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Journal of Paleolimnology 25: 455–465, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.
210
Pb and
137
Cs dating methods in lakes: a retrospective study
Gaboury Benoit & Tim F. Rozan* Yale School of Forestry and Environmental Studies, 370 Prospect Street, New Haven, CT 06511, USA (E-mail:
[email protected]) *Current address: College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, DE 19958, USA Received 30 October 1998; accepted 11 March 2000
Key words: 210Pb, 137Cs, metals, mobility, geochronology, Connecticut
Abstract 210
Pb has been used for more than two decades to provide the geochronology of annually deposited sediments and to construct pollution histories. Evidence from some lakes suggests that this radionuclide may be adequately mobile to compromise dating reliability. This study provides one test of that possibility by comparing recent measurements of 210Pb and trace metals to ones carried out more than 20 yrs in the past. 137Cs dating is used to confirm sediment accumulation rates in the recent cores. In the three Connecticut, USA, lakes studied, sediment accumulation rates changed abruptly to higher values between 40–50 yrs ago (increasing by factors of 2.2, 2.9, and 3.0). In all three lakes, rates calculated from 210Pb distributions both above and below this horizon agreed, within measurement uncertainty, in recent and older cores. Furthermore, when the older data were corrected for 20 yrs of burial, the changes in slope in 210Pb distributions occurred at the same depth in each pair of cores. The depth of sharp peaks in concentrations of trace metals also matched. In general, this evidence supports the idea that sediments in these lakes have simply been buried, without significant diagenetic remobilization of 210Pb and trace metals . Nevertheless, some important differences were also observed. For two of the three lakes, there was a significant difference in average sediment accumulation rate during the past 33 yrs as calculated from 137Cs and 210Pb in the recent cores. Most potential causes for this difference can be ruled out, and it appears that one of the two nuclides is remobilized compared to the other. There were also significant differences in the total inventories of both 210Pb and trace metals (both up to 2 ×) between recent and older cores in some cases. This may be due to dissimilar sediment focusing, since it is not known for certain whether the new cores were collected at exactly the same sites as in the past. Introduction The distribution of a variety of substances in annually deposited sediments has been used to provide information on pollution chronologies and paleoenvironments. Metals have received special attention because they are persistent, so uncertainty introduced by compound degradation is eliminated. Furthermore, they can pose ecotoxicological risks at low concentrations. Natural and artificial radionuclides (e.g. 210 Pb, 137 Cs) are commonly used to provide geochronologies in such studies.
Several investigations have raised the possibility that either trace metals or radionuclides might be sufficiently mobile in sediments to undermine conclusions about pollution histories in some cases (Erten et al., 1985; Wan et al., 1987; Benoit & Hemond, 1991). Indirect evidence is also provided by the common occurrence of anomalous 210Pb distributions in sediments, such as levels that decline towards the sediment water interface (Brush et al., 1982; Dillon et al., 1986; Baskaran & Naidu, 1995), as well as remobilization to the water column (Benoit & Hemond, 1990). At the same time, there is clear evidence that radionuclides are
456 immobile in sediments of some lakes where varves form (Appleby et al., 1979; Crusius & Anderson, 1995), though arguably sediments in such lakes are atypical. Now that some lakes were analyzed with 210Pb dating more than 20 yrs in the past, an historical approach can be used to further investigate the question of radionuclide and metal mobility. Specifically, lakes that were studied by 210Pb methods in the past, and where considerable sedimentation has occurred since, can be re-sampled to see whether distributions in sediments have changed significantly. We report here on three lakes that were among the first to which the 210Pb method was applied, and which we have revisited in an effort to provide further evidence on this important topic.
Study sites The three studied lakes (Figure 1) are all in the vicinity of New Haven, CT, USA, and were sampled in the mid 1970s by Brugam (Linsley Pond, 1978) and Bertine & Mendeck (Lakes Whitney and Saltonstall, 1978). All three are moderately deep, dimictic lakes, and are described in the earlier references. In addition, Linsley Pond is one of the most well-studied water bodies in the world, because it was the subject of research for many years by Hutchinson and his students (e.g. 59 topics in Hutchinson, 1975). Some characteristics of the lakes are summarized in Table 1. Sediment cores were collected by SCUBA divers at the locations indicated in Table 1 on 11 June (Whitney) and 19 June 1996 (Saltonstall and Linsley). These sites were selected to match, as nearly as possible, those described by the original authors. Curiously, for both Lakes Whitney and Saltonstall, the coordinates given in the original reference did not match the verbal descriptions of the coring sites. In fact, latitudes and longitudes listed in the earlier paper are actually for locations near the lakes, but on land.
