees in the outer delta than in the middle delta. Frequent erosional events in the outer delta likely prevent levee development, which could partly exp lain the lower levee heights in the outer delta than in the middle delta.
Estimates of erosion rates in connected lakes in the outer delta varies between 0.1 and 1.0 g/cm2*y.
On average, 50% of the annual sediment input into lakes is eroded and reentrained in the channel network (table 3.15 B.). The estimate of gross sedimentation rates on levees ranges from 0.4 to 1.1 g/cm2*y, wh ile it varies between 0.4 and 1.2 g/cm2*y in the middle delta (table 3. L5 A). This suggests that the annual sediment input to connected lakes in the middle delta is similar to the one in the outer delta, white the loss of sediments from lakes is much more important in the outer delta. The large range of erosion rates (between 14 and 86%) is probably explained by different lake sill elevations. The loss of sediments from lakes is probably higher in low sill elevation lakes than in higher sill elevation lakes.
However the sill elevations of lakes sampled in the outer delta are unknown and this assumption cannet be verified.
It should be stressed that erosion rates determined in this study are simply estimates because the relative uncertainty of mes inventories is quite important (on average ± 10%). mes activity was not measured in ail samples and thus 137es inventories were calculated by interpolating 137es activities. The uncertainty attached to erosion rates is not symmetrical and increases with sedimentation rates. It includes the uncertainty of the regression tine representing the deposition line, the uncertainty of sedimentation rates and the uncertainty of 137es inventories in sediment cores. The proportion of sediment eroded from levees and lakes (erosion expressed in % in table 3.15) is believed to be estimated with an absolu te uncertainty of about ± 20%.
Although the uncertainty attached to erosion rate estimates is substantial, these results should be considered for the following reasons:
• the relationship between sedimentation rates and 137es inventories is verified in 4 cores (lakes 6 and 7 and levees 6 and 8). The intercept of the regression tine with the 137es inventory axis corresponds to the atmospheric 137es inventory (2.93 pei/cm2);
• the cores containing 137es inventories lower than the atmospheric mes inventory indicate that the sedimentary record is incomplete and that lake deposits have been disturbed by erosional events.
• the mes inventories and sedimentation rates resulting from the simulation of 10 different erosion rates in lake core 7.1 can be fitted by a regression li ne (or erosion li ne) connecting the origin to the point representing the non-eroded core located on the deposition line (fig. 3.26).