Annual growth banding in a cave stalagmite

The presence of microscopic luminescence banding in cave calcite deposits (speleothems) has recently been reported^1-3. The luminescence seems to be caused by excitation of humic and fulvic acids derived from the overlying soil and subsequently incorporated into the calcite^3,4. We have tested whether such luminescence corresponds to annual growth layers, using high-precision thermal-ionization mass-spectrometric ²³⁸U-²³⁴U-²³⁰ Th ages from banded sections of a Holocene stalagmite. We report here that the timespans yielded by counting bands, on the assumption that they are annual, agree within error limits with those obtained by our uranium-thorium dating. This demonstration that the banding is annual should make banded speleothems valuable resources for providing high-resolution records of past climate (in which precipitation can be estimated from growth rates^5,6 and temperatures from stable-isotope measurements⁷) and solar-terrestrial correlations (as the luminescence intensity can be related to the solar sunspot cycle^1-3).