Methods Taking care to avoid disturbing the sediment-water interface, divers collected the cores with 12.5 cm I.D. PVC tubing. The large diameter reduces core compression and provides adequate material for radionuclide analysis by non-destructive gamma counting with a planar Ge detector. Core tubes were presectioned into rings at appropriate intervals (typically 1.5 cm) and taped back together with waterproof electrical tape. This design facilitates core sectioning and circumvents the need for extrusion. All cores were immediately returned to the laboratory and processed within 2 h of collection. Sediments were dried to constant weight at 85 °C, then ground in a mortar and pestle and subsamples sealed in aluminum cans for counting. Metals in sediments were extracted by microwave digestion in Teflon bombs (Kingston & Jassie, 1988) with concentrated HNO3 and HF, followed by measurement using a Perkin Elmer 3300 simultaneous inductively coupled plasma atomic emission spectrometer (ICP-AES). Metals measured included Al, Fe, Mn, Ni, Co, Ag, Cd, V, Cr, Pb, Cu, and Zn. Results were checked by simultaneous measurement of standard reference materials. For trace metals in sediments the standards used were NIST SRM 2704 (Buffalo River Sediment) and SRM 1646 (Estuarine Sediment), which have been certified for all of the metals of interest, except Ag. Recoveries in all cases were 100 ± 5%. Radionuclides were measured by gamma spectrometry. Approximately 50 g (dry wt) samples were sealed in gas-tight, aluminum ‘tuna can’ sample containers (100 cm2) and counted with a low-background planar Ge detector. Total and excess 210Pb were measured by the method of Cutshall et al. (1983) following a ≥ 21 day equilibration period to allow ingrowth of 222Rn daughters, particularly 214Bi, which is used to indicate 210 Pb supported by 226Ra. For consistency with the earlier studies, sedimentation rates were calculated by the C.I.C. method (Krishnaswami et al., 1971) from
Table 1. Sampling locations and characteristics of the three lakes Sampling site Whitney Saltonstall Linsley *Typical values.
longitude
Maximum depth (m)
Coring depth (m)
Area (ha)
pH* (meq/l)
ANC*
latitude 41 ° 20′ 13′′ N 41 ° 17′ 43′′ N 41 ° 19′ 04′′ N
72 ° 54′ 37′′ W 72 ° 50′ 52′′ W 72 ° 47′ 06′′ W
6 32 14.7
6 18 13
72 166 9.3
7.7 7.3 8.4
0.7 1.4 1.5
457
Figure 1. Map showing the watersheds and relative locations of the three lakes. The lower part of Whitney’s watershed is characterized by heavy suburban development. The watersheds of the other two lakes are only lightly developed.
portions of the core exhibiting log-linear decreases with depth. Short-lived 7Be was measured immediately following can sealing to confirm integrity of core tops and to assess short time-scale mixing. In all cases, 7Be was measurable only in the topmost core section. 137 Cs, which was measured simultaneously, provided a marker for the 1963 horizon of maximum atmospheric weapons testing, and thus an average sedimentation rate since that time (Pennington et al., 1973; Robbins & Edgington, 1975). For radionuclides, self absorption was evaluated with point sources of 137Cs, 226Ra, and 210 Pb purchased from Isotope Products Laboratories. Efficiency corrections were evaluated with standards designed to reproduce the geometry of the counting
system. These comprised liquid radioactivity standards of the same three radionuclides (from the same vendor) that had been evenly coated onto kitty litter and sealed in a 100 cm2 can. Radionuclide and metals data from the earlier studies were extracted from graphs and replotted with an offset in depth corresponding to the recent sediment accumulation rate multiplied by the time between core collection. Activities of 210Pb were also reduced by an amount appropriate for radioactive decay during the period between collection of the two sets of cores (21 yrs for Whitney and Saltonstall, and 23 for Linsley Pond). This facilitated comparison of the recent and original data. Since the earlier plots were in terms of
458 distance (cm) rather than accumulated mass depth (g cm–2), we followed that same convention. Variations in organic matter content and bulk density with depth tended to be small, so the two scales are almost directly proportional to one another in these lakes. For example, in Lake Whitney bulk density varied randomly except in the top four sections, where it increased steadily (Figure 6). Compaction of this part of the sediment profile to the deeper average value would cause a final displacement of only 2.3 cm vertically. In the original paper by Bertine and Mendeck, metals were normalized to Al in an effort to correct for variations in clay content with depth. We elected to correct their metal data back to raw total values before comparison with our more recent measurements. Since Al distributions in the sediments varied only slightly with depth, the metals data are similar whether normalized to Al or not.
