12.3     Daughter excess methods

 

12.3.1  234U dating of carbonates

 

238U decays via two very short-lived intermediates to 234U (Fig. 12.2). Since 234U and 238U have the same chemical properties, it might be expected that they would not be fractionated by geological processes. However, Cherdyntsev and co-workers (1965, 1969) showed that such fractionation does occur. In fact, natural waters exhibit a considerable range in 234U/238U activities from unity (secular equilibrium) to values of 10 or more (e.g. Osmond and Cowart, 1982). Cherdyntsev et al. (1961) attributed these fractionations to radiation damage of crystal lattices, caused both by " emission and by recoil of parent nuclides. In addition, radioactive decay may leave 234U in a more soluble +6 charge state than its parent (Rosholt et al., 1963). These processes (termed the ‘hot atom’ effect) facilitate preferential leaching of the two very short-lived intermediates and the longer-lived 234U nuclide into groundwater. The short-lived nuclides have a high probability of decaying into 234U before they can be adsorbed onto a substrate, and 234U is itself stabilised in surface waters as the soluble UO2++ ion, due to the generally oxidising conditions prevalent in the hydrosphere.

 

            The variety of weathering conditions prevailing in the terrestrial environment leads to very variable 234U/238U activity ratios in fresh water systems. However, the long residence time of uranium in seawater (> 300 kyr, Ku et al., 1977) maintains seawater 234U/238U within narrow limits, corresponding to an activity ratio of ca. 1.14 (Goldberg and Bruland, 1974). A major uranium sink in the oceans is calcium carbonate, with which uranium is co-precipitated. This is deposited in shallow water by marine organisms, and in deep water as an authigenic mineral (i.e. by direct chemical precipitation). At the time of deposition, this material takes on the ‘daughter excess’  234U/238U activity ratio of seawater, but once isolated, the excess decays away until secular equilibrium with the parent is regained (Fig. 12.4). Given an estimate of the original 234U/238U fractionation, and given subsequent closed system behaviour, the system can be used as a dating tool until it returns to within analytical error of secular equilibrium.

Fig. 12.4. Plot of 234U/238U activity against time showing the return to secular equilibrium after isolation from seawater. Arrows show the amplification of analytical errors as the system approaches equilibrium.

 

            Unfortunately, many problems are encountered in the practical application of this method. As noted above, the variable uranium isotope fractionations observed in fresh-water systems preclude its application there. In addition, pelagic sediments are ruled out by open-system behaviour of uranium after deposition (Ku, 1965), while mollusc shells also tend to take up uranium after deposition (Kaufman et al., 1971). However, the method has been applied with reasonable success to the dating of corals (e.g. Thurber et al., 1965).

 

            The decay of excess 234U can be expressed by the fundamental decay equation [1.5]. Although this equation was derived in section (1.4) for atomic abundances, it is also true for activities (on dividing both sides by the decay constant, e.g. 8234U):

 

                        n          n0

                        )  =     )) e!8t                                                 [12.2]

                        8          8

 

                        A  =     A0 e!8t                                                  [12.3]

 

In order to date a carbonate sample by the decay of excess 234U (Fig. 12.5), we can substitute into equation 12.3 to yield:

 

            234UXpresent  =  234UXinitial e!8234 t                                     [12.4]

 

where ‘x’ signifies excess activities above secular equilibrium, and ‘initial’ signifies the activity at the time of precipitation.

 

            As noted above, all nuclide quantities in this chapter will be presented in terms of activities (unless stated otherwise). However, absolute activities are not as readily measurable as activity ratios, so it is convenient to divide through by 238U activities. But because of the very long half-life of 238U, the activity of 238Upresent is the same as 238Uinitial. So:

 

            (234UX/238U)present  = (234UX/238U)initial e!8234 t                              [12.5]

 

            Since these quantities are in the form of activities, the excess 234U/238U activity is equal to the total activity ratio minus one (that part corresponding to secular equilibrium). So:

 

            (234U)total                       | (234U)total         |

            ())))  !  1   =            | ())))  !  1    |  e!8234 t                       [12.6]

            (238U)present                    | (238U)initial       |

 

            Hence, if we assume that the initial activity ratio of the sample is given by present-day seawater, we can calculate the age of a coral simply by measuring its present-day activity ratio. Chen et al. (1986) showed that modern seawater in the Pacific and Atlantic oceans has a homogeneous 234U/238U activity ratio, with values of 1.143 and 1.144 respectively. Given the > 300 kyr residence time of uranium in seawater (Ku et al., 1977), this gives us a strong expectation that the activity ratio should have been close to this value within the 1.2 Myr theoretical dating range of the method. This was indeed demonstrated by more recent work (Henderson, 2002), which indicated a 234U/238U activity ratio of 1.145 for the last 350 kyr. This value is often expressed as * (excess 234U), with a value of 145 per mil (Fig. 12.4).

