9.2.3. The L/L method and implications for routine U-series analysis

using the original electroplating method suggested that no immediate improvement to

chemical yields could be made (section 4.5.3). The modification and subsequent adoption of the Hallstadius method (Hallstadius, 1984) in April 1994 dramatically increased yields

from <10% for thorium and <20% for uranium to between 40 and 90% for both

isotopes. The modified Hallstadius method is now part of the routine uranium-series

procedure in the Liverpool laboratory. The improvement in chemical yields means that

the sample count rates are easily distinguishable above counter background thus

satisfying the criteria of Komura and Sakanoue (1964). They suggested that the

detection limit is four times the standard deviation of the background count rate i.e. 4(b/T)'/j where b is the background countrate and T is the count time of the blank run.

9.2.3. The L/L method and implications for routine U-series analysis.

Dates of selected Chinese sub-samples calculated using the MLE data treatment technique of Ludwig and Tittering,on (1994) were disappointing. The reasons and

potential remedies for these poor results have already been discussed (4.9.2). Despite

these problems the potential o f the method is e.w , t /r

moa ls such that L/L analysis should almost certainly be undertaken routinely when dealino v, •, y QealinS Wlth heavily contaminated samples. The L/L method, with dates calculated usine the MT p Qi _ g ne M LE aigonthm can considerably reduce the doubt attached to a series o f ages corrected „ ,s corrected using only an estimated initial thorium ratio Despite the dating inaccuracies of sub-samples in this study, the correlations between similarly aged features in a) the individual Chinese records (Fignte 6.30) and b) in other contemporaneous records (Figures 6 31a and u- c- „ , , , * a and b, Figure 7.14) corroborates the overall dating accuracy.

These limited L/L results, particularly those from analyses of SSJ2 (section 4.9.1) suggested an interesting possibility that may arise during routine uranium-series analysis

of young « 1 0 ka) and contaminated sub-samples. If a sub-sample has a “ W ^ h ratio of <25 then it is deemed to be detritally contaminated. A routine uranium-series date is normally obtained from a single sub-sample and in effect, this represents a single leach. If

the sub-sample is detritally contaminated then the acid used for dissolution will affect the

quantity of detrital thorium and uranium carried over into the leachate. This in itself will

increase the 230Th/234U ratio and produce an apparently increased age. Because detrital

contamination is independent of the authigenic isotopes any changes in the measured

230Th/232Th ratio will result in differing degrees of correction throughout a series of

‘dirty’ samples. Figure 9.2 shows this effect using a plot of 230Th/234U versus 230Th/232Th

from L/L analyses of coeval sub-samples from SSJ2 (section 4.9.1).

Figure 9.2. Effects of different acid strength on the measured » W * ™ ,■ , c coeval samples from Mexican speleothem « n n Th ratio of three changing * W » T h ratio p r in c e s dT ffcriJg ^ different calculated dates (Table 4.4) actors and subsequently In the case of SSJ2, for each of the HNO? leaches (deemed to be more reliable than the HC1 leaches in this case; section 4.9.1) the 230Th/232Th ratio decreased with increasing acid strength (the 230Th/234U ratio increased because of the increased input of detrital thorium). Thus, in a routine uranium-series procedure the 230Th/234U and 23°rh/232Th ratios (and hence the age) observed in analyses of single sub-samples would be dependant upon the strength of the acid used. For example, Table 4.4 shows that the uncorrected date from each coeval leach from SSJ2 becomes older with more detrital