A high-resolution electron microscopic and energy-dispersive spectroscopic study on the molecular mechanism underpinning the natural preservation of 2300 Y.O. naturally-mummified human remains and the occurrence of small-sized [Zn][Al]Carbon spheres

In this paper we use Scanning and Transmission Electron Microscopy, and Energy Dispersive Spectroscopy to characterize the surface of the skin of a 2300 YO, naturally-preserved mummy, belonging to a two-year and eight month girl (" Pepita" ), found inside a cave located in Altamira, Querétaro, México (21°40'-20°01'; 99°03'-100°36'). The cave was found in Sierra Gorda, an orographic region with a relief of sedimentary origin from the Gulf of Mexico, composed by high mountains with altitude values surpassing 3000 m above sea level, with ample and steep canyons, and a prominent role on the exploitation and distribution of cinnabar (HgS). The skin showed the presence of small-sized spherules, containing Al (≤43%) in the most exposed region (5-10 μm depth). Thin layers and structural microdomains covered small and large spheres. Structures conformed by stacked, nano-sized particles located far-from-the bunches contained C (≤45%), Zn and Si (≤10%), and minor amounts of Ca (≤2.6%). By contrast, regions between spheres contained high amounts of Ca (≤23%) and Al (≤15%), but lacked Zn and Si. Carbon spheres showed two distinctive composition, a signature that their formation might have occurred in a least two different stages via concentric growth mechanisms, with the incorporation of Zn and Al at a later stage. [Zn][Al]Carbon spheres showed morphology and growth patterns that compared to those resulting from the hydrothermal carbonization by Fe²⁺ ions under mild conditions, suggesting a common mechanism of formation. Textural changes of thin films found between [Zn][Al]Carbon spheres were attributed to differences in viscosity, which might have contributed to increases in functionality and specific surface area (by means of decreases in size) and, in turn, facilitating the sequestration of biomolecules. We propose that the presence of [Zn][Al]Carbon spheres provides protection against bacterial and UV attack. The physical properties of these spheres helped entrap biomolecules. Taken together, these factors contributed to the preservation of Pepita.