 

            Because the seawater 234U/238U value is relatively close to secular equilibrium, a small error in the 234U measurement leads to a large error in the calculated age (Fig. 12.4). Hence, the 230Th deficiency method (section 12.4) yields more precise ages below 400 kyr. However, using mass spectrometric analysis, the 234U method allows the possibility of dating back to 1 Myr with tolerable precision. This was demonstrated by Ludwig et al. (1991), who used 234U to date submerged coral terraces off NW Hawaii. Comparison of 234U ages with terrace depth led to a subsidence curve which is approximately linear for the last 500 kyr, at a rate of 2.6 mm/yr (Fig. 12.5). Small undulations on the subsidence curve represent the calculated effect of eustatic sea-level fluctuations. These cause development of coral terraces by periodically neutralising subsidence (to create a sea-level ‘stand’) and then exacerbating subsidence, to drown the reef.

Fig. 12.5. Plot of terrace depth against mass spectrometric 234U age for corals off NW Hawaii showing the good fit to a cooling subsidence curve (modulated by eustatic variations). After Ludwig et al. (1991).

 

            The good fit of data points to a linear subsidence model in Fig. 12.5 provides evidence of the reliability of the 234U/238U dating method, including the closed-system assumption for uranium systems in coral. This is attributed to the good preservation of submarine coral systems. In contrast, coral which has suffered fresh-water percolation is very susceptible to open-system behaviour. For example, Bard et al. (1991) found that many coral specimens over 50 kyr old which had been dated by 230Th at the Lamont laboratory had calculated initial 234U activities above the seawater value of 1.14 (Fig. 12.6). These corals come from raised terraces on the island of Barbados, which is undergoing net tectonic uplift with time.

 

            Since the evidence from Hawaii is consistent with a constant 234U activity in seawater (at least for the past 500 kyr), the high apparent initial ratios found by Bard et al. must be attributed to open-system behaviour of uranium. Unfortunately this sub-aerial redistribution of uranium will also affect the 230Th ages calculated from these samples. For example, the sample with an apparent age of 528 kyr in Fig. 12.6 comes from a lower terrace on Barbados, and hence must be younger than the samples with apparent ages of 230 and 418 kyr.

Fig. 12.6. Plot of initial 234U/238U activities in Barbados coral terraces, against calculated 230Th ages, to show apparent initial ratios above the seawater value. After Bard et al. (1991).

 

 

12.3.2  234U dating of Fe–Mn crusts

 

Another application of 234U is to the dating of ocean floor ferro-manganese crusts. These crusts grow over long periods of time on the ocean floor, and provide very useful archives of past seawater chemistry, if they can be dated accurately. In the first U-series dating study on this material, Chabaux et al. (1995) analysed two crusts dredged from 1900 m depth on a West Pacific seamount. For both crusts, the 234U and 230Th daughter excess methods gave consistent growth rates of ca. 7.6 mm/Myr and 6.7 mm/Myr respectively (Fig. 12.7). This suggested that closed-system conditions were preserved, and that initial uranium and thorium isotope ratios remained constant (within error) during the 150 kyr period of deposition.

Fig. 12.7. The use of  230Th/232Th and excess 234U activities in a ferromanganese crust to determine the growth rate and the zero-age surface of the crust before abrasion. Modified after Chabaux et al. (1995). NOTE: the y axis of the lower diagram has been incorrectly labelled. Figures should read: 0.04, 0.10, 0.16.

 

            A problem with the sampling of Fe–Mn crusts is that the outer surface can be abraded during dredging operations, preventing a determination of the absolute age of the crust from its growth rate. This is specifically a problem with the 230Th method, because the initial thorium isotope ratio of seawater is variable. Therefore, in the absence of an ‘initial ratio’ determination from the surface of the crust, an absolute date is not possible. On the other hand, the initial 234U/238U ratio of seawater is constant in space and time. Therefore, it should be possible to use 234U to determine the original growth surface of the crust by projecting the excess 234U activity back to the known seawater composition (Fig. 12.7b).

