The unique contextual situation of the Ca’ Granda burial ground: from taphonomic observations towards a new type of anthropogenic soil

Soil is a dynamic matrix that can rapidly respond to disturbance events, such as the death and the subsequent deposition of an organism. Concurrently, it can be considered an archive of evidence due to its ability to record the signals of disturbance events. Such a condition turns the biogeochemical analysis of geopedological samples into a valuable tool for the study of decomposition processes, especially when they are flanked with the examination of the remains. The aim of the present research is to present the unique contextual situation of the Sepolcreto (i.e., burial ground) under the crypt of the ancient Ospedale Maggiore of Milan Ca’ Granda (Italy). The sepulchre hosted an estimated amount of 150000 buried individuals, 10000 of which are still buried in one of the underground chambers, named “chamber O”, and whose remains underwent various type of post-mortem transformative processes, both disruptive and preservative. In this study microscopic and ultramicroscopic analysis have been carried out in order to detect any evidence of material exchange between the bone tissue, from three skeletal remains collected from the “chamber O” of the Sepolcreto, and the surrounding pedosedimentary matrix. The specimens were analysed by the mean of a polarizing microscope and a SEM-EDS, which pointed out the presence of a mutual exchange of material between the two substrates, underlying the intensity of the interaction between organisms (even after their death) and the environment. Finally, this burial context permitted to observe an inedited type of soil, mainly composed of organic matter transformed by thanatological processes, bone tissue fragments and some other evidence of anthropic origin and/or activity. Therefore, it has been considered a new type of anthropogenic soil named “Thanatogenic soil”.