PETROGRAPHIC AND ISOTOPIC CHARACTERISTICS ACROSS A 35,000 YEAR HIATUS IN A STALAGMITE FROM CREVICE CAVE, MISSOURI

Among the various paleorecords, stalagmites are outstanding in their combined traits of high-resolution (approaching seasonal), longevity of deposition (tens of kyrs), and accurate dating using U-Th methods. Hiatus events are not uncommon, however, and the accurate identification of these hiatus events is essential in the proper interpretation of age-interpolated stable isotope records. Potential hiatus events are generally recognized by distinct petrographic evidence, including: (1) periods of uneven dissolution where growth layers are visibly truncated; (2) prominent changes in the stalagmite fabric; (3) abrupt color changes; and (4) sediment accumulation. The petrographic and isotopic characteristics across a hiatus event in stalagmite CC-94-D from Crevice Cave were studied to gain a better understanding of this specific period of non-deposition. U-Th dates of 45,230 ± 560 and 9,810 ± 100 yr BP (taken < 3 mm apart) indicate a significant period of non-deposition.

A macroscopically distinct color change is visible at the hiatus. Microscopically, the hiatus is characterized by the euhedral terminations of elongate columnar calcite crystals overlain by a thin layer of detrital material, including detrital-rich spheroidal growths concentrated toward the apex of the stalagmite. Euhedral terminations indicate there likely was not dissolution before the deposition of the detrital material and the knobby spheroids are consistent in form with what is commonly called “popcorn”. This uneven, spheroidal form has been associated with water condensation, consistent with a shut-down of the drip source of the stalagmite. A new stage of clear, elongate columnar competitive growth calcite radiates from the spheroidal growths and detrital-rich substrate.

Because it cannot be assumed that all hiatuses in deposition are readily visible petrographically, micro-sampling at two resolutions (0.05 and 0.3 mm) was carried out to examine the isotopic characteristics of the hiatus. At 0.3 mm resolution, carbon isotope ratios increase abruptly at the hiatus. At 0.05 mm resolution, carbon isotope values change more gradually with small scale shifts that are not visible in the abrupt changes of the 0.3 mm sampling. This highlights the importance of sampling resolution when collecting isotope data from stalagmite records.