Structural and thermodynamic factors which may influence burnt bone survivorship in archaeological contexts have not been fully described. A highly controlled experimental reference collection of fresh, modern bone burned in temperature increments 100–1200˚C is presented here to document the changes to bone tissue relevant to preservation using Fourier transform infrared spectroscopy and X-ray diffraction. Specific parameters investigated here include the rate of organic loss, amount of bone mineral recrystallization, and average growth in bone mineral crystallite size. An archaeological faunal assemblage ca. 30,000 years ago from Tolbor-17 (Mongolia) is additionally considered to confirm visibility of changes seen in the modern reference sample and to relate structural changes to commonly used zooarchaeological scales of burning intensity. The timing of our results indicates that the loss of organic components in both modern and archaeological bone burnt to temperatures up to 700˚C are not accompanied by growth changes in the average crystallite size of bone mineral bioapatite, leaving the small and reactive bioapatite crystals of charred and carbonized bone exposed to diagenetic agents in depositional contexts. For bones burnt to temperatures of 700˚C and above, two major increases in average crystallite size are noted which effectively decrease the available surface area of bone mineral crystals, decreasing reactivity and offering greater thermodynamic stability despite the mechanical fragility of calcined bone. We discuss the archaeological implications of these observations within the context of Tolbor-17 and the challenges of identifying anthropogenic fire.

Duration of an anthropogenic fire event is one aspect of fire use and maintenance that is linked to combustion feature function but has low archaeological visibility. In this study, we describe the transformations to fresh, modern cortical bone with prolonged exposure to heat in order to evaluate the utility of archaeological bone for the recognition of long duration thermal alteration. Cores of bovid cortical bone were heated exposed to air at 300, 550, and 750 °C in a sequence of experimental trials in a Nabertherm muffle furnace for periods of 10 minutes, 9 hours, and 48 hours, plus an extensive cooling period on heat retaining sediments (gravel or gravel compacted with fine quartz sand) to mirror the smoldering and extinguishing of actualistic fires. After heating, bone cores were analyzed with a color tool, Fourier-transform infrared spectroscopy, and X-ray diffraction to evaluate changes in structure, composition, and crystallinity of bioapatite as a function of different temperature thresholds and time. Results indicate that prolonged heating in air induces specific structural and chemical changes in bone compared to shorter duration burned counterparts. Coloration changes also demonstrate that white coloration, a primary characteristic utilized by zooarchaeologists to record information about burning intensity, is not an exclusive indicator of calcination at moderate to high temperatures but may also result from long duration exposures at low temperatures.

The open-air site of Tolbor-17 (T17), located in northern Mongolia, contains one of few faunal assemblages in the region from early Marine Isotope Stage 3 (MIS 3, ~57-29 ka cal BP). During this period, humans migrated into northern Asia as part of the Initial and Early Upper Paleolithic (IUP and EUP). Much is still unknown about these populations, including the adaptive strategies that facilitated their success at the geographic margins of hominin dispersal. This paper presents the preliminary analysis of the T17 faunal assemblage, a valuable source of data on early human subsistence and paleoecology. Although small (n = 836) and fragmentary, the assemblage illuminates several aspects of early human behavior, namely the butchery of large and extra-large ungulates as well as the use of osseous tools and personal ornaments. In addition to ungulates, humans also exploited marmot, ostrich (eggshell) and at least one large carnivore, attesting to the wide range of human-animal interactions at the site. In addition to humans, carnivores also modified the assemblage, likely as scavengers of human butchery waste. These results provide a basis for future analyses at the site, focusing on the agents of accumulation, taphonomic processes, and implications for early human adaptations in Central Asia.