 

            Fe–Mn crusts are quite porous and have very slow growth rates. Therefore, there was concern that diffusion could cause open system behaviour of U-series nuclides in the crust after deposition. The work of Chabaux et al. (1995) appeared to allay these fears, but subsequent work by Chabaux et al. (1997), Neff et al. (1999) and Henderson and Burton (1999) has confirmed that U diffusion is a problem in Fe–Mn crusts. These later studies gave Fe–Mn growth rates from 7 to 19 mm/Myr, based on excess 234U measurements, but only 3 to 4 mm/Myr based on excess 230Th. The discrepancy between these growth rates, especially for the very high value of 19 mm/Myr, must be explained by diffusional redistribution of uranium. By comparing the observed decay profile of 234U with a theoretical decay curve based on excess 230Th dates, Henderson and Burton were able to calculate effective diffusion coefficients for uranium in various Fe–Mn crusts (e.g. Fig. 12.8). These values were in the range from 10-6 to 5 H 10!8 cm2/yr.

Fig. 12.8. Plot of 234U activity (parts per mil above secular equilibrium) against depth in a Fe–Mn crust from the North Atlantic. The measured 234U ‘decay curve’ is compared with the predicted decay curve for zero diffusion. After Henderson and Burton (1999).

 

            Henderson and Burton argued that the magnitude of U diffusion in the crust must be controlled by the ability of uranium to be exchanged from the solid crust into pore water within the crust. Hence, the diffusion coefficient must be dependent on the partition coefficient of uranium between seawater and the Fe–Mn crust. The solubility of U in surface water has already been discussed. In contrast, thorium is said to be ‘particle reactive’, meaning that it is readily adsorbed onto the surface of detrital grains, and has a very short residence time in natural waters. Thus, uranium has a seawater residence time exceeding 300 kyr, whereas thorium has a seawater residence more than 1000 times shorter than this.

 

            Based these principles, Henderson and Burton estimated the diffusion coefficients of other elements from their relative concentrations in the crust and in seawater. This calculation places these species in two groups (Table 12.2; Henderson, pers. comm.). Uranium, together with strontium, has a relatively large diffusion coefficient in crusts, consistent with these elements being non-particle-reactive, and hence conservative in seawater. At the other extreme, thorium has a partition coefficient about six orders of magnitude lower than uranium, consistent with it being the most particle-reactive of its group. In addition, other members of the particle-reactive group also have sufficiently low diffusion coefficients in Fe–Mn crusts that they can be considered immobile. This is encouraging for the use of Fe–Mn crusts as an inventory of the past seawater signatures of these tracers (sections 4.5, 5.6, 9.3 and 14.3). However, it is concluded that the 234U method is not reliable for dating this material.

 

Table 12.2 Partition and diffusion coefficients in Fe–Mn crusts

)))))))))))))))))))))))))))))))))))))))))

Element            Partition coefficient       Diffusion coefficient

                        (crust / seawater)          in crust, cm2 / yr

)))))))))))))))))))))))))))))))))))))))))

Particle reactive species

Th                    2.0 H 109                      2 H 10!12

Nd                   2.6 H 108                      2 H 10!11

Pb                    1.6 H 108                      3 H 10!11

Be                    4.0 H 107                      1 H 10!10

Hf                    5.2 H 106                      9 H 10!10

Os                    1.7 H 105                      3 H 10!8

 

Conservative species

U                     4.0 H 103                      1 H 10!6

Sr                     2.1 H 102                      2 H 10!5

)))))))))))))))))))))))))))))))))))))))))

 

            Another area where excess 234U activity data would be very useful is in the dating of  planktonic foraminifera, since these are the basis of the seawater oxygen isotope record (section 12.4.2). However, forams have low U contents (typically 20 ppb), which tend to be swamped by the U contents of ferromanganese diagenetic overgrowths. Henderson and O’Nions (1995) showed that dithionite solution (a reducing agent) could be used to clean recent forams in order to recover normal seawater uranium isotope ratios. However, a test on 2 Myr old forams showed excess 234U activities above the seawater value, which must have been introduced from pore waters after sedimentation. This suggests that forams do not remain a closed system for uranium, and therefore cannot be used for dating or to constrain the uranium isotope evolution of seawater.

 

 

12.3.3  230Th sediment dating

 

The differing behaviour of uranium and thorium in seawater causes U/Th fractionation during the formation of different sediment types, leading to systems out of secular equilibrium. As noted above, 238U decays via two very short-lived intermediates to 234U in seawater. This in turn decays to 230Th, but the latter is almost immediately adsorbed onto the sediment surface. Because it is preferentially enriched on the sediment surface, relative to its (234U) parent, 230Th is ‘unsupported’ and out of secular equilibrium. However, after isolation from the sediment!water interface, this unsupported 230Th begins to decay back to secular equilibrium with its parent. Hence, this method should allow the dating of sedimentary deposition.

