PRECESSION- AND LONG-ECCENTRICITY-SCALE VARIABILITY IN AN EARLY PALEOGENE TERRESTRIAL ORGANIC D13C RECORD (WILLWOOD FORMATION, CLARKS FORK BASIN, WY)

The bulk organic component of terrestrial sediment in the geologic record may be derived from disparate sources, such as decayed plant remains or residual organics of eroded bedrock. Carbon isotope records derived from bulk organics likely conflate patterns in atmospheric δ¹³C, floral composition, erosion rates, and preservational processes, among others. Climate change impacts these phenomena, potentially synchronizing their contributions to bulk organic δ¹³C records through Milankovitch-scale climate forcing. Detection of Milankovitch-scale patterns in bulk organic isotope records is beneficial for regional and global correlation of terrestrial deposits, even if the primary source of the bulk organics is unknown. We present a high-resolution 614-meter δ¹³C record measured from dispersed organic carbon (DOC) of the alluvial Willwood Fm. (late Paleocene-early Eocene, Clarks Fork Basin, WY). Results from MC-Clean spectral analysis and Empirical Mode Decomposition (EMD) were combined due to expected heterogeneity in short-term accumulation rates, which could cause responses to high-frequency forcing to appear arrhythmic in the thickness domain.

MC-Clean results reveal a statistically-significant, thickness-domain periodicity consistent with a 405 ky, long-eccentricity scale pattern based on time-domain conversion, cycle amplitude, and stratigraphic positions of local extrema relative to the Paleocene-Eocene Thermal Maximum. EMD results show a significant arrhythmic cycle with a median thickness of 9.6 m, interpreted as a precession-scale signal based on (1) long-term accumulation rates and (2) high correlation between cycle counts and duration estimates derived from paleosol maturities. Comparison with early Eocene inorganic carbon isotope records and lithologic characteristics of Paleogene hyperthermal events suggests the majority of the 405 ky pattern is consistent with eccentricity-scale perturbations to the global carbon cycle. However, low correlation between the precession-scale δ¹³C_DOC pattern and paleosol stacking is inconsistent with previously observed precession-scale lithostratigraphic patterns in the Willwood Fm., suggesting greater degree of conflation of sources of organic carbon isotope variability at smaller temporal scales in the Clarks Fork Basin.