GEOCHEMISTRY OF FOSSIL BONE FROM THE PIPE CREEK SINKHOLE (EARLY PLIOCENE, GRANT COUNTY, INDIANA)

The Pipe Creek Sinkhole (PCS) records a diverse late Neogene terrestrial biota from eastern North America. Most PCS bone is well preserved in surface view, suggesting limited pre-burial weathering. Thin sectioning, scanning electron microscopy, and energy-dispersive (EDX) spectroscopy were used to compare the composition of Recent bone with that of corresponding PCS material. Modern bone examined included frog (Rana sp.) limb material, turtle (Chrysemys picta) shell, rodent phalanx (Marmota monax) and incisor tooth (Sciurus carolinensis), and deer (Odocoileus virginianus) rib. PCS bones examined included frog limb elements, urostyle, and scapula, turtle and tortoise shell, rodent incisors, and a probable large mammal rib. EDX analyses were done by the standards method, with elemental weight percentages summed to 100 % on a carbon-free basis.

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.