Determining isotopic life history trajectories using bone density fractionation and stable isotope measurements: A new approach

A number of recent studies have attempted to trace diet at different stages of an individual's life by comparing isotope ratios of bone from different gross anatomical sites within the skeleton. In this study we develop this approach further by separating bone of differing mineral densities within o ne skeletal element, where each density fraction represents a different per iod of time. Isotope ratios are measured for these fractions. Each density fraction represents a period of bone formation and maturation, where younge r (more recently formed) bone is less well-mineralized and therefore less d ense than relatively older packets of bone. In an adult, bone is therefore able to partition approximately the last 15 years of life. Bone fractions w ere recovered by stepped ultracentrifugation in a series of organic solvent s of increasing density, and then collagen was recovered by decalcification in dilute acid, and stable carbon isotope ratios (C-13/C-12) were measured . Bone density microstructure was checked for bacterial remodelling using b ackscattered electron imaging in a scanning electron microscope. Our result s indicate that the bone density fractionation method is applicable to arch aeological material, here extending to a maximum of 5,000 years BP, and tha t collagen can successfully be extracted from such fractions. The carbon is otope values for bone fractions of different densities patterned out as exp ected in one modern control bone and in specimens from five archaeological human skeletons, including three precolonial hunter-gatherers and two 18th/ 19th century individuals. The latter two are known (from previous assessmen ts) to have undergone marked changes in diet during their lifetimes. Postmo rtem alteration was evident in some of the specimens examined. The bone den sity fractionation approach has allowed greater resolution of diet than has hitherto been possible and has provided access to the elusive last years a nd months of an individual's life. (C) 2001 Wiley-Liss, Inc.