GIeochimica et Cosmochimica Acta, 1973, vol. 37, pp. 1171 to 1187. Pergamon Press. Printed in NorthernIreland Th-228/Th-232 and Pb-210 geochronologies in marine and lake sediments MINORU KOIDE, KENNETH W. BRULAND and EDWARD D. GOLDBERG Scripps Institution of Oceanography, University of California, La Jolla, California 92037 U.S.A. (Received 18 July 1972; accepted in revisedforln 18 October 1972) Abstract-Two dating techniques, applicable to coastal marine and lacustrine sediments over periods ranging from years to a century, evolve from the natural radioactive series: Th-2281 Th-232 from the Th-232 series and Pb-210 from the U-238 series. There is an excess of Th-228 over and above that supported by the parents Th-232 or Ra-228 in such deposits. The amounts of Th-232-supported and Ra-23%supported Th-228 are similar, indicating that both radium and thorium isotopes are removed from solution in these coastal zones quite rapidly after introduction or formation. The radium isotopes are probably trans- ferred from the overlying waters to the sediments via phytoplankton, where they are reportedly enriched. In the surface levels of the deposits analyzed, there is nearly an order of magnitude more unsupported than supported Th-228. This excess Th-228 can be used both for dating purposes over time periods of the order of a decade and as permissive evidence that the upper- most levels of the deposit were obtained during the coring operation. Preliminary results from several lakes indicate the possibility that Th-228/Th-232 geochronologies are held by their sediments. Average rates of accumulation of lake sediments have been obtained from decreases in Pb-210 activity as a function of depth, although surface layers of the deposits may not have been recovered in the coring operation. Finally, Pb-210 geochronologies have been extended from varved to unvarved marine sediments. THE OCCURRENCES of the natural radioactive series of uranium and thorium in the marine environment are characterized by the non-equilibrium concentrations of their members. A variety of nuclides with differing half-lives and differing chemical properties are involved. Some have been extremely valuable in the formulation of radioactive dating techniques and in elucidating the chemical behaviors of elements in the oceans. The uranium-238 series has provided, through its members, entries into both of these areas. Less attention has been directed towards the thorium-232 series, most probably because of the short half-lives of the daughters and because of the difficulties encountered so far in explaining the distributions of these daughters in waters and sediments. The relevant part of the series follows: Th_232 141x10’oYrt Ra_228 6”Yrk AC_228 @13hr, Th_228 l*QlYr+ Ra_224 In surface and deep waters of the Atlantic ocean, MOORE and SACKETT (1964) found a fifteen-fold excess of unsupported Th-228 over the amount expected from radioactive equilibrium with Th-232. Similarly, SOMAYAJULU and GOLDBERG (1966) found pronounced disequilibria (the Th-228/Th-232 activity ratio varied between 10 and 25) in open ocean Pacific deep waters. On the other hand, the Th-228/Th-232 1171
Transcript
GIeochimica et Cosmochimica Acta, 1973, vol. 37, pp. 1171 to 1187. Pergamon Press. Printed in Northern Ireland
Th-228/Th-232 and Pb-210 geochronologies in marine and lake sediments
MINORU KOIDE, KENNETH W. BRULAND and EDWARD D. GOLDBERG
Scripps Institution of Oceanography, University of California, La Jolla, California 92037 U.S.A.
(Received 18 July 1972; accepted in revisedforln 18 October 1972)
Abstract-Two dating techniques, applicable to coastal marine and lacustrine sediments over periods ranging from years to a century, evolve from the natural radioactive series: Th-2281 Th-232 from the Th-232 series and Pb-210 from the U-238 series.
There is an excess of Th-228 over and above that supported by the parents Th-232 or Ra-228 in such deposits. The amounts of Th-232-supported and Ra-23%supported Th-228 are similar, indicating that both radium and thorium isotopes are removed from solution in these coastal zones quite rapidly after introduction or formation. The radium isotopes are probably trans- ferred from the overlying waters to the sediments via phytoplankton, where they are reportedly enriched. In the surface levels of the deposits analyzed, there is nearly an order of magnitude more unsupported than supported Th-228. This excess Th-228 can be used both for dating purposes over time periods of the order of a decade and as permissive evidence that the upper- most levels of the deposit were obtained during the coring operation. Preliminary results from several lakes indicate the possibility that Th-228/Th-232 geochronologies are held by their sediments.