Results and discussion
Comparison of sediment accumulation rates calculated from 210Pb and 137Cs In general, the cores had smooth distributions of 210Pb excess with depth, and lacked unusual features near the sediment water interface (Figures 2–4). In each case, 137 Cs exhibited a sharp peak, suggesting that substantial mixing of the sediments had not occurred. Table 2 is a comparison of sediment accumulation rates derived by the two radionuclide methods. In these calculations, the resolution of the 137Cs dating method was assumed to be two of our standard sections (2 × 1.5 cm) so that the uncertainty was ± 1.5 cm/33 yr, or 0.045 cm yr–1. The uncertainty of the 210Pb method was derived from the standard error of the linear regression. To propagate the uncertainties, they were combined (column B, Table 2) in quadrature as is appropriate for random independent errors (Taylor, 1982 p. 52). Table 2. Comparison of recent sedimentation rates as determined by 210
Whitney Saltonstall Linsley
Pb
Average sediment accumulation rates for the previous 33 yrs, based on 137Cs, agreed within measurement uncertainty with values derived from 210 Pb analysis for Lake Saltonstall (Table 2). (The difference was less than twice the combined uncertainty, a criterion that corresponds to p = 0.05). For Lake Whitney and Linsley Pond, the sediment accumulation rates were significantly different at the 95% confidence threshold. The difference cannot be explained by simple mixing, since that would cause 210 Pb rates to appear higher, not lower than ones from 137 Cs. Arguably, the discrepancy for Linsley Pond is not extreme, since the t value of 2.4 is not very different from the threshold for p = 0.05, which occurs at t = 2.0. Also, the uncertainty for the 137Cs-calculated sediment accumulation rate is only a constrained estimate. On the other hand, the difference measured for Lake Whitney seems to be real. Resolution was greater for Lake Whitney because its higher sediment accumulation rate meant that each core section corresponded to a shorter period of time than in the other lakes. As a consequence, we believe that the uncertainty assumed for the 137Cs resolution is an overestimate, so the difference may be even more significant than calculated here. The t value of 3.5 for Lake Whitney corresponds to p = 0.0005. The difference is also not resolved by switching to the C.R.S. dating model, since this yields a slightly lower sediment accumulation rate (averaged from the surface to the depth of the 137Cs maximum) than the C.I.C. model (0.58 vs. 0.76). Delayed delivery of 137Cs from the watershed also cannot be invoked, since that would imply an even higher sediment accumulation rate for 137Cs. Based on this evidence, we believe that either 210Pb or 137Cs has migrated differentially compared to the other by some process other than physical mixing in these sediments. Blais et al. (1995) found in several lakes that 210Pb and 137Cs dates disagreed, which they attributed to 137Cs migration. Since there is no way to be certain which radionuclide is more reliable in these lakes, we have taken a straight mean of the two values 210
C.I.C. 2
0.58 0.35 0.63
0.76 ± .032 0.47 ± .047 0.57 ± .072
137
Cs dating
137
(A) Diff.
(B) Comb. uncert.
(A)/(B)
0.95 ± .045 0.40 ± .045 0.77 ± .045
0.19 0.068 0.20
0.055 0.065 0.085
3.5 1.0 2.4
Cs
C.R.S. 1
Pb and
Sediment accumulation rate units: cm yr_1. 1C.R.S. – Constant Rate of Supply model. Average value between the surface and the depth of the 137Cs maximum; 2C.I.C. – Constant Initial Concentration model. Value for log-linear zone near surface.
