GEOCHEMISTRY OF FOSSIL BONE FROM THE PIPE CREEK SINKHOLE (EARLY PLIOCENE, GRANT COUNTY, INDIANA)
Relative weight percentages of Ca, P, and O are fairly uniform among both Recent and PCS bones, and between Recent and PCS bones. PCS bone differs from modern bone, however, in the routine presence of 3-4 % F, yielding stoichiometries associated with typical fluorapatites. Elemental composition of fossil bone is uniform throughout, showing no differences from exterior to interior regions of bone, or between cortical and internal trabecular bone; there is no evidence of compositional zoning.
Unlike Recent bone, PCS bone consistently has small (1 % or less by weight) concentrations of Mn and Fe. Many PCS bones show little or no crystalline filling of pore spaces, and commonly remain open. However, partial permineralization of pore spaces by crystals of iron-manganese carbonate occurs in some bone, and large amounts of both Mn and Fe occur in amorphous material in some pore spaces. Mn and Fe are likewise abundant in nodules that occur throughout the PCS deposit.
Pore spaces of some PCS bones that lack crystalline filling are dotted by hemispherical, often paired structures about 80 microns in diameter. The hemispheroids are fairly regular in shape, and have sharp, seemingly erosive contacts with the bone. Their elemental composition differs from that of PCS bone, but is similar to that of material found in bone pore spaces. We tentatively interpret the hemispheroids as mineralized microbial (?bacterial) colonies.