Average rates of accumulation of lake sediments have been obtained from decreases in Pb-210 activity as a function of depth, although surface layers of the deposits may not have been recovered in the coring operation. Finally, Pb-210 geochronologies have been extended from varved to unvarved marine sediments.
THE OCCURRENCES of the natural radioactive series of uranium and thorium in the marine environment are characterized by the non-equilibrium concentrations of their members. A variety of nuclides with differing half-lives and differing chemical properties are involved. Some have been extremely valuable in the formulation of radioactive dating techniques and in elucidating the chemical behaviors of elements in the oceans. The uranium-238 series has provided, through its members, entries into both of these areas. Less attention has been directed towards the thorium-232 series, most probably because of the short half-lives of the daughters and because of the difficulties encountered so far in explaining the distributions of these daughters in waters and sediments. The relevant part of the series follows:
In surface and deep waters of the Atlantic ocean, MOORE and SACKETT (1964)
found a fifteen-fold excess of unsupported Th-228 over the amount expected from radioactive equilibrium with Th-232. Similarly, SOMAYAJULU and GOLDBERG (1966) found pronounced disequilibria (the Th-228/Th-232 activity ratio varied between 10 and 25) in open ocean Pacific deep waters. On the other hand, the Th-228/Th-232
1171
1172 MINORU KOIDE, KENNETH W. BRUL~WD and EDWARD D. GOLDBERQ
ratio had a value of 1-O for a water sample taken from the Scripps Institution of Oceanography pier where the total thorium content was an order of magnitude or two higher than the values for the open ocean samples. MOORE (1969) has submitted that river influx of dissolved Ra-228 or generation of Ra-228 from the particulates in the water column with a subsequent leaching of this nuclide by the water is unable to account for the high Th-228/Th-232 ratios in the open ocean. He suggests that the most probable mechanism for explaining these observations is an upward diffusion of Ra-328 from nearshore and continental shelf and rise sediments in contact with surface waters.
The Th-228/Th-232 activity ratios in the acid leachates from marine sediments are often less than one. In the authigenic minerals, phillipsites and barites, the activity ratios are always greater than 1 and sometimes as high as 23 (SOMAYAJULU and GOLDBERG, 1966; BERNAT and GOLDBERG, 1969; CHURCH and BERNAT, 1972). Explanation of these results involves a movement of Ra-228 from detrital minerals to the interstitial water and a subsequent and preferential uptake of it and/or its daughter Th-228 by the mineral in question.
This investigation is an extension of our geochronological studies with Pb-210 on the rapidly depositing (0.4 cm/yr) varved Santa Barbara Basin sediments (KOIDE, SOUTAR and GOLDBERG, 1972). A decrease in the Th-288/Th-232 ratio with depth in the core was observed in the upper levels from which leached thorium isotopes were assayed. Subsequently, we decided to investigate this observation in somewhat greater detail, hoping that some insights into the geochemistries of thorium and lead might be realized. The time parameter in these sediments is well-established, both by the yearly varves and by the Pb-210 measurements. Further, a box core from the coast off Baja California and gravity cores from lakes were also available from continuing studies with Pb-210.
METHODS The Pb-210, Th-228, Th-232 and Th-230 analyses were performed using previously described
techniques (KOIDE et al., 1972; GOLDBERCJ and KOIDE, 1902) using 6 N HCI leachates of the 400° furnace heated samples, which were previously oven dried at 110’. Analyses of the total samples were carried out by combining the leachate solution with that of the residue following its treatment with HF and with a concentrated HCl/HNO, mixture.