459
Figure 2. Comparison of radionuclide data for cores collected from Lake Whitney, Connecticut, in 1975 and 1996. Vertical error bars are depth increments, while horizontal error bars are one sigma counting uncertainties. 210Pb data for the earlier core (open circles) have been offset downward by the average of sediment accumulation rates calculated from both 137Cs (filled squares) and 210Pb (filled circles) in the recent core. They are also adjusted in activity to compensate for radioactive decay during the 21 yrs between core collections. Once this is done, both cores exhibit a change in slope in 210Pb trends near 43 cm depth. Above this point the two cores have nearly identical 210Pb levels and slopes that are statistically indistinguishable. Below the inflection point, slopes differ, but results for the newer core rely on only three data points, so the slope is highly uncertain. Dotted lines are the 95% confidence interval for the regression on the three recent data points below the inflection point. They indicate that there is not a significant difference between the two regressions in this region. In the recent core, sediment accumulation rates calculated from 137 Cs and 210Pb (either C.I.C. or C.R.S. models) are significantly different at the 0.0005 probability level. 226Ra data, which are constant with depth, are shown for comparison. Note the 210Pb values are already corrected for support by 226Ra.
in further analyses. Table 2 also compares C.R.S. and C.I.C. model sediment accumulation rates to each other and to 137Csderived values. In Whitney, the C.I.C. rate is closer than the C.R.S. value to the 137Cs one, though the 210Pb and 137 Cs results are significantly different, as noted above. For Saltonstall, the C.I.C. and C.R.S. rates are about equally close to the 137Cs value though one is higher and
the other lower. For Linsley, the C.R.S. rate is slightly closer to the 137Cs-derived value, and both 210Pb rates are lower. The higher sediment accumulation rates calculated by the C.I.C. model for Whitney and Saltonstall match the observation of Blais et al. (1995) that C.I.C. ages tend to be younger than those derived from the C.R.S. model, but the Linsley results differ in the opposite sense.
460
Figure 3. Radionuclides in cores from Lake Saltonstall. Symbols and error bars are the same as in Figure 1, and depths and activities of the older 210Pb data have been adjusted in the same manner. Again, the depth of the inflection in the 210Pb distribution is the same in the two cores and calculated sediment accumulation rates are equivalent. Rates calculated from 137Cs and 210Pb in the upper portion of the new core are the same. Different absolute amounts of 210Pb in the two cores may reflect dissimilar levels of sediment focusing at the two coring locations. 226Ra data again are constant with depth.
Comparison of current and past
210
Pb dating
In each lake the apparent sediment accumulation rate changed to a lower value at a distance below the surface ranging from 18–45 cm, which corresponded in time to 40–50 yrs in the past (Figures 2–4). It was possible to calculate sediment accumulation rates above and below this boundary from both older and more recent data. For the shallow portion of Linsley Pond sediments, there may not have been enough linear points above the inflection to reliably derive a sedimentation rate, but we have adopted the original author’s assigned rate of 0.94 ± 0.24 cm yr–1 for this zone. Table 3 presents comparative data for the lakes and shows that sediment accumulation rates are not significantly different between studies, except in one case. Deeper sediments in Lake Whitney in the more recently collected core
appear to have accumulated more slowly than in the earlier core from the same lake. However, this conclusion is based on 210Pb measurements on only three sections from the recent core. A change in the value for the single deepest (and least certain) section could significantly alter this conclusion. A change in slope in log-linear 210Pb plots, as observed here, can be caused by a variety of factors other than varying sediment accumulation rates. For example, a zone of near-surface partial mixing or radionuclide migration could produce similar results. It is interesting then that this zone has increased in depth at precisely the rate at which sediment is accumulating, since this implies that burial, and not some other process, is responsible. Another comparison between cores concerns 210Pb inventories. In Lake Whitney, the total amount of 210Pb
461
Figure 4. Radionuclides in cores from Linsley Pond. Symbols are the same as in Figure 1, and depths and activities of the older 210Pb data have been adjusted in the same manner. The depth of inflections in the 210Pb distributions match as do sediment accumulation rates measured 20 yrs apart in both zones. There is a significant difference between sediment accumulation rates measured by 137Cs (filled squares) and 210Pb (filled circles) in the recent sediments of the modern core. 226Ra data again are constant with depth.
