DEVELOPING A METHOD FOR THE EXTRACTION OF 26AL FROM CARBONATE SEDIMENTS TO DETERMINE BACKGROUND EROSION RATES OF DRAINAGE BASINS

Measurements of cosmogenic nuclides in detrital sediments can be used to determine the spatially-averaged, background erosion rate of a drainage basin; so far, this application has largely been limited to the measurement of ²⁶Al and ¹⁰Be in quartz-bearing lithologies. Measurement of ³⁶Cl in calcite has been used infrequently to determine erosion rates in carbonate terrains; however, the three production pathways of ³⁶Cl in calcite create additional complexity and uncertainty in these calculations. The ability to extend cosmogenic nuclide techniques to a greater variety of minerals and the ability to pair nuclides with different half-lives would greatly expand the utility of these isotopes.

Recent research (Merchel et al. 2010) has demonstrated the potential for extracting ²⁶Al from well-crystalized calcite, offering an alternative to ³⁶Cl as an erosion rate monitor in carbonate lithologies. Our research aims to refine a method for isolating ²⁶Al from calcite for use with detrital sediments, which would allow the determination of spatially-averaged, basin-wide erosion rates in carbonate terrains using ²⁶Al.

The primary challenge associated with extracting ²⁶Al from detrital sediments is clay mineral content. Clay minerals contain abundant stable ²⁷Al, which can lower the ²⁶Al/²⁷Al ratio below the detection limit. Our initial results show that acetic acid is the most effective reagent for preferentially dissolving calcite without affecting clay crystallinity, thus limiting the amount of ²⁷Al that is released into solution. In dissolution tests of carbonate sediment samples from Oman, measured concentrations of Al in replicate samples dissolved with hydrochloric acid and nitric acid were 30X to 100X higher than the samples dissolved in acetic acid; the concentrations of Al in the samples dissolved in hydrochloric and nitric acids were between 14-72 ppm and 19-83 ppm respectively, while the samples dissolved in acetic acid measured only 0.4-1 ppm Al. We have successfully isolated ~7-11% of the total Al in solution in our initial attempts, but thus far have been unable to isolate a majority of the Al in solution, likely due to high levels of Ca prohibiting the trace amounts of Al from precipitating. We are refining this method and plan to apply the ²⁶Al in carbonate sediments technique to measuring erosion rates in Cuba.