The activities of the thorium isotopes were determined by alpha spectroscopy. The Th-228
levels were measured through the 5.43 and 5.34 MeV energies which were unresolved under one peak. The peak also contains the 6.45 MeV Ra-224 alpha particles which constitute 6 per cent of the alpha emissions from this nuclide. Since all of the samples could not be counted immedi- ately after plating, the build-up of the Ra-224 depended upon the time between plating and assay (half-life of Ra-224 is 3.64 days) and upon the amount of Th-228 present. The main Ra-224 activity (5.68 MeV) was not used for this correction inasmuch as there was a small con- tamination in some of our detectors at this energy.
All radiometric assays were carried out to at least 1000 counts giving a maximum counting error of slightly greater than 3 per cent.
DESCRIPTION OF SAMPLES The coastal marine samples included box cores from the Santa Barbara Basin (No. 262-1,
34’14.0’N; 120°01.5’W; 575m depth) and from Baja California (No. 244, 25’13eS’N; 112O 40.6’W; 520 m depth). Pb-210 dating (KOIDE et al., 1972) confhmed the ‘annual varve’ concept for the Santa Barbara basin deposit (Fig. 1 and Table 1) and dates can be assigned to the various strata upon such a basis.
Th-228/Th-232 and Pb-210 geochronologies in marine and lake sediments 1173
Fig. 1. The Pb-210 activity in Santa Barbara Basin core as a function of time of deposition of varve. The solid curve is drawn with the half-life of Pb-210. The
average sedimentation rate is 0.4 cm/yr.
Table 1. Pb-210 activities in Santa Barbara basin sediments (Box Core 262-I)
Amount of dried sample
Amount of remaining after Time of Amount of sample sample remaining 6 N HCI leach
Depth in deposition remaining after after igniting and drying Pb-210 sediment (from drying at 1lO’C at 4oo”c at llOV* activity
* The CaCOs levels in these sediments is about 6%. t Based upon a 30% counting efficiency.
1174 MINOBU KOIDE, KENNETH W. BRULAND and EDWBRD D. GOLDBERG
lo@E
Fig. 2. Pb-210 activity in Baja California Core as a function of depth in the deposit. The constancy of Pb-210 between 9 and 15 cm may reflect a slump; there is evi- dence for this on physical grounds (A. Soutar, personal communicrttion). There is & lower water content in these levels (Table 2) 89 additional support for this concept.
The Pb-210 concentrations showed an exponential decrease with depth in the unvarved Baja California sediments. Utilizing the 22.3 yr half-life of Pb-210, dates of deposition can be assigned to the various sedimentary levels (Fig. 2 and Table 2).
The water contents in both sediments are clearly higher in the upper ten centimeters with compaction becoming evident at deeper levels (Tables I and 2 and Fig. 3). On the other hand, the loss in weight following ignition at 400%, & measure of the organic matter and of the more strongly bound water molecules, was essentially independent of depth. Therefore, over the short time intervals, there appears to be no measurable bseterial degradation of the organic phases in the deposits.
Comparison of properties of the Baja California and Santa Barbara basin deposits, about one thousand kilometers apart on the western coasts of the North American Continents, provides a background for understanding the distributions of thorium and lead isotopes. Both basins have similar morphologies; the Baja California, depositions1 area experiences less rainfall and a smaller influx of detrital solids (Soutar, personal communication).
An average of 3.6 times more Th-232 was found in the Santa Barbara bulk sediments in comparison with the Baja California value (Tables 7 and S), reflecting most probably a larger continental run-off in the former are+~
Since the heights of the overlying waters for both deposits are similar, the potenti& supplies of Th-228 and Th-230, borne of Ra-228 and U-234 respectively in the sea water, are about the same in both oases. Hence, one would expect lower Th-228]Th-232 and Th-230/Th-232 ratios in the Santa Barbara Basin and this is found to be the case.
The Pb-210 contents in both box cores are similar. This isotope primarily arises from the decay of Ra-226 in the water column (KOIDE, SOUTAR and GOLDBERG, 1972). Since both de- posits are at similar depths and since the processes that bring Pb-210 to the sediments most probably operate with similar effectiveness, the like Pb-210 activities are not unexpe&&.