supplied to both cores was the same, and distributions are nearly identical in both. In Lake Saltonstall, 210Pb supply was greater in the more recently collected core, and activities are higher by about a factor of two at all depths even though slopes are the same. This effect appears not to be one of an error of vertical displacement of the older data, since this would cause a serious
mismatch of the inflection points in the two cores. Instead, it seems likely that this incongruity reflects a dissimilarity in the location from which the two cores were collected, and hence a difference in sediment focusing of 210 Pb. As mentioned previously, the coordinates of the coring location in Bertine and Mendeck place it on land. Our core was collected at a
Table 3. Comparison of sedimentation rates determined in previous studies to those in the current study
Whitney Saltonstall Linsley
shallow deep shallow deep shallow deep
This study
Earlier study
(A) Diff.
(B) Comb. uncert.
(A)/(B)
0.76 ± .032 0.133 ± .019 0.47 ± .047 0.14 ± .011 0.57 ± .072 0.26 ± .028
0.93 ± .096 0.26 ± .026 0.41 ± .062 0.15 ± .011 0.94 ± .24 0.32 ± .012
0.17 0.13 0.06 0.01 0.37 0.06
0.101 0.032 0.078 0.016 0.251 0.031
1.7 4.0 0.8 0.6 1.5 2.0
Sediment accumulation rate units: cm yr–1.
462 similar water depth and as close to this location as possible, but may have been at a slightly different site. Differences in absolute 210Pb levels over time were the most difficult to explain in Linsley Pond. Here, the older core had lower levels in shallow sediments, but higher levels in deeper sediments when compared to the newer core. No vertical displacement of the profiles would resolve this problem, so it seems not to be related to an error in assigning sediment accumulation rates. Linsley Pond is very small, and we are reasonably confident that the older and newer cores were collected from nearby locations. The difference may be the result of short distance scale areal variability in bottom sediments, or diagenetic processes. Comparison of metal distributions in sediments Trace metal distributions in sediments provide a final method to evaluate diagentic processes from a historical perspective. We remeasured profiles of the same set of metals evaluated by Bertine & Mendeck (1978) in
Lakes Whitney and Saltonstall. Copper, Pb, and Zn distributions all had sharp peaks that were conducive to intercomparison (Figures 5 & 6). As in the figures for 210Pb and 137Cs, the older data have been displaced downward by a distance equal to the sediment accumulation rate multiplied by the difference in time between sampling dates. The data for Lake Saltonstall show good agreement between cores collected in the 1970s and 1990s, with similar trends as a function of time. Absolute metal levels were sometimes slightly different. For example, Zn is generally greater in the older core (112% for the average of overlapping sections), while Cu and Pb were usually lower (48 and 83%, respectively, for overlapping sections). This deficiency in stable Pb (Figure 5) is in accord with the observed lower inventory of 210Pb described earlier (Figure 3). Slight differences between metal profiles in the two cores may again reflect dissimilar sampling sites. In general, the concordance of trace metal profiles supports the idea that they are simply buried, with little other diagentic
Figure 5. Trace metal distributions in cores collected from Lake Saltonstall in 1975 (open circles) and 1996 (filled circles). The older profile has been shifted downward to account for an average sediment accumulation rate of 0.44 cm yr–1. The shapes of the profiles are the same and absolute concentrations are similar, though the earlier core was sub-sampled with higher resolution.
463
Figure 6. Trace metals in sediments of Lake Whitney. Symbols are the same as on the previous figure, and solid squares indicate bulk density in the recent core. Below the top four depth increments, bulk density varied randomly around an average value of 0.35 g cm–3 (solid line). Depth distributions are generally similar, but locations and shapes of peaks are not identical. These differences are unexplained, since the cores should have been collected from closely similar locations.
