PALEOCENE-EOCENE CARBON ISOTOPE EXCURSION AND MAMMALIAN FAUNAL CHANGE IN THE HONEYCOMBS AREA OF THE SOUTHERN BIGHORN BASIN, WYOMING

The Paleocene-Eocene boundary is defined chemostratigraphically by the onset of a global carbon isotope excursion (CIE), recognized in marine and continental strata in both carbonates and dispersed organic carbon (DOC). In continental sections the CIE is important as a proxy for climate change affecting mammalian faunal succession during the Paleocene-Eocene thermal maximum (PETM). At Polecat Bench in the northern Bighorn Basin, Wyoming, the CIE spans four faunal zones from latest Clarkforkian Cf-3, through earliest Wasatchian Meniscotherium or Wa-M, Wa-0, and Wa-1. The CIE at Polecat Bench has been studied at high resolution in both pedogenic carbonate and DOC, where there is good agreement between records in both. Here we report a new high resolution study of the CIE in relation to Wa-M and Wa-0 faunal change in the Honeycombs area of the southern Bighorn Basin.

In the Honeycombs area 46 isotope sample sites span a vertical section of 52 meters, including a Meniscotherium-and-perissodactyl-bearing interval at Halfway Hill (UCMP V99557), the rich Wa-0 wash site of Castle Gardens (UCMP V99019), and a higher Wa-0 site of UCMP V99557 with the small perissodactyl Hyracotherium sandrae. The latter site is at the base of the most salient marker bed in the sequence, a thick, deeply weathered red paleosol called Big Red. Total organic carbon is in the range of 0.1-0.3% and CaCO₃ is notably lacking except in the wash site interval where it reaches ca. 3%. Pre-excursion values range from δ¹³C (PDB) = -26.2 to -24.7‰ and excursion values range from δ¹³C = -28.8 to -25.9‰. As at Polecat Bench, the Meniscotherium zone in the Honeycombs area is in the brief initial onset of the CIE, and the Wa-0 sites are in the longer recovery during or after the maximum negative excursion.

The Honeycombs area is important because the Halfway Hill locality is the first to yield a perissodactyl from the Meniscotherium zone, and the Castle Gardens wash site is the first to yield abundant evidence of Wa-0 microvertebrates. This area is different from Polecat Bench in preserving mammals in channel deposits.