Graves under the microscope: micromorphological study of sediments in archaeological burials

The InterArChive project investigated the sediment of the grave fills of archaeological burials. This study applied micromorphological analysis, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and image analysis on sediments of seventeen graves spanning from 4th C BC - 15th C AD and three experimental piglet burials (2009-2013). A new standardized and rapid method was developed for the measurement of porosity. The grave types comprised wooden coffin, absence of coffin and chamber tombs. The sediments were sandy clay soils, variably affected by water-logged conditions, from temperate oceanic climates; sandy loam soil and limestone and sandstone deposits in Mediterranean climates. The microstructure of the backfill, features produced by the decomposition of the corpse and interactions with the surrounding sediment, secondary products related to the environmental conditions in the grave, degree of weathering of bone fragments in the different contexts, preservation of organic components from the graves and biological activity during and after the corpse decomposition were investigated. The results showed that corpse decomposition within the soil produced characteristic microstructures and features according to the type of soil and climate. Neoformed minerals, such as vivianite, siderite and leucophosphite formed in water-logged soils and anaerobic conditions. Amorphous phosphates were preserved only in water-logged soils. In all environments examined redoximorphic pedofeatures formed in the area around the skeleton in the absence of a coffin or in the layers above the skeleton in the presence of a coffin. Organic components and bone fragments were rarely preserved, especially in limestone and sandstone deposits from warm climates. Fungal and mesofauna activities were better represented in aerobic conditions and Mediterranean climate. Changes in porosity and segregation of mineral grains showed downward movement of soil particles and fluids from the layers above the skeleton and they highlighted the role of the coffin as barrier to this movement.