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.

            The principal obstacle in the 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’ analyzer 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 analyzers 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 modeling, 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 enters the aquifer):

                           1                   36Clsample  36Clequilib

t    =               –––          ln   –––––––––––––––                                [14.7]

                          836                 36Clrecharge  36Clequilib

            The 36Cl groundwater ages calculated from this equation are shown in Fig. 14.44 for four transects across the basin, two approximately NS 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 groundwaters in the western part of the basin, this implies that a somewhat radial water flow from basin margins to the center is imposed on the general NE–SW flow direction deduced from hydrological modeling.

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 problematic 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.


There are over 100 cosmogenic isotopes with masses over 40 and half-lives over one year, which are, therefore, potentially useful geochemical tracers or dating tools (Henning, 1987). However, most of these elements are metals, and they are not suited to AMS analysis due to the difficulty of forming negative ions. One of the few heavy isotopes to have found significant application is 129I, which is formed in modest abundance in the atmosphere by spallation of Xe, and which, as a non-metal, forms good negative ion beams.

            129I analysis by AMS is relatively straightforward since the only isobaric interference (129Xe) does not form stable negative ions (Elmore et al., 1980). The principal interference is 127I, which at isotope ratios above 1012 forms a peak tail that must be removed by time-of-flight analysis in addition to magnetic and electrostatic analysers. The 129I/127I detection limit under these conditions is about 10!14.

            As in the case of 36Cl, the 129I tracer has been used to study the entry of anthropogenic material into natural systems. In a study of a marine sediment core from the continental slope off Cape Hatteras (North Carolina), Fehn et al. (1986) found 129I/127I levels at the sediment surface that were two orders of magnitude higher than the relatively constant abundances at depth. This has been confirmed by more recent studies. For example, studies of Mississippi delta sediments (Oktay et al., 2000) gave 129I depth profiles very similar to those for 36Cl. In a similar way, anthropogenic 129I signatures in surface ocean waters mimic the radiocarbon signatures of these waters (Fig. 14.46). Maximum 129I signatures are seen at temperate latitudes, where water mixing is lowest, and minimum concentrations at the poles, where upwelling of deep water swamps the anthropogenic signal (Fehn and Snyder, 2000).

Fig. 14.46 Plot of iodine isotope ratio against latitude for surface ocean waters, showing peak signals of anthropogenic iodine at around 40o north and south. After Fehn and Snyder (2000).

            129I has a much longer half-life (15.7 Myr) than the other scientifically useful cosmogenic nuclides. It is therefore applicable to much older systems, but its geological applications are complicated by the significant radiogenic iodine production from in situ uranium fission. This was examined in case studies of the Great Artesian Basin of Australia and the Stripa granite of Sweden (Fabryka-Martin et al., 1985; 1989).

            Groundwaters in the Great Artesian Basin range up to ca. 1 Myr in age on the basis of hydrological and 36Cl evidence (above), so that negligible decay of 129I is expected. Therefore, in the absence of contamination by radiogenic iodine or extraneous water sources, 129I/127I ratios should be constant across the basin. Analytical data bear out this prediction to a reasonable extent, consistent with the very low uranium content of the aquifer rocks and the hydrostatic overpressure of the artesian basin relative to potential contaminant water bodies. Since the near-surface water is itself old relative to human activity, there is no anthropogenic signature. However, excess 129I/127I ratios (above normal cosmogenic levels) were seen in the water of ca. 150 and 500 kyr age. Fabryka-Martin et al. attributed the latter result to contamination by radiogenic 129I from the granitic basement, which forms the floor of the aquifer at its distal end. The cause of the other high value is unknown.

            Very different conditions were found in studies of the Stripa granite groundwater (Fabryka-Martin et al., 1989). In this case, radiogenic 129I is present at levels two orders of magnitude higher than cosmogenic iodine. The 129I systematics at Stripa can be seen most clearly when plotted against 36Cl/Cl (Fig. 14.47). Except for one shallow water sample with a prominent anthropogenic 36Cl signature, the data form an array trending from estimated meteoric recharge towards a pure radiogenic component. This array could result from mixing between two end-members, but it could also result from variable but correlated production of radiogenic 129I and 36Cl in the granite since both are controlled by the uranium content of the rock. Hence, the main role for 129I is to gauge in situ radiogenic perturbation of 36Cl ages in ground-water systems.

Fig. 14.47. Plot of absolute 129I abundance against the 36Cl/total Cl ratio for groundwaters from the Stripa mine, Sweden. Radiogenic and anthropogenic signatures are shown. After Fabryka-Martin et al. (1989).


Adkins, J. F. and Boyle, E. A. (1997). Changing atmospheric ) 14C and the record of deep water paleoventilation ages. Paleoceanography 12, 33744.

Anderson, E. C. and Libby, W. F. (1951). World-wide distribution of natural radiocarbon. Phys. Rev. 81, 64)9.

Andree, M., Oeschger, H., Broecker, W., Beaven, N., Klas, M., Bonani, G., Hoffman, H. J., Suter, M., Woelfli, W. and Peng, T.-H. (1986). Limits on ventilation rates for the deep ocean over the last 12,000 years.  Climate Dynamics 1, 53–62.

Andrews, J. N., Davis, S. N., Fabryka-Martin, J., Fontes, J. C., Lehmann, B. E., Loosli, H. H., Michelot, J-L., Moser, H., Smith, B. and Wolf, M. (1989). The in situ production of radioisotopes in rock matrices with particular reference to the Stripa granite. Geochim. Cosmochim. Acta 53, 1803)15.

Arnold, J. R. (1956). Beryllium-10 produced by cosmic rays. Science 124, 584)5.

Arnold, J. R. (1958). Trace elements and transport rates in the ocean. 2nd UN Conf. on Peaceful Uses of Atomic Energy 18, 344)6. IAEA.

Arnold, J. R. and Libby, W. F. (1949). Age determinations by radiocarbon content: Checks with samples of known age. Science 110, 678)80.

Bard, E., Arnold, M., Fairbanks, R. G. and Hamelin, B. (1993). 230Th)234U and 14C ages obtained by mass spectrometry on corals. Radiocarbon 35, 191)9.

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