Advances in micromilling techniques: a new apparatus for acquiring high-resolution oxygen and carbon stable isotope values and major/minor elemental ratios from accretionary carbonate

Abstract

A computer-controlled micromilling apparatus that permits discrete sampling of accretionary biogenic carbonate specimens with micron-scale resolution has been developed for the purpose of acquiring high-resolution δ 13C and δ 18O values, and major/minor elemental chemistry. Secular variation in stable isotope ratios and major/minor elemental composition records inter-annual and intra-annual changes in the environmental parameters or animal behavior for extant and extinct species. A polished specimen is attached to a stage beneath a fixed micro-milling head, and viewed on a large-screen monitor via a color digital camera. Growth bands (analogous to tree rings) are generally a result of variable accretion rates in biogenic carbonates. These growth features are first digitized in real-time as a series of three-dimensional coordinates. To better characterize complex growth features, intermediate coordinates are interpolated using a cubic spline fit through the digital points. Intermediate sampling paths, which mimic less visible daily growth banding, are in turn calculated between digitized curves. Sampling path arrays serve to guide three high precision actuators, which position the sample stage relative to the fixed micromilling head. A fourth actuator provides vertical control of the digital color camera (compensating for vertical movement of the z-axis stage actuator) keeping the specimen image focused. This new micromilling device permits high-resolution sampling of complex internal structures via a user-friendly program interface.