Th-228/3h-232 and Pb-210 geochronologies in marine and lake sediments
Table 2. Pb-210 activities in Baja California sediments (Box Core 237-l-4)
1175
Amount of dried sample Amount of sample Amount of dried sample remaining after
Depth in remaining after mmaining after igniting 6 N HCl leach and Pb-210 sediment drying at 110°C at 400% drying at llO°C* activity
* The CaCO, levels in these sediments is about 24%. + Based upon a 30% counting efheiency.
L&e Tahoe, located 1900 m above sea level in Northern California and Nevada, has an area of 530 km2 with a maximum dopth of 501 m. Its basin is formed by a graben near the crest of the Sierra Nevada Mountains.
Lake Titicaea on the Peru-Bolivia frontier which has a maximum depth of 281 m and an area of 7600 km2 is at an elevation of 2500 m. It was formed during the process of mountain building.
Trout Lake and Lake Mendota have been especially well studied by the limnologists over the past half-century (FREY, 1963). Lake Mendota, adjacent to Madison, Wisconsin, is an eutrophic, hard-water lake with an area of 40 km2 and a maximum depth of 26 m. Trout Lake, in the northeastern part of the state, has an area of 15 km2 with a maximum depth of 35 m.
Pb-210 GEOCHRONOLOGIES IN MARINE SEDIMENTS
The Pb-210 contents in the varves of the Santa Barbara Basin Box Core 262-l extending to 1971 dovetail with those reported previously (KOIDE et al., 1972) for Santa Barbara Box Core 239 with varves up to 196 2. The unvarved Baja CaIifornia sediments displayed both Pb-210 levels in their surface layers and decreases in the Pb-210 concentrations with depth like those of the Santa Barbara deposit,
5
1176 MINORU KOIDE, KENNETH W. BRWLAND and EDWARD D. GOLDBERG
Fig. 3. Varves in Santa Barbara deposit. The greater varve thicknesses in upper levels reflect greater wLater contents.
There appears to be a mobility of dissolved lead in the surface layers of these coastal sediments. The Pb-210 concentrations for both the Baja California and Santa Barbara basin sediments have maximum vdues near, but not at, the sediment water interface (Figs. 1 and 2). In the Santa Barbara core the low surface values extend to a depth of about 1.6 cm (one year) while in the Baja California deposit they involve the upper 2 cm, corresponding to about 6 years. Apparently there is a migration of the nuclide in fhe deposit--a movement which could arise in the following way. The Pb-210 is initially deposited under oxidizing conditions and distributes itself between the solid phases and the interstitial water. The sediments slightly below the surface become anaerobic with the consequential formation of hydrogen sulfide (EXERY, 1960). The subsequent precipitation of lead, perhaps as PbS, decreases the activity of the dissolved lead species in Lhe interstitial waters of the reducing zone, creating a concentration gradient with the waters in the aerobic strata. fixation downward into the anoxic zone or upward into the water can then take place, resulting in minimal Pb-210 concentrations in the upper strata of the sediments.
The shape of the Pb-2lO/depth curves for coastal sediments of this type may be used to establish not only the recovery of the upper portion of the deposit but also that it has not suffered disturbances either in the retrieval of the material or through physical or biological processes.
There does not appear to be similar concentration~epth profiles for the thorium isotopes, which decrease with depth into the sediments. Rapidly accommodated in the sedimentary phases, the thorium becomes immobilized with presumably very low levels in the inters%al waters.
Th-228/Th-232 and Pb-210 geochronologies in marine and lake sediments 1177
Pb-210 IN LAKE SEDIMENTS
Pb-210 geo~~onologies of lake sediments appear to provide reasonable rates of segmentation in some cases (KBISHNASWAMY et al., 19’71) ; still, there remains some uncertainty about the systematics of the method due to the possible loss of the top portions of the deposits during the coring operations (KOIDE, SOUTAR and GOLDBERGI, 1972). Lake sediments are often quite flocculant, due to their high con- tents of siliceous plant tests (the lake deposits analyzed in this work contained up to one third opaline silica by weight). The high velocity impact of small dia.meter, open barrel corers at the lake floor may so disturb the surface strata of these sediments that their recovery is not complete.