activity taking place that would alter their distribution in the sediments. Saltonstall also had a sharp peak in the distribution of Mn a short distance below the surface in 1975 (Figure 7). One possibility is that this was the result of redox processes, such as migration of soluble reduced Mn(+II), and precipitation of relatively insoluble manganese sulfides or Mn(+IV) oxides. If this were the case, the expectation would be for the peak to remain at a nearly constant depth over time. In fact, the Mn peak has moved downward at precisely the same rate as inferred sediment accumulation, implying that the Mn distribution at this site was a function of varying input rather than redox processes. In Lake Whitney, the shape of trace metal profiles over time did not match as well as those in Lake Saltonstall, though there was overall similarity. Of the three, Zn was the only metal for which the shape of the distribution did not show significant differences between the two cores. The Pb peaks were similar in shape and magnitude, but the maximum in the older core occurred 7.5 cm deeper. Lake Whitney is where
the sediment accumulation rate inferred from the 210Pb distribution was significantly slower than that from 137 Cs. Use of this slower rate to displace the older Pb profile, rather than the average, places the peaks closer together but does not resolve the misalignment. Matching the Pb peaks would require a sediment accumulation rate of only 0.51 cm yr–1 as opposed to the average value of 0.86 cm yr–1 (c.f. 0.76 cm yr–1 for 210 Pb alone). The Cu peak in the older Whitney core was broader, shorter, and again deeper than that in the recent core. The Cu profiles are especially difficult to reconcile because of these several differences. The description of the original coring site (‘immediately upstream of the dam in approximately 6 m of water’, Bertine & Mendeck, 1978) leaves little chance that cores were collected from dissimilar locations. But, perhaps in the vicinity of the dam, spatial heterogeneity is great. This is clearly an example where the positional accuracy of a technique like differential GPS would be of great value. An alternative explanation is that sediments in these locations display temporal (as opposed to spatial)
464 ≈ 6%) by compaction. Overall for this site, 210Pb and Zn seem very similar in cores taken 21 yrs apart, while Cu (and to a lesser extent stable Pb) show substantial, unexplained differences.
Conclusions
Figure 7. Mn distribution in the Lake Saltonstall cores. Symbols are the same, as is treatment of data from the earlier core. A sharp peak in Mn has migrated downward at exactly the same rate as burial, so it appears not to have been the result of diagentic redox processes. Iron (data not shown) shows a similar pattern.
variations due to small turbidity flows and erosion events. This is most likely if they were located above the mud deposition boundary depth (DBD) of the lake (level below which soft sediments permanently accumulate, Rowan et al., 1992). In fact, the calculated mud DBD for Whitney at the coring site is only 1.3 m, much shallower than the water depth at the coring site. This does not rule out the possibility that sediment accumulation at this site varies over time, but it indicates that it is unlikely. At Linsley and Saltonstall, the mud DBDs were also much shallower than the coring depths. There was a trend of increasing bulk density with depth in the recent core (Figure 6), and if this same compaction occurred in the earlier core, it would explain some, though not all, of the difference. For example, it could account for a shortening of the core by 2.3 cm, bringing the peaks for Pb that much closer to being aligned. It could not, however, explain the much broader peak in Cu in the earlier core, which actually would have been sharpened slightly (2.3/40 cm
Cores collected more than 20 yrs apart in each of three lakes were compared in terms of 210Pb distribution, sediment accumulation rate, and trace metal profiles (Cu, Pb, Zn, Mn). Some differences between cores were evident, but generally sedimentary features seem to have been buried at a constant rate that is not significantly different from values estimated two decades ago. Other diagenetic processes that might cause redistribution of metals within sediments have been largely inactive. In two of three instances, sediment accumulation rates derived from 210Pb were significantly different from those inferred from 137Cs in the same core. For the highest resolution core, the difference was highly significant and independent of whether the C.R.S. or C.I.C. 210Pb dating model was applied. Significant mixing, loss of the core top, and delayed delivery of 137 Cs from the watershed can all be ruled out as causes of the discrepancy. It seems likely that migration of either 210Pb or 137Cs may have caused the inequality, but it is impossible to determine which radionuclide is mobile from this data. Total amounts of 210Pb and metals sometimes differed between cores, and this may be attributable to variable sediment focusing or other input-related differences on short spatial scales. The inability to verify that cores were collected at exactly the same sites in the 1970s and 1990s left uncertainty when differences were found between cores of different ages.
Acknowledgments Jeff Albert and Rahul Krishnaswamy assisted with core collection. Financial support was provided by the Connecticut Sea Grant College Program.
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