tivity on the carmel faul

3.3.3 Cluster dating
Osmond isochron dating is based equation [2] according to Broecker (1963):
[2] [ 230 Th/ 238 U] = (1-e -λ230t ) + {[ 238 U/ 234 U] - 1}*{λ 230 /(λ 230 -λ 234 )}*[1-e -(λ230-λ234)t ]
The calculation of age is done using Isoplot3.7 (Ludwig, 2008) with a first derivative
estimation for samples of similar ages plotted on isochron lines (sample age clusters)
Here too the analytical error (±2σ) is given.
The Isoplot program creates a three dimensional plot where x= 230 Th/ 238 U, y= 234 U/ 238 U
and z= 232 Th/ 238 U. This plot, based on the U-series evolution diagram of 230 Th/ 238 U vs.
234 U/ 238 U, incorporates the detrital end member (DEM) of the dated sample and using the
z-axis corrects for it and calculates an average age for all samples plotted on the isochron
line (Ludwig and Titterington, 1994).
Age clusters were determined for the seismite samples, which were then plotted on an
isochron line. These clusters are derived from the ages of seismites from the cave, which
tended to cluster at very specific intervals, as well as from criteria based on the amount of
detrital matter within the dated sample, structural features within the cave and
stratigraphic considerations in the sample laminae.
Criteria for establishing a reasonable isochron plot are as follows: Foremost is the
relationship between samples within speleothems. When dated samples came from the
same lamina, and yet yielded varying ages because of the varying amounts of detrital
matter which they contain, they were considered as being of the same age. Another
consideration takes into account the very high detrital content in a few of the samples
observed during analysis. This indicated that they might plot on an isochron plot of
younger ages. When the sample added to the accuracy of the plot and consequently the
age it yielded, it was considered an indication that the correction the program gave for its
age was valid.
An important aspect of determining ages of seismic events is based on the types of
seismites representing it (Table 5), as well as on their locations within the cave (Fig. 10).
If all seismite samples of age clusters were found in the same place in the cave, they
might indicate a localized breaking event that might not be seismic in origin. By the same
token, seismites of the same age, which are located in different parts of the cave,
represent an event which affected the whole cave and is, therefore, most likely seismic in
origin.
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