Our results (Tables 3, 4, 5 and 6 and Figs. 4, 5, 6 and 7) are presented with the sense that the upper few centimeters or more may be missing. A suggestive, but not definitive, check may be made through the presence of Cs-137 (an artificial radio- active isotope introduced to the global environment from fission reactions in nuclear weapons tests). A maximum fallout of such materials to the Earth’s surface appears
Table 3. Pb-210 activities in Trout Lake sediments. Gravity core taken Augnst, 1966
Depth in sediment
(cm)
O-5 5-10
20-25 25-30
35-40 40-45 45-50
76-75
Amount of sample Amount of dried sample remaining after igniting remaining after 6 N Opal in
dried sample at HCl leach and drying Pb-210 sample dried 4oo”c at llO°C activity at 110’
W%) W%) (dpmlg) wt%)
68.2 47.0 85.7 30 70.5 49.2 842 31
69.5 53.7 42.3 33
69.4 54.6 23.6 39
58.2 41.7 20.6 26 55.6 41.7 13.3 20
60.5 45.7 9.55 35
62.2 43.0 3.71 26
Table 4. Pb-210 activities in Lake Mendota sediments. Gravity core taken August, 1966
Depth in sediment
(cm)
Amount of sample Amount of dried sample remaining after igniting remaining after 6 N Opal in
dried sample at HCl leach and drying Pb-210 sample dried 4oo”c a;t 11o*c activity at 110’
to have occurred about 1963 (SCHREIBER et al., 1968) and the event is recorded in both marine and lake sediments. Gamma-ray spectra on bulk sediments and chemi- cal isolates of cesium indicated that two of our Lake cores possessed Cs-137 in their upper levels; Trout Lake and Lake Mendota. Both of these cores contain excess Pb-210 levels rea&Iy measurable over about four or so half-lives, times up to a
century {Figs. 4 and 5). On the other hand, Lake Titicaca in the southern hemisphere and Lake Tahoe do not contain measurable amounts of Cs-137 and the excess Pb-210 is measurable only over about two half-lives. Perhaps, there has been a loss during coring of the upper few centime~~ of these more slowly depositing solids with a consequential loss of Cs-137 and excess Pb-210 activities. The loss of strata deposited over the past fifty years would correspond to losses of centimeters of sediment (Figs. 6 and 7).
This argument is supported by the distributions of thorium isotopes. Both Trout Lake and Lake Mendota surface layers possessed an excess of Th-228 relative to that required for radioactive equilibrium with Th-232 whereas no excess was
Th-228/Th-232 and Pb-210 geochronologies in marine and lake sediments 1179
Fig” 4. Pb-210 activity in Trout Lrtke sediments as a fxmction of depth.
o.l~ 0 40 80 120 169 200 240 280
Depth in Sediment in cm
Fig. 5. Pb-210 wtivity in Lake Mendota sediments as 8 function of depth.
1180 ~NORU KOIDE, KENNETH W. BRUITAND and EDWARD D. GOLDBERG
iooor
z ; .- v \
CE -A b 100 - \
45 yew*
E \ e 0.1 cm/yew P
: ! \ \
k - \ v \
.f
21 ‘b .Z
.z I\
; \ a \
10 _- 0 - \ N _
d . a
: ~
\
\ \
\--
\
\
\
\
I I I 1 1 I\ I I -4 0 4 8 12
DeDih in Sediment in cm
FIG 6. Pb-210 activity in Lake Tahoe sediments zw a fun&ion of depth.
Depth in ‘Sedimenf in cm
Fig. 7. Pb-210 ctctivity in Lake Titicaca, sediments aa a function of depth.
Th-228/Th-232 and Pb-210 geochronologies in marine and lake sediments 1181
found in Lake Titicaca and Lake Tahoe. Thus, the presence of this excess is con- firmatory that materials deposited within the last 8 years have been recovered by the coring procedure for the former two lakes but not in the latter two.
