New Mexico Geological Society Annual Spring Meeting — Abstracts

Since the advent of ¹⁴C dating in the early 50's, there has been a continuing effort to assess the effect ofsecular variation of production of cosmogenic nuclides on atmospheric ¹⁴C activity. It is now widely accepted that long-term variations production of cosmogenic nuclides are caused by fluctuations in the strength ofthe earth's magnetic field. The secular variation of production of one ofthese nuclides, ³⁶Cl, could provide an excellent too for soil moisture and groundwater tracing because CI- is conservative in the subsurface. If a recharge system is advectiondominated, ³⁶Cl concentration variations along a flow path should preserve the record of production of the isotope. To test this hypothesis, we have measured ³⁶Cl/Cl ratios in 23 groundwater samples from the San Jaun Basin in northern New Mexico. Results are compared to a hypothetical reconstruction of production variations over the last 35 ka based on global paleointensity records or Tric et al. [1992].

After eliminating samples believed to have been affected by in-situ addition or dilution of chloride, the pattern of ³⁶Cl/Cl variation appears to agree well with the paleointensity-based signal. This supports the hypothesis that long-term variations in production of cosmogenic nuclides are caused by variations in the strength of the earth's magnetic field. Moreover, the preservation ofthis signal in the groundwater archive suggests times suggests that it may, indeed, prove a useful groundwater tracer for aquifers with residence times on the order oftens of thousands of years.

For comparison with other archives of cosmogenic nuclides, we summarize the results of previous investigations ofsecular variation, including atmospheric ¹⁴C, ¹⁰Be in polar ice and our recent study of the ³⁶CI/CI ratios preserved in fossil middens of packrats in Nevada.