Hunting for Paleoclimatic Periodicities in a Geologic Time Series With an Uncertain Time Scale

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Abstract
An 8‐m section of a mid‐Cretaceous (Albian) deep‐sea sediment core from Piobbico, Italy contains ∼2 cycles/m compositional oscillations. As postdepositional chemical migration in the core is weak, the relative amount of carbonate is a primary indicator of Cretaceous paleoclimate variability. We report results of spectral analysis of two series, CaCO3wt % measurements taken at roughly 2 cm intervals and a more densely sampled photodensitometer record of light‐dark variation. The uncertainty in the age‐depth relation (i.e., unknown fluctuations in sedimentation rate) hampers a detailed assessment of the suggested eccentricity correlation. We fit for long‐term (time scale greater than 75 kyr) fluctuations in sedimentation rate by tracking the local frequency modulation of the dominant harmonic in the data series, assuming the true input signal to be monochromatic. We estimate uncertainty in the observed local frequency by means of a statistical jackknife. We fit a smoothed cubic spline to the set of discrete frequency modulation estimates, specifying a χ2=Nmisfit criterion, whereNis the number of frequency estimates. From this analysis we infer fluctuations of a factor of two in the sedimentation rate, which occur predominately on a time scale of 400 kyr. We generate “tuned” series using several candidate age‐depth relations from the carbonate and densitometer series. The tuned carbonate series returns greater correlation with the modern Milankovitch periodicities. The 2 cycles/m energy resolves principally into two harmonic components whose frequency ratio corresponds (to an observational accuracy of 1–2%) to that of the 97.0‐ and 127.6‐kyr eccentricity oscillations. At frequencies lower than 2 cycles/m, we find harmonic oscillations whose phase correlates with the “beating” envelope of the 2 cycles/m energy in a manner consistent with the modern Milankovitch insolation. We also observe a sinusoidal oscillation of lower amplitude with period (scaled by those of the presumed eccentricity oscillations) of 39.2 ± 1.1 kyr, which agrees with the period of the modern obliquity cycle after correction for the effects of tidal friction. Oscillations corresponding to Milankovitch precessional frequencies are not established, owing possibly to low signal level and unknown short‐term deposition fluctuations. The tuned densitometer series recovers few new periodic features, although its inferred age‐depth relation correlates well with that of the carbonate series. We calibrate the depth variable downcore to the orbital cycles to estimater=5.0 ± 0.15 m/m.y. as the effective sedimentation rate.