Latest Miocene to earliest Pliocene sedimentation and climate record derived from paleosinkhole fill deposits, Gray Fossil Site, northeastern Tennessee, U.S.A.

Abstract

Lacustrine sediments deposited within a paleosinkhole at the Gray Fossil Site (GFS) in northeastern Tennessee, USA, provide a latest Miocene and earliest Pliocene (4.5–7 Ma) record of sedimentation and paleoclimate. The basal graded facies consists of mm- to cm-thick, normally graded layers of primarily locally derived terrigenous silts and fine sands with low organic content, which record storm flow influxes into a 40 m-deep paleosinkhole lake. The graded facies is overlain by the laminated facies, which is characterized by mm thick, non-graded “A–B couplets” of abundant macerated terrestrial organic matter and fine to coarse quartz sand (A), alternating with quartz and carbonate silt (B). Isotopic evidence for establishment of either C4-dominated or mixed C3/C4 terrestrial ecosystems is not evident at this site; δ 13C values of bulk organic matter from the basal graded facies range from − 24‰ to − 26‰ PDB, and decrease upsection to − 28‰ to − 30‰ in the organic-rich laminated facies. Isotope values covary with changes in both total organic content (TOC: 0.25–1 wt.% in graded facies, 2–12 wt.% in laminated facies) and in carbon–nitrogen ratio (C / N: 1–2 in graded facies, 20–50 in laminated facies). These upsection changes in sedimentary facies and organic geochemistry within the GFS are attributed to either: (1) a climate shift characterized by increased precipitation and concomitant increased vegetation within the paleosinkhole watershed over time, or (2) progressive shallowing and eutrophication of the paleosinkhole lake, coupled with organic diagenesis within either the watershed or the paleosinkhole sediments, in which there was isotopic discrimination associated with organic matter decomposition. Subaerial exposure of the paleosinkhole lake is recorded by paleosol development.