 

            Adsorption of thorium onto detrital grains is so much more effective than uranium adsorption that for young sediments the uranium-supported component (i.e. the component in secular equilibrium) can be effectively ignored. In other words:

 

            230Thexcess  .  230Thtotal                                                   [12.7]

 

Therefore, we can use the method as a dating tool by means of the simple decay equation:

 

            230Thpresent  =   230Thinitial e!8230 t                                     [12.8]

 

Since the 230Th excess method is used to study sedimentation, it is convenient to formulate t in terms of sediment depth, D, (in a core), and sedimentation rate, R:

 

                      t   =   D / R                                                         [12.9]

 

If we substitute this into equation [12.8] and take the natural log of both sides, we obtain:

 

            ln (230ThP)  =  ln (230ThI)  !  D (8230 / R)                        [12.10]

 

This corresponds to the equation for a straight line:

 

                        y     =       c     !  x  m                                      [12.11]

 

Hence, if the natural log of the present-day 230Th activity is plotted against depth in the core, the sedimentation rate can be obtained from the reciprocal of the slope (solid line in Fig. 12.9):

 

                                                D

            R   =   ! 8230  @ )))))))))))))                            [12.12]

                                    ln 230ThP ! ln 230ThI

Fig. 12.9. Schematic plot of log 230Th activity (decays per minute / gram) against depth to show behaviour expected in a core formed by a constant sedimentation rate. Solid line = young sediments; dashed line = older sediments with U-supported 230Th.

 

            Although the effects of U-supported 230Th may be negligible near the sediment surface, this component becomes increasingly important as the system approaches secular equilibrium with increasing burial depth (dashed line in Fig. 12.9). Two possible sources of U-supported 230Th may be present. Authigenic minerals such as calcite contain no 230Th at the sediment surface, but their uranium budget generates 230Th until this reaches secular equilibrium with the parent. This fraction is best removed physically by mineral separation. On the other hand, detrital grains contain 230Th which is in secular equilibrium with 234U and 238U, even at the sediment surface. This fraction is removed by subtracting 234U activity (in secular equilibrium with the 230Th daughter) from total 230Th activity (e.g. Ku, 1976). This leaves the ‘excess’ 230Th activity of the clay fraction:

 

            230Thexcess  =  230Thtotal ! 234U                                         [12.13]

 

The corrected (excess) activities determined in this way are substituted into equations [12.10] and [12.12] above to determine sedimentation rates. Since the concentration of 230Th in the oceans is expected to be constant through time, and the adsorption process is expected to be of constant efficiency, the initial concentration of 230Th in the detrital sediment fraction should be constant. Then, if the bulk sedimentation rate (R) remains constant with time, excess 230Th activity will decrease as a log function with depth. Figure 12.10a shows data from a Caribbean core which fit this model (Ku, 1976), yielding a linear fit (of log activity against depth). The regression slope yields a sedimentation rate R of 25 " 1 mm/kyr for the last 300 kyr.

 

            Within the decay chain of 235U, the species 231Pa (protactinium) is another particle- reactive species that behaves very similarly to thorium in seawater. Therefore it also develops excess activities at the sediment surface relative to its parent isotope of uranium. 231Pa has a half-life of 32.76 kyr (Roberts et al., 1969), and is therefore used in an analogous way to 230Th. However, because the parent (235U) has a much lower abundance than 238U, analytical errors are larger. Therefore, 231Pa is usually used only as a concordance test for 230Th dates, to check that the dating assumptions have been upheld. This application is shown in Fig. 12.10b.

Fig. 12.10. Plots of (a) excess 230Th activity, and (b) excess 231Pa activity, against depth in a sediment core, yielding two independent estimates of average sedimentation rate. After Ku (1976). NOTE: sedimentation rates given are cm/kyr, not cm/yr.

