The Late Neogene represents warm Earth conditions immediately prior to the development of
extensive northern hemisphere glaciation, and this period in Earth history may therefore provide the best
available analog for the projected outcome of continued global warming. There are few interior continental
sites of Late Neogene age from the eastern half of North America and subsequently very little is known
about the conditions characterizing climate. The Early Pliocene (~5 Ma) Pipe Creek Sinkhole (PCS) includes
the sediment fill of a complex karst environment that developed in north-central Indiana, USA (Lat. 40° 27'
25.4", Long. 85° 47' 37.2"). The site includes more than 3 m of high-chroma, red-colored silty-clay sediment
interpreted to be terra rossa. The δ13 C values PCS terra rossa average -20 ±0.7‰ PDB, and are
interpreted to represent sediment deposited in a closed cave system under high temperatures and with welldrained
soils. An in-situ paleosol at the top of the terra rossa represents a transition from a closed cave to an
open environment that eventually flooded, thereby becoming a small pond. δ13 C values from lacustrine
sediments with organic matter derived dominantly from algae average -20.6 ‰ and suggest the pond was
stagnant and enriched with bicarbonate from the underlying limestones or via aquifers. Pond sediments
include abundant vertebrate fossils, which are broadly consistent with those inhabiting an open ecosystem
such as a savannah or parkland. However, the PCS pollen includes low taxonomic diversity that is
dominated by pine with some hickory and flowering plants, but no grass pollen. It is likely that the pollen
assemblage represents a local pine dominated ecosystem associated with the pond paleoenvironment, such
as a riparian community, and that the greater landscape was drier and open. An alternative hypothesis is
that the climate became wetter and initiated the formation of the pond, and an early succession forest
ecosystem developed. |
