Orbitally Induced Climate and Geochemical Variability Across the Oligocene/Miocene Boundary

To assess the influence of orbital-scale variations on late Oligocene to early Miocene climate and ocean chemistry