 

 

12.3.4  230Th)232Th

 

Unfortunately, not all cores yield such good results as that in Fig. 12.10, because 230Th and 231Pa are sometimes variably diluted in sediments. Picciotto and Wilgain (1954) suggested that this problem could be avoided by using 232Th as a reference isotope to normalise for variable absolute levels of adsorbed Th. They justified this approach on the basis that 230Th and 232Th (t1/2 = 14 Byr) are chemically identical, so they should be removed from seawater at the same rate. Because 232Th has such a long half-life, it suffers no significant decay within the dating range of 230Th. Therefore, if we assume that initial 230Th/232Th activities at the sediment surface remain constant at any given locality through time, we can divide both sides of equation [12.8] by 232Th (where X signifies excess activities):

 

            (230ThX)                        (230ThX)

            ()))))        =             ()))))          e!8230 t                [12.14]

            (232Th)present                   (232Th)initial

 

On applying this to the activity versus depth plot, we obtain:

 

            (230ThX)            (230ThX)            8230

        ln ()))))   =   ln ()))))   !  D  )))                            [12.15]

            (232Th)P            (232Th)I R

 

            Picciotto and Wilgain pointed out that, for this method to work, effectively all of the Th in the sediment must have been chemically precipitated, rather than being detrital. However, 30% or more of the total 232Th budget in a pelagic sediment is normally within the detrital phases (Goldberg and Koide, 1962). Consequently, Ku et al. (1972) argued that the effect of dividing by 232Th is similar to the effect of dividing by the detrital (non-carbonate) fraction in the analysed sample. If the detrital fraction in the sediment is constant then this does not cause a problem, but if it varies with depth, this will perturb the initial 230Th/232Th ratios and hence lead to erroneous ages and sedimentation rates. This problem is illustrated in Fig. 12.11 using data for a core from the Mid Atlantic Ridge. The 232Th/230Th plot (Fig. 12.11a) yields an age for the 12 cm deep horizon (arrowed) which is more discordant from the 14C age of 17 kyr than the simple 230Th plot (Fig. 12.11b).

Fig. 12.11. Thorium isotope results from the ZEP 15 core (Mid Atlantic Ridge) showing interpretations of sedimentation history using a) the 230Th/232Th method and b) the simple 230Th method. After Ku (1976).

 

            In order to reduce the perturbing effect of the detrital component on 230Th/232Th ages, Goldberg and Koide (1962) used a technique by which authigenic minerals and adsorbed Th were leached from the detrital component with hot hydrochloric acid. This led them also to adopt a different correction for U-supported 230Th. On the assumption that no detrital 230Th component was leached, they excluded the component in secular equilibrium. Instead they corrected for U-supported 230Th in the authigenic (carbonate) component, which is expected to grow with time. This is equivalent to the 230Th daughter deficiency method, and will be dealt with in detail below (section 12.4.1). If the immediate parent (234U) is assumed to be in equilibrium with 238U (an approximation) then the growth of U-supported Th is given by equation [12.24]. This is subtracted from total 230Th activity to determine excess 230Th:

 

            230Thexcess  =  230Thtotal  !  238U (1 ! e!8230 t)                               [12.16]

 

Ku(1976) argued that this method also had drawbacks, since thorium leaks from detrital phases during the acid leaching process. Hence, it is concluded that normalising with respect to 232Th can sometimes improve 230Th data, but sometimes has a degrading effect. Therefore, it tends to be used on an ad hoc empirical basis.

 

 

12.3.5  230Th sediment stratigraphy

 

In view of the difficulties described above, the 230Th dating method should probably be regarded as semi-quantitative in most circumstances. However, 230Th data may be a powerful tool for stratigraphic correlation of Quaternary sediments. An example of this application is provided by the study of Scholton et al. (1990) on a 5-m core from the Norwegian Sea near Jan Mayen (Fig. 12.12). In general, excess 230Th activity data from this study fitted an average decay curve equivalent to a sedimentation rate of 1.9 cm/kyr. This is in reasonable agreement with the rate of 1.6 cm/kyr calculated from oxygen isotope stratigraphy. However, the data display large short-term variations superimposed on the mean decay curve.

Fig. 12.12. Plot of excess 230Th activity (on a log scale) against depth in core 23059 from the Norwegian Sea. Regression line indicates average sedimentation rate. After Scholten et al. (1990).

 

            Traditionally, variations of this type have been attributed to changes in sedimentation rate. However, this is clearly impossible for some segments of core 23059, which define a positive slope of excess activity against depth (opposite to the effect of radioactive decay). In order to examine these short-term activity variations, Scholten et al. corrected the data for radioactive decay since burial (using the mean decay curve), and then ratioed these initial (excess) 230Th activities against 232Th to correct for variable carbonate contents. The resulting values display variations with depth which are correlated with *18O (Fig. 12.13). Scholten et al. attributed these variations to the influence of climatic factors on the 230Th deposition rate. Climatic changes affect the productivity of plankton, and hence the amount of sinking organic matter.