The two shallow, sea-level lakes from Wisconsin, Trout and Mendota, have rates of accumulation about six times higher than Lakes Tahoe and Titicaca at several kilometers elevation. The importance of the river-borne solids to the sedimentary deposits, as opposed to phases brought in by wind transport, are emphasized in such data.
For Lake Tahoe, the only river entering the basin is the Truckee which contains between 100 and 250 mg/l. of dissolved solids (CRIPPEN and PAVE~KA, 1972). If these materials uniformly deposit to the lake floor (the residence times of the waters entering Lake Tahoe are probably of the order of several hundred years at least), then a sedimentation rate of between 0.072 and 0*18 rnrn~~ is computed. These values are substantially higher than those derived from radiocarbon measurements on the upper meter of the sedimentary column (HYNE et aZ., 1972) and somewhat lower than the values reported here.
An understanding of the geochemical behavior of Pb-210 in lacustrine systems is dependent upon (1) a knowledge of the annual fluxes of Pb-210 in rivers and rains and (2) Pb-210 profiles in sediments where the uppermost portions of the deposit have been recovered (now possible with the use of such devices as box-corers). Up to the present time, such data has not appeared in the literature. This information would allow the relative contributions of rain-delivered and river-delivered Pb-210 to the lake systems to be determined. In addition, comparative geochemical studies of different types of lakes could be carried out.
Th-228/Th-232 GEOC~~ONOLOQIES IN MARINE SEDIMENTS
The Th-228 activities in leachates from surface layers of the Santa Barbara and Baja California sediments are far above those required by radioactive equilibrium with Th-232 (Tables 7 and 8). In deeper strata, such as those deposited about a decade or so previously, the Th-228 activities can be significantly below those required by radioactive equilibrium.
Table 7. Thorium isotopes in Santa Barbara Basin sediments (Box Core 262-l recovered 23 September 197X; chemical separations in April 1972)
Depth in Deposition measured Ra-228 sediment period at time &h-.8ao Thorium growth
(ON (Y@w of plating -4TLesz fPP+ unsupported growth R = 0.9
* From Box Core 260.1 taken in same wea end st 881118 time ~a Box Core 262-l. These surf&w values have been averaged in subsequent graphs and calculetions.
1182 MINORU KOIDE, KENNETH W. BRULAND and EDWARD D. GOLDBERQ
Table 8. Thorium isotopes in Baja California sediments (time of collection 1 July 1969; time of plating, May 1972). Average sadimentation rate in top 5 cm from Pb-210 data of 0.35 cm/yr
Depth in Deposition measured Ra-228 sediment period t at time ATh-220 Thorium Th-232 growth
(cm) (Yews) (years) of plating &h--232 (PPN unsupported growth R = 0.90
The total Th-228 in the sediment can be related to its predecessors in the follow- ing manner. One component exists in the detrital phases which is probably in radioactive equilibrium with both Ra-228 and Th-232. Experimentally, we assume that this Th-228 would be retained in the HCl insoluble phase. A second component is the authigenic Th-228 which is that measured in the HCI leachate. This can have three sources, each of which is independent of the other: (1) from sea water where it is derived from the excess Ra-228 there-the Th-228/Th-232 activity ratio appears to be of the order of 1.0-20; (2) from the Th-232 in the sediment-this Th-232 is precipitated from sea water in the absence of its daughters Ra-228 and Th-228; and (3) from unsupported Ra-228 deposited in the sediments from sea water in the absence of its daughter Th-228. This model of the sources of Th-228 is described in the following three equations.
A Th-ZZS,,,,~ = A Th-228unsupported + A~h22~~~4sa erorth + ATh-228Ba_zzs growth
or dividing by the activity of Th-232 found in the leachates
utilizing the Bateman Equations for the growth of Th-228 from its parents Ra-228 and Th-232 where the lambdas are the decay constants of the subscripted isotopes and R is the activity ratio (A&+,,,/A,,_,,,) with A&228 representing the activity of radium-228 at the time of sediment deposition. The data for the Santa Barbara and Baja California deposits are given in Tables 7 and 8.
