14.4 Chlorine-36

³⁶Cl is analogous to ¹⁰Be in its atmospheric production, in this case by spallation of ⁴⁰Ar rather than ¹⁴N, and like ¹⁰Be it is quickly swept from the atmosphere by precipitation. However, unlike ¹⁰Be, ³⁶Cl is not removed from groundwater by adsorption onto particulates, but remains in the aqueous medium as it travels through geological strata. This fact, coupled with its relatively short half-life of 0.301 Myr, makes ³⁶Cl potentially very useful in the dating or tracing of Quaternary groundwater systems. Cosmogenic ³⁶Cl can also be generated in the surfaces of exposed rocks by in situ production. However, this subject will be covered under section 14.6.

The principal obstacle in AMS analysis of ³⁶Cl is isobaric interference by ³⁶S. This forms abundant negative ions and is not removed by the charge-stripping process. It can be resolved by its lower energy loss in the gas counter, but this is most effective at energy levels above 48 MeV, requiring an accelerator of at least 6 MV potential. This rules out ³⁶Cl analysis with lower-energy (2 MV) tandetrons (Wolfli, 1987). A ‘time-of-flight’ analyser may also be used before the gas counter (Fig. 14.41) in order to eliminate peak tailing from the relatively very large ³⁵Cl and ³⁷Cl ion beams, which are not adequately resolved by the preceding magnetic and electrostatic analysers in the system. Time-of-flight analysis can only be performed on pulsed ion beams, which are controlled by pulsing the sputter source. This analysis relies on the fact that lighter masses are accelerated to slightly higher velocities than heavier ones, so that after traversing a distance of a metre or so, they arrive at the detector a few nanoseconds earlier. Hence ³⁶Cl is resolved from both ³⁶S, ³⁵Cl and ³⁷Cl (Fig. 14.41).

Fig. 14.41. Analyser segment and output data of an AMS instrument designed for ³⁶Cl determination, showing the use of time-of-flight analysis to resolve ³⁶Cl from ³⁵Cl and ³⁷Cl and energy loss detection to resolve from ³⁶S. After Wolfli (1987).

The first use of ³⁶Cl as a hydrological tracer was not based on the cosmogenic isotope at all, but on anthropogenic bomb-produced ³⁶Cl. This resulted from seven large nuclear tests conducted on the sea surface from 1952 to 1958, which caused neutron activation of marine chlorine. Profiles of anthropogenic ³⁶Cl against time were determined in a Greenland ice core (Elmore et al., 1982), in Canadian groundwater (Bentley et al., 1982) and in a soil profile from New Mexico (Phillips et al., 1988). All of these measurements showed a very sharp spike in ³⁶Cl, with a duration of 15 ) 20 years (Fig. 14.42). It is anticipated that in the near future anthropogenic ³⁶Cl will be a useful hydrological tracer, replacing bomb-produced tritium as the latter becomes extinct.

Fig. 14.42. Profiles of anthropogenic ³⁶Cl as a function of depth in different environments. a) Ice (Dye 3 station, central south Greenland); b) groundwater (Borden landfill, Ontario); c) desert soil (New Mexico). After Gove (1987) and Fabryka-Martin et al., (1987).

As seen for other cosmogenic isotopes, the production of natural ³⁶Cl is expected to have varied in the past due to modulation of the cosmic-ray flux by the solar wind and Earth’s magnetic field. The most easily measured inventories of ³⁶Cl were the ice cores from Greenland and Antarctica which were also studied for several other environmental tracers. However, it was shown that ³⁶Cl and ¹⁰Be abundances in these cores are better correlated with climatic variations than with past variations in geomagnetic field intensity (e.g. Beer et al., 1988). This result caused considerable puzzlement at the time, but is not really surprising since the cosmogenic isotope flux in polar snow is largely of local (polar) origin, where cosmic-ray intensity is not significantly shielded by the Earth’s magnetic field. Therefore, to obtain more representative records of past changes in global ³⁶Cl production it was necessary to find a suitable inventory from a non-polar source. Such an inventory was discovered by Plummer et al. (1997) in the form of fossil packrat urine from Nevada.

Packrats obtain all of their water from the desert plants that they eat, and these plants in turn derive their water from surface-infiltrated rainfall. Therefore the abundant chlorine in packrat urine accurately reflects the ³⁶Cl/Cl ratio of recent rainfall. Furthermore, this urine may be preserved for thousands of years in underground middens, and can be dated by the radiocarbon method. Hence, this material represents an ideal inventory of past cosmogenic ³⁶Cl production. ³⁶Cl/Cl ratios for packrat urine up to 40 kyr old are presented in Fig. 14.43, along with a record of past ¹⁴C production compiled from several sources. The two data sets are relatively well correlated, especially at the present day and at the peak of cosmogenic isotope production around 30 kyr ago. Since geomagnetic modulation is the principal cause of past ¹⁴C variations, it follows that ³⁶Cl production is subject to the same controls.

