Reports: AC8
46858-AC8 Biotic Change in the Bighorn Basin, Wyoming, Before, During, and After the Paleocene-Eocene Thermal Maximum (PETM): Consistent with Orbital Forcing?
Our main goal is to determine whether the terrestrial Paleogene geological record preserved in the Bighorn and Clarks Fork Basins, Wyoming, exhibits cyclical patterns consistent with orbitally-forced changes in solar energy (insolation). More specifically, we are interested in comparing patterns in (1) sedimentary sequences (i.e., stratigraphic spacing of depositional environments); (2) stable isotope records; and (3) faunal sequences (i.e., patterns of first and last appearances of mammalian taxa); to (4) expected precessional (19/23 ky), obliquity (41 ky), and eccentricity (100/400 ky) scale periodicities. We measured and logged over 675 meters of stratigraphic section in the late Paleocene-early Eocene Willwood Formation, sampling for dispersed organic carbon (DOC) every ~ 1 m (on average) for stable carbon isotopic analysis. All 628 isotope samples have been analyzed, resulting in a 610 m, high-resolution composite isotope record covering over 1.4 million years of geological time. This new stable carbon isotope record has been tied into the biostratigraphic framework of the two contiguous basins, enabling direct comparisons of isotopic and faunal dynamics.
No significant peaks in power spectra were found using conventional spectral analyses, but this may be due to applying an average depositional rate to convert stratigraphic thicknesses to quantitative ages. Consequently, we used the empirical mode decomposition (EMD) method of Huang et al. (Proc. R. Soc. Lond. 454A: 903-995, 1998) to extract hierarchical oscillatory patterns regardless of linearity. Our lower-frequency extracted patterns exhibited periodicities consistent with the 100 ky eccentricity cycle, indicating that perturbations to the carbon cycle during the late Paleocene and early Eocene are orbitally influenced. However, faunal boundaries do not consistently coincide with maxima, minima, or zero-crossings in any extracted pattern, providing no evidence that major changes in faunal composition are associated with orbital forcing mechanisms. The autecologies of individual mammalian lineages may make them more susceptible to orbitally-influenced environmental factors. We are currently working to determine if this is the case by comparing individual morphological time series to the 100 ky periodicity in the DOC isotope record.