 

            Biogenic particle fluxes were argued by Mangini and Diester-Haas (1983) to control the downward flux of radionuclides off NW Africa, and hence 230Th activity variations in sediment cores. Therefore Scholten et al. argued that the low initial excess 230Th/232Th activity ratios in isotope stages 2 and 6 (Fig. 12.13) were due to a widespread reduction of biogenic paleo-productivity during these cold periods. This regional climatic control of radionuclide deposition allows the opportunity of correlating 230Th variations between different sites in an ocean system. Similar results may be obtained using 231Pa/230Th activity ratios (Kumar et al., 1993), and using the cosmogenic isotope 10Be (section 14.3.4).

Fig. 12.13. Comparison of the depth-dependence of excess initial 230Th/232Th and *18O in core 23059. Numbered intervals are stages based on 18O stratigraphy. Stages 1 and 5 represent the holocene and the 120 ) 130 kyr interglacials. After Scholten et al. (1990).

 

            The rapid adsorption of 230Th onto particulate matter makes it a very useful oceanographic tracer. Hence, several studies have been directed at understanding its behaviour in seawater, including its ocean residence time. The activity of 230Th in North Atlantic seawater was determined by Cochran et al. (1987) by pumping large volumes of seawater, at various depths, through a filter system designed to scavenge 230Th. Two profiles showed increasing activity with depth, both on particulates and in solution (Fig. 12.14). High levels of dissolved 230Th at depth were attributed to attainment of sorption equilibrium between particulates and seawater. In addition, riverine supply of 230Th causes slight enrichment in shallow seawater off Cape Hatteras, but makes a negligible contribution to the total 230Th inventory.

Fig. 12.14. Plot of total 230Th activity as a function of depth in waters off Cape Hatteras ( " ) and north of Puerto Rico ( ! ). After Cochran et al. (1987).

 

            Yu et al. (1996) used these results to make a new estimate of the 230Th residence time in North Atlantic seawater. This value can be determined from the 230Th inventory per unit volume of water (n = activity / 8230), divided by the supply flux per unit volume. Since riverine supply of 230Th is considered insignificant, the supply flux is equal to oceanic 234U decay. Hence, in terms of activities:

 

                                                 1                     230Th

            tau Th-230           =         )))     .           ))))                          [12.17]

                                                 8 230                234U

 

Based on profiles of activity against depth, Yu et al. estimated an average 230Th activity of 0.65 d.p.m. / m3 in the North Atlantic at 25 oN. This compares with a 234U activity of 2700 d.p.m. / m3 which is constant throughout the oceans due to the long residence time of uranium. Plugging these values into the above equation gave a tau value of only 26 yr, much shorter than previously estimated.

 

 

12.3.6  231Pa–230Th

 

Similarities between the chemistries of Pa and Th prompted Sackett (1960) and Rosholt et al. (1961) to suggest their use in conjunction as a dating tool. Three factors suggested that the adsorbed initial 230Th/231Pa activity ratio should be a constant (~11) defined by the production ratio of the two species: firstly, the isotope ratio of their parents is relatively constant in seawater (as demonstrated by the concordance of 231Pa and 230Th dates); secondly, they are both adsorbed rapidly compared with their half-lives; and thirdly, the direct river-borne contribution of 231Pa and 230Th to the oceans is negligible (Scott, 1968). In this case, equation [12.8] can be divided by the corresponding equation for protactinium, yielding:

 

            (230Th)excess        (230Th)excess

            ()))))        = ()))))        e!(8230 ! 8231) t                    [12.18]

            (231Pa)P            (231Pa)I

 

Equation [12.18] can then be solved for t by assuming the initial (production) ratio to be 11. The early work of Sackett (1960) and Rosholt et al. (1961) appeared to bear out the assumption. However, subsequent work has yielded variable excess 230Th/231Pa activities at the sediment surface. Sediments often have surface ratios much higher than 11 (e.g. Sackett, 1964), while manganese nodules may have ratios much lower than 11 (e.g. Sackett, 1966). Hence, it is concluded that variable fractionation between 231Pa and 230Th occurs during sedimentation, rendering the method useless as a dating tool.

 

            The variable fractionation between 231Pa and 230Th can now be explained by the different seawater residence times of these species. Because of its extremely particle-reactive behaviour, very little 230Th can be transported laterally (advected) before it is scavenged and sedimented. In contrast, 231Pa can be advected by ocean currents before it is scavenged in locations with a high flux of sinking particles. As a result, 230Th/231Pa ratios vary across ocean basins, normally with high ratios in the centre of the basin, where sedimentation rates are low, and low ratios near the margins where sedimentation rates are high (Yang et al., 1986).