The left-hand term in equation (3) is the measured activity ratio. The second right-hand term is evaluated directly where the time, t, is taken as the period between chemical separation and deposition. The Santa Barbara samples were recovered on
Th-228/Th-232 and Pb-210 geochronologies in marine and lake sediments 1183
23 September 1971 and were plated in the month of April 1972. The time period between plating and collection is taken as O-5 years and combined with the time since deposition. The third right-hand term in the equation is evaluated in a similar way, except at first the ratio R was taken as 1.
In order to have the unsupported Th-228 decay with a half-life of 1.91 years, the value of R has been approached through trial and error. The most reasonable figure appears to be about 0.9 as is illustrated in Fig. 8. The relative contributions of the
0 I 25 4.5 6-5 85 103 years
Fig. 8. Calculated values of the ratio ATh-228 (unsupported)/Am_2s2 for various values of R for Santa Barbara Basin sediments. Note for R = 0.90 the ratio de-
creases in a straight line with a half-life of I.90 years.
sources of Th-228 are plotted in Fig. 9. There are other approximations that might be used to resolve the amounts of these contributions. For example, the last two terms in equation (3) may be approximated by the lowest measured values of the Th-228/Th-232 activity ratio found at depths in the core. The rationale for this approach may be seen in Fig. 9, where at depth in the core this lowest value of the ratio is nearly equivalent to the combined Th-232 and Ra-228 supported Th-228. The results for the Santa Barbara Core are plotted in Fig. 10.
These rather high values of R strongly suggest that the residence time of radium in these inshore waters does not differ drastically from that of thorium. Such a situation could arise from the transport of radium to the deposits in phytoplankton, a possibility in line with recent observations of SHANNON and CHERRY (1971) who find marked accumulations of this alkaline earth metal in algae. Clearly, measurements of the Ra-228 contents are necessary to confirm our model as well as to extend our knowledge of the marine geochemistry of radium.
1184 MINORTJ KOIDE, KENNETH W. BRULAND and EDWARD D. GOLDBERGI
Fig. 9. Relative contributions to the Th-228/Th-232 activity ratios in the Santa Barbara Basin sediments. Note that for times after about 6 years theratioinvolving supported Th-228 can be approximated by the lowest measured value of the ratio.
Fig. 10. Activity ratios, Th-228/Th-232 for leached materials from Santa Barbara Basin sediments. The measured values are given by the hatched lines. The meth- ods to compute the unsupported Th-228 are: (1) by using a value of R = O-9; and (2) by using the lowest measured value of the Th-228/Th-232 ratio.
Th-228/Th-232 and Pb-210 geochronologies in marine and lake wdiments 1185
Implicit in our model is the assumption that there is no diffusion of radium over the time periods involved or disturbances in the deposits from organisms or near bottom currents.
For the unvarved Baja California core, the times of deposition for the various levels were obtained from the Pb-210 data and the Th-228/Th-232 ratios approached in a way similar to the one above for Santa Barbara (Table 8 and Fig. 11).
100 E !-
h
cm
Fig. 11. Activity ratios, Th-228/Th-232, for leached materials from the Baja California sediments. The measured values are given by the hatched lines. The
methods to compute the unsupported Th-228 are: (1) by using a value of R = 0.9; and (2) by using the lowest measured value of the Th-228/Th-232 ratio.
Most probably, Th-228/Th-232 activity ratios can give reliable rates of accumula- tion for coastal marine sediments for periods up to a decade, even in the absence of radium values. This is especially evident in the case of the Baja California deposit where the extrapolated value of the ratio is about 13. In addition, a combination of Pb-210 accumulation rates with such Th-228/Th-232 rates will provide, where concordant results are found, detailed geochronologies for periods up to a century.
