The effect of depositional conditions on mineral transformation, chemical composition, and preservation of organic material in archaeological Hg-enriched bone remains

Archaeological bone remains are exposed to depositional conditions that profoundly affect their preservation. This manuscript reports on the mineral transformation, chemical composition, and preservation of organic material in pre-Hispanic archaeological Hg-enriched bone remains exposed to different depositional conditions, namely, open and confined environments. Here, we studied 2 vertebrae specimens collected inside Temple XIII of the archaeological site of Palenque, Mexico. Specifically, the 2 vertebrae were collected inside and outside a sarcophagus (VIS and VOS) located in a burial chamber and belonged to a 30-yr-old woman and an 11-yr-old boy, respectively. Subsections of the vertebrae specimens were characterized by radiocarbon dating, high-resolution electron microscopy (HRSEM), and energy-dispersive X-ray spectroscopy analyses (EDS). VIS and VOS showed conventional radiocarbon ages dating of 5050±30BP and 5440±30BP respectively. HRSEM-EDS analysis for VIS showed a high content of Hg (~30%) in the upper layer and below the detection limit in the lower layer. In contrast, HRSEM-EDS analysis for VOS confirmed the lack of a layered structure, for which the accumulation of Hg was as high as 30%. Furthermore, VIS was the only vertebrae that showed the presence of S. This was because VIS belonged to a pre-Hispanic character exposed to HgS, typically used in Mayan royal rituals; VOS did not contain S because the Hg source in this case was probably from cooking utensils, the elaboration of paintings or murals, or the ingestion of contaminated food or water. In addition, VOS contained a particularly low content of P, which was attributed to hydroxyapatite undergoing higher rates of transformation in the open environment. Moreover, the incorporation of Hg in hydroxyapatite via Hg-for-Ca substitution further accelerated the mineral transformation pathway(s). Finally, the pre-Hispanic archaeological Hg-enriched bones preserved the genetic material, regardless of the depositional environments (exposed or unexposed). Therefore, the prevailing preservation mechanisms differ from those underpinned by physical protection by collagen fibrils or natural antibacterial mechanisms.