 

            Yu et al. (1996) proposed that the different seawater residence times of Pa and Th should allow their activity ratios to be used as monitors of ocean circulation. For example, the present day Atlantic Ocean is dominated by a ‘conveyer belt’ which transports North Atlantic Deep Water (NADW) southwards to the Antarctic. Radiocarbon evidence (section 14.1.6) suggests that NADW has a residence time of 200–300 yr in the Atlantic. Comparison of this value with the ocean residence times of 231Pa and 230Th indicates that about 50% of 231Pa, but only 10% of 230Th produced in the Atlantic will be exported to the Southern Ocean. These predictions were supported by activity measurements on (recent) core tops from ocean floor sediment (Fig. 12.15). These data are presented in the reciprocal form (231Pa/230Th), and reveal an average activity ratio of only 0.06 in the Atlantic, but 0.17 in the Southern Ocean (relative to a production ratio of 0.09).

Fig. 12.15. 231Pa/230Th activity ratios for the South Atlantic and the Southern Ocean, showing values both above and below the production ratio of 0.09; a) present day; b) last glacial maximum. 231Pa/230Th values: " = below 0.1;  ! = 0.1 – 0.2; <> = over  0.2. After Yu et al. (1996).

 

            Yu et al. made the critical observation that sediments deposited at the time of the last glacial maximum had exactly the same distribution pattern of 231Pa/230Th activity ratios (Fig. 12.15b) as present day sediments. From this observation they concluded that the ocean conveyor belt operated at a very similar rate during the glacial maximum. This result cast doubt on the widely favoured model in which the conveyer belt was thought to have partially or completely ceased during the last glacial maximum (section 14.1.7). Hence, it was critically important to evaluate the result obtained by Yu et al., to see whether the conclusions were robust, or whether factors other than ocean currents could have produced similar patterns at the present day and during the last glacial maximum.

 

            The circulation model of Yu et al. (1996) made a prediction that could be tested, as one way of assessing its robustness. Because the high 231Pa/230Th ratios in sediments from the Southern Ocean were attributed to high sedimentation rates from Circum Polar Deep Water (CPDW), it should be expected that sediment 231Pa/230Th ratios should decrease again near the Antarctic shore, where sedimentation rates are low.

 

            Walter et al. (1997) tested this expectation by collecting a much larger set of data from waters and sediments right across the Antarctic Circumpolar Current (ACC), which extends from 50o S to 60o S, and is equivalent to the range of CPDW. Their data (Fig. 12.16a) showed that high ratios of excess 231Pa/230Th in sediments continued south of 60o into the Weddell Sea, despite the low sedimentation rates in this area near the Antarctic continent. However, analysis of suspended particulates revealed very high 231Pa/230Th activity ratios in this zone (Fig. 12.16b), which were related to the presence of biogenic opal. Therefore, Walter et al. suggested that biogenic opal was much more effective at scavenging Pa than clay minerals. Hence, it is concluded that 231Pa/230Th ratios can be influenced by sediment type, so that they are a less reliable measure of paleo-productivity and ocean circulation than had previously been expected (Luo and Ku, 1999; Chase et al., 2002).

Fig. 12.16. Plot of excess 231Pa/230Th activity ratios as a function of latitude across the Southern Ocean, showing values increasing towards the Antarctic coast. a) recent sediments; b) particulate matter in water. After Walter et al. (1997).

 

            The short ocean residence time of 230Th also makes this tracer useful in constraining the deposition fluxes of other species, such as 10Be, with longer residence times. Frank et al. (1995) illustrated this approach in a study of sediment stratigraphy from the Weddell Sea, Antarctica. Samples were collected at intermediate water depths to avoid contamination by resuspended sediment (> 1500 m below the surface and > 500 m above the sea floor). Both 10Be abundances and excess initial 230Th activities exhibited large variations in concentration between glacial periods (stages 2, 4 and 6) and interglacials (stages 1, 5e and 7).

 

            Frank et al. calculated average burial fluxes for each climatic stage, based on initial radionuclide abundances, dry bulk density, and sedimentation rate. These fluxes were positively correlated with sedimentation rate (Fig. 12.17), and varied by more than an order of magnitude. Since 230Th activities vary to the same extent as the abundance of 10Be, these variations cannot be attributed principally to boundary scavenging of advected nuclides (although this may play some role). Instead, Frank et al. attributed the variations to sediment transport. High radionuclide fluxes during interglacial periods were attributed to focussing of ‘marine snow’ (radionuclide-bearing diatoms) by strong bottom currents. Low radionuclide fluxes during glaciations were attributed to ‘bulldozing’ of sediments by grounded ice shelves, which replaced young (isotopically hot) sediment by old (isotopically dead) material.