Th-228/Th-232 RATIOS IN LAKE SEDIMENTS
Th-228/Th-232 ratios were determined in Lake Mendota and Trout Lake sedi- ments whose upper levels appear to have been retained in good part during the coring process on the basis of the Pb-210 results (Table 9). It should be pointed out that these analyses were performed before we developed the concept that the thorium activity ratio might be useful for dating lacustrine sediments. Thus, the sampling intervals are very large, as the materials were obtained for Pb-210 studies. Also, we exhausted our supplies of material and could not make additional analyses. Our
1186 MINORU KOIDE, KENNETH W. BRULAND and EDWARD D. GOLDBERG
Table 9. Thorium isotopes in lake sediments. For Lake Mendota, HCI leached separates were
assayed; for Trout Lake the analyses are for the total sample
Depth in sediment
(cm)
Th-232
(ppm)
ATh-228/ATh-232 Corrected to
At plating collection time*
LAEE MENDOTA o-5 4.7 1.30 1.357 3.9
lo-15 38 1.09 1.01
20-25 4.8 1.07 1.04
30-35 3.1 1.06 0.86
40-45 2.2 1.04 1.03
TROUT LAKE o-5 3.3 1.10 2.66 14.1
5-10 3.4 1.14 2.36 11.7
* This correction was made by assuming that the measured activity of Th-228 consisted of
an unsupported component and of a Th-232 supported component. In the latter case, an amount of Th-228 in radioactive equilibrium with Th-232 was taken.
t A redundant experiment was performed to establish the Th-228 excess. A parallel O-5 cm sample was counted one year following plating and the Th-228/Th-232 activity ratio dropped to a value of 0.98 f 0.3. Correcting this value back to the time of plating gives an activity ratio of 1.4, in agreement with the measured 1.35 value.
discussion is presented to indicate the potential of the ratio in developing lake geochronologies.
For both the Trout Lake and Lake Mendota samples, the thorium contents a’re quite reasonable with respect to levels in crustal rocks (several parts per million) but the corrected activity ratios of Th-22S/Th-232 are unusually high (Table 9). Since we have integrated over perhaps several decades with the possibility of mixing in the sediments, detailed studies of these preliminary results do not appear war- ranted here. On the other hand, it is quite clear that the high excesses of Th-228, over and above that required by radioactive equilibrium with Th-232,” indicate the possibility that a Th-228/Th-232 geochronology may be held within such deposits. More detailed sampling over shorter time intervals on appropriately collected cores should reveal the applicability of such a method.
CoNCLusIoN
Pb-210 dating of sedimentary deposits (lacustrine and coastal marine) appears valid for periods up to a century or so. Th-228/Th-232 geochronologies may be applicable for times up to a decade. Both techniques depend upon the removal from the overlying water column of chemically reactive decay products of radium iso- topes-Pb-210 from Ra-226 and Th-228 from Ra-228. In rapidly depositing sedi- ments (mm to cm/year) the Th-228/Th-232 technique may be especially valuable. The use of these geochronologies together provides insights into the geochemistries
* The corrected Th-228/Th-232 ratios in the totally dissolved samples from Trout Lake would have been even more impressive if leached samples had been used instead.
Th-228/Th-232 and Pb-210 geocbronologies in marine and lake sediments 1187
of lead, radium and thorium. Further, the presence of excess Th-228 in the upper- most layers of the deposit is indicative that materials accumulated over the last ten years have been recovered at least partially, if not totally. The immobility of thorium isotopes in the deposits makes the Th-2ZS~Th-~3~ geoc~onologies especially useful where reducing environments can alter the distribution of lead isotopes and hence interfere with Pb-210 geochronologies.
~c~n~le~ge~~~s_The authora are indebted to Mr. AXDREW SOWAX of the Scripps Institution of Oceanography who provided the box-cores from Baja California and from the Santa Barbara Basin.
Mr. JOHN C&RIB-FIN provided us with the opal contents of the sediments. Professor G. FRED LEI of the University of Wisconsin made available to us the sediments
from Lake Mendota and Trout Lake, for which we were most grateful. Dr. CHARLES GOLDMAN and Mr. ROBERT RICHARDS of the University of California at Davis
graciously made available facilities at Lake Tahoe for the coring activities there. The investigation was carried out; under a grant from the U.S. Atomic Energy Commission,
Division af Biology and Medicine, AT(O4-31-34, Project 84.
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