Collagen turnover and isotopic records in cortical bone

Bone collagen of modern and ancient animals is a useful tissue for isotope analyses because it is stable over time. However, uncertainty regarding metabolic turnover processes of bone collagen can make isotope analysis difficult to correlate with relevant life history information. We used radiocarbon (¹⁴C) dating to examine turnover within cortical bone and to investigate retrospective isotope ratios along the growth direction of mid-shaft femurs of several large mammals with long life-span, including brown bear (Ursus arctos), sika deer (Cervus nippon), Japanese serow (Capricornis crispus), and Japanese macaque (Macaca fuscata). The individuals examined died between 1971 and 1986, and their bones were thus expected to contain radiocarbon generated by nuclear bomb testing, which led to an atmospheric ¹⁴C spike around 1964. Therefore, ¹⁴C dating could be used to date sections of cortical bone at fine scale. The ¹⁴C ages in the bone sections of all specimens except the Japanese serow specimen showed similar trends; perimedullary bone sections contained younger carbon, which has depleted ¹⁴C after the peak of the ¹⁴C spike, whereas ¹⁴C ages became rapidly older in midcortical sections before gradually becoming younger towards the bone surface. We observed metabolic turnover of collagen driven by bone remodelling in perimedullary bone sections, but we observed no evidence of remodelling in midcortical and pericortical sections. Thus, our results confirm that bone collagen in femoral cortical bone records retrospective isotopic information during skeletal growth of mammals and suggest that entire femoral cortical bone of aged terrestrial mammals represents isotopic values during adolescence rather than an average value from several years prior to death.