EARLY PLIOCENE PALEOCLIMATE RECONSTRUCTION FROM THE PIPE CREEK SINKHOLE, INDIANA, USA

The Early Pliocene (4-5 Ma) Pipe Creek Sinkhole (PCS) includes the sediment fill of a complex karst environment that developed in north-central Indiana, USA. Well-preserved vertebrate fossils are used to date the site and have been used to indicate that the PCS fossils are broadly consistent with those inhabiting an open savannah. PCS pollen are dominated by the family Pinaceae (58%), whereas the second highest percentage (10%) were Juglandaceae, with minor palynomorphs (5%) that include Asteraceae, Plygalaceae and Chenopodiaceae; there were no grass pollen within the pollen counts. The PCS pond includes abundant charcoal grains that are attributed to fire. Thus, the PCS pollen record is consistent with a coniferous forest with records of fire, but that also included some hardwoods such as hickory. The mean ä13C value of PCS tree fossil wood is -25.2‰ PDB, which is consistent with either somewhat water-stressed growing conditions or more 13C in the Early Pliocene atmosphere, but supports the hypothesis that trees did not grow in a closed canopy. Geochemical analysis of PCS pond sediment total organic carbon (TOC) have ä13C isotope values (PDB) averaging -22‰ and C/N ratios averaging 17.5, which conventionally would seem to indicate that the sediment TOC represents a mixture of algae and land plant-derived organic material that includes contributions from C-4 grasses. However, details of the geochemistry of the PCS pond sediments strongly suggest that autochthonous algae using bicarbonate (HCO3-) as a C source for photosynthesis produced organic matter with ä13C values as great as -20.6‰, which resulted in ä13C isotope of TOC that falsely resemble isotope values from C-4 plants. The presence of algae using bicarbonate for photosynthesis suggests that the pond was stagnant or maintained high rates of algal productivity. More than 3m of high-chroma, red-colored silty-clay sediment interpreted to be terra rossa underlies the pond facies. ä13C values from terra rossa TOC have not been previously published, and PCS terra rossa averages -20 ±0.7‰ PDB. The terra rossa is interpreted to represent sediment deposited in a closed cave system, under dry conditions. A paleosol at the top of the cave sediments represents a transition from a closed cave to an open environment that eventually flooded, thereby becoming the small pond.