Environmental Signatures of Authigenic Mineral Formation on Modern and Fossil Bones

Fossil bone is an important tool for paleoenvironmental studies, because it retains information about the prevailing conditions before and during fossilization. Most studies have focused on differences in surface features or the chemical composition of bones, correlating certain features with particular environmental contexts. Though the presence of mineral growth has been noted on modern, archeological and fossil bone surfaces, it is commonly overlooked. Many of the minerals form under particular conditions (pH, Eh, porewater chemistry), making them suitable (paleo)environmental indicators. Despite the observed relationship between environmental conditions and mineral formation on bone, little is understood regarding how bone interacts with the burial environment during diagenesis. Here I describe a 14-month controlled laboratory experiment used to investigate the effect of different environmental conditions on the decay and diagenesis of buried bone. Two hydrologically different sediments were created: a porous sand-silt combination and a less permeable silt-peat-sand combination. Bone is found to buffer sediment pH changes and contribute to local anoxia. Mineral growth is observed on bones from silt-peat-sand sediment treatments only, where acidic and anoxic conditions prevailed. Examination with SEM-EDX shows these crystals to be an iron phosphate, most likely vivianite. Although vivianite has been found on modern and archeological remains under similar conditions, this experiment is the first to show an empirical connection between low pH, anoxia, and vivianite formation on bone. On fossil bones, minerals such as pyrite are common and appear closely connected to sediment types representing acidic and/or anoxic environments. Comprehensive examination of minerals growing on bone surfaces therefore provides a sensitive indicator of the hydrological and biogeochemical conditions surrounding a bone. This is yet one more way in which buried bone can be used to understand environmental conditions from 1 year to millions of years in the past.