Comparison of bulk and sequential sampling methodologies on mammoth tooth enamel and their implications in paleoenvironmental reconstructions

Abstract

Mammoth teeth have been widely investigated using stable-isotopic analysis for paleoenvironmental and paleoecological reconstructions due to their large size and frequent discoveries. Many past investigations sampled the tooth enamel with the "bulk"method, which involves drilling one sample from the occlusal surface to the root for each tooth. Some of the more recent studies applied the "sequential"method, with a sequence of samples drilled following the dominant enamel growth direction to produce a time series of isotopic oscillations that reflects high-resolution environmental changes, as well as changes in mammoth dietary behavior. Although both the bulk and mean sequential δ18O values are expected to represent the averaged signal over the time of tooth formation, it is uncertain whether their paleoenvironmental records were formed during similar periods of time. In this study, we applied both sampling methods (sequential drilling first followed by a thin layer of bulk drilling) on the same enamel ridges of multiple mammoth teeth and compared their respective δ18O values. The results indicated that, in most enamel ridges, the bulk samples have more negative δ18O values compared to the average sequential values, and some of the bulk values even fall outside the range of sequential values. The most likely explanation for the differences is the structure and formation stages of enamel that caused uneven distributions of different seasons recorded in the samples. This finding provides insights into current limitations of the two sampling methods and the applicability of cross-method data comparison from past studies.