Blood-induced biomineralized calcium carbonate used for binding in the Huashan rock art of Guangxi, Southern China

Abstract

Rock art is the physical evidence of early human symbolism, distributed across the globe as a consequence of human dispersal. In this study, we analyzed the Huashan rock art, which contains more than 80 sites and is in good condition. Created at least 2000 years ago, it is located in Guangxi, southern China, an extremely hot and humid subtropical monsoon zone. The process for producing the rock art and how they have survived the weathering remain mysteries. Rock art fragments from three sites in the Huashan rock art belt were collected and studied. They were analyzed using Raman spectroscopy, energy-dispersive spectroscopy (SEM-EDS), Fourier infrared spectroscopy (FTIR), and proteomics to determine their structure and mineral and organic composition. Our study indicates that red clay rich in nano-hematite was used as the pigment, and a composite mortar made from lime and blood served as the binder in the rock art. Biomineralized calcium carbonate induced by blood proteins tightly encapsulates nano-hematite and strongly bonds to the limestone. This stable organic-inorganic composite bonding system reveals the complex technological processes used by the creators and explains why the Huashan rock art has survived so well in such challenging conditions for over two millennia. The blood used may come from the females who gave birth, which could be linked to the worship of female fertility of the creators' community, a matriarchal fisher-hunter-gatherer society. These findings provide new insights into the creation of prehistoric rock art, showing that it was a technically intensive project rather than a labor-intensive one.