Fig. 14.43. Plot of chlorine isotope variation against age in samples of packrat urine from Nevada, USA. The data are compared with a normalised curve of radiocarbon production compiled from several sources. After Plummer et al. (1997).

Following this evidence for the geomagnetic modulation of global ³⁶Cl production, it was found that appropriate corrections for the variable accumulation rates of Greenland snow did yield a record of past ³⁶Cl variations that was well correlated with geomagnetic field strength (Baumgartner et al., 1998). This suggests that a significant fraction of the precipitation in Greenland is actually derived from more temperature latitudes. However, the large corrections that must be applied for variable snow accumulation rates mean that ice cores are not reliable as prime records of past cosmogenic isotope production. Instead, the known variations in past cosmogenic isotope production may be more useful to calibrate the variable ice accumulation rates in these cores (Wagner et al., 2001).

The most important application of cosmogenic ³⁶Cl (as opposed to anthropogenic ³⁶Cl) is the dating of ancient groundwater, hundreds of kyr in age. For simple sedimentary aquifers this has been quite successful, as demonstrated by studies on the Great Artesian Basin of eastern Australia by Bentley et al. (1986) and Torgersen et al. (1991). These studies presented a total of five ³⁶Cl transects across the basin, reaching up to 800 km from the recharge area. Comparisons were made with average groundwater ages from hydrological modelling, and examinations were also made between three different ways of calculating the ³⁶Cl age. However, the simplest method is probably the most reliable in most circumstances. This calculates the groundwater age from the total abundance of ³⁶Cl (above secular equilibrium) in an unknown sample relative to the total ³⁶Cl abundance above secular equilibrium at the recharge site, (where water inters the aquifer):

1 ³⁶Cl_sample – ³⁶Cl_equilib

t = – ––– ln ––––––––––––––– [14.7]

8₃₆ ³⁶Cl_recharge – ³⁶Cl_equilib

The ³⁶Cl groundwater ages calculated from this equation are shown in Fig. 14.44 for four transects across the basin, two approximately N–S and two E–W, and compared with the average age profile from hydraulic measurements. These results show that the two N–S transects, which are in the westerly part of the basin (open symbols), generally have younger ³⁶Cl ages than predicted from hydraulic measurements. This can be explained by additional water input into the system along the length of the basin, which dilutes old basin water with young recharge water. On the other hand, the E–W transects (solid symbols), which span the easterly half of the basin, generally have older ³⁶Cl ages than predicted. This implies that basin water tends to accumulate in this area and develop older ages.

Fig. 14.44. Plot of ³⁶Cl groundwater ages for four transects across the Great Artesian Basin against average age since recharge based on hydraulic modelling. ³⁶Cl ages: ( " ) = N–S transects; ( ! ) = E–W transects. Data from Torgersen et al. (1991).

The ages from the transects are used to calculate a contour diagram of ³⁶Cl groundwater age in Fig. 14.45, where the results are compared with streamlines based on hydraulic measurements. The latter data imply that water flows mainly in a NE to SW direction across the basin from Queensland to South Australia. However, the contour plot shows the presence of old ages in the middle of the basin, implying that water tends to pool here in what is also the deepest part of the basin. When coupled with the evidence for younger ground-waters in the western part of the basin, this implies that a somewhat radial water flow from basin margins to centre is imposed on the general NE–SW flow direction deduced from hydrological modelling.

Fig. 14.45. Map of the Great Artesian Basin showing, (a) Groundwater age contours from ³⁶Cl dating and, (b) Groundwater flow directions based on hydraulic modelling. After Torgersen et al. (1991).

The ³⁶Cl method has been more problematical in studying groundwater ages in igneous rocks, due to interference by local radiogenic ³⁶Cl production. These problems have been evaluated in a case study of the Stripa granite, Sweden, which has unusually high uranium contents of ca. 40 ppm. This generates a substantial neutron flux, which produces ³⁶Cl by the n,( reaction on ³⁵Cl. The U content of metasedimentary country rocks (5 ppm) also generates significant, if much lower, levels of radiogenic ³⁶Cl. Analysis of Stripa groundwater yields ³⁶Cl/Cl ratios between the in situ radiogenic production in the two dominant rock types (Andrews et al., 1989). These values are so high that they exceed and swamp normal cosmogenic ³⁶Cl/Cl ratios. ³⁶Cl is therefore only a viable dating method for waters in uranium-poor rocks such as sedimentary aquifers.

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