Fig. 12.17. Plots of normalised fluxes of excess 230Th and 10Be against normalised sedimentation rate to show similar correlation lines for the two nuclides. ( " ) = glacial, isotope stages 2, 5d, 6; ( ! ) = interglacial, stages 1, 5e, 7. After Frank et al. (1995).

 

 

12.3.7  210Pb

 

Within the 238U decay chain, the daughter product of 226Ra is the rare gas 222Rn. This escapes into the atmosphere from the whole land surface. However, 222Rn has a half-life of only three days, and is followed by four intermediates with half-lives of minutes to seconds, ultimately yielding longer-lived 210Pb. This is estimated to remain in the upper atmosphere for a few days, before the majority returns to the surface in precipitation. Thereafter, unsupported 210Pb decays away with a half-life of 22.3 yr. The use of 210Pb was first suggested as a tool to date snow accumulation by Goldberg (1963). However, it can also be used to date very recent fresh-water and marine sedimentation (e.g. Krishnaswamy et al., 1971; Koide et al., 1972) because 210Pb has an aqueous residence time of only a year or two before adsorption onto sediment.

 

            If the 210Pb concentration in newly precipitated snow or sediment remains more or less constant with time at a given locality (as expected), then the system will behave in exactly the same way as the 230Th excess method. We can then use 210Pb activity at the present-day surface to determine initial 210Pb, and solve for the age of a buried ice or sediment sample:

 

            210Pb  =   210Pbinitial e!8210 t                                            [12.19]

 

            As with 230Th, if we plot the log of 210Pb activity against depth, the slope yields the sedimentation rate. The first application of the method was to snow chronology (Crozaz et al., 1964). The calculated sedimentation rate of snow at the South Pole in water equivalents (6 " 1 cm/yr) compared well with a rate determined from yearly ‘ice varves’.

 

            The short half-life of 210Pb also makes it ideally suited to the dating of historical-age sediments. For example, the method has become an important tool in studying the history of heavy metal pollution of coastal waters and lakes. Bruland et al. (1974) used the method in a study of metal pollution of the Santa Monica basin off Los Angeles. A log plot of total 210Pb activity against depth (Fig. 12.18) yields a linear fit at shallow depths, but the profiles flatten out at ca. 8 cm depth due to the effect of 210Pb supported by 226Ra. However, this can be corrected by subtracting 226Ra activity, yielding excess 210Pb activities:

 

                 210Pbexcess = 210Pbtotal ! 226Ra                                    [12.20]

 

When the data are plotted in this form, the usable range of the method is extended to ca. 150 yr. For the Santa Monica basin the corrected (excess) 210Pb data yield a sedimentation rate of 0.7 mm/yr (Fig. 12.18).

Fig. 12.18. Plot of 210Pb activity against depth in recent sediments of the Santa Monica basin. Solid symbols: total 210Pb activity, including 226Ra-supported fraction. Open symbols: excess 210Pb only. After Bruland et al. (1974).

 

            A particularly appropriate application of the 210Pb method is to studies of anthropogenic Pb contamination of sediments. Shirahata et al. (1980) applied the method to a remote sub-alpine pond in Yosemite National Park, in order to assess the regional atmospheric fallout of Pb from car exhausts. A sedimentation rate of 0.6 mm/yr was calculated from 210Pb data (Fig. 12.19). Bioturbation of the sediment was ruled out because all bomb-produced radionuclides remained within ca. 2 cm of the sediment surface. Total Pb concentrations in the sediment were found to have increased four-fold over the past hundred years, and this change was accompanied by a change in 206Pb/207Pb ratio from a natural local value of 1.15 to an exotic value of 1.2 . The latter was typical of the sources of Pb ore used in the USA for the manufacture of leaded gasoline (section 5.6.1).

Fig. 12.19. Plot of excess 210Pb activity against depth in a sub-alpine pond from Yosemite National Park, California. After Shirahata et al. (1980).

 

            Despite these achievements with the 210Pb method, caution must be exercised in the interpretation of data, since some studies (e.g. Santschi et al., 1983; Benoit and Hemond, 1991) have shown that 210Pb can be re-mobilised from the surfaces of sediment grains into sediment pore waters, and thence into the overlying water column. Benoit and Hemond showed from theoretical modelling that 210Pb re-distribution could occur by diffusion of pore-water, without the need for particle reworking.

 

 

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