ACTUALISTIC ASSESSMENT OF EARLY-DIAGENETIC ALTERATION OF BONES BURIED IN HIGHLY-MATURE GLAUCONITIC GREENSANDS

Little remains known about how the peculiar nature of glauconitization and long-term burial in greensand affect the fossilization of bone. A recent study found that many fossil bones recovered from the Maastrichtian–Danian Hornerstown Formation (HF), one of the world’s purest greensand deposits, exhibit spatially-variable discoloration and histologic alteration. Altered regions of the bones, which appear light tan, were found to exhibit complete loss of histologic integrity and original soft-tissues (e.g., osteocytes and blood vessels) whereas unaltered regions, which appear dark brown, retain them. This peculiar alteration appears to affect fossils at random. To evaluate whether acidic groundwaters percolating through the HF could account for this enigmatic diagenetic alteration, two sets of modern cow, chicken, and turkey bones were buried beneath the groundwater table in greensands of the HF within a stream bed in Mantua Township, New Jersey. One set was exhumed after three months of exposure to the percolating groundwater, while a second was exhumed after six months. Diaphyseal (for the limb bones) and transverse (for the vertebrae) thin sections of the buried bones revealed a change in color of the outer margin of the external cortex yet no signs of bacterial/fungal damage or other histologic alteration. In the 3-month bones, the outermost ~220 µm of the external cortex had turned a light brown, which gradationally faded into light tan internally. The 6-month bones exhibited a similar change but with a typically ~130 µm thicker rim. While our results do not pinpoint a cause of the alteration patterns observed in many HF fossils, lack of histologic alteration of the experimental bones implies that the pH of current local groundwater (~4.5) is too weak to cause rapid inorganic dissolution of bone mineral yet still high enough to severely limit microbial activity. Rather, our results imply that the discoloration and histologic alterations of HF fossils were more likely acquired during early diagenesis after burial on the Paleocene seafloor, prior to glauconitization of the entombing sediments. The stochastic nature of the alterations may reflect slight variations in initial burial depths on the seafloor, sediment permeability, and/or pore-fluid pH within and around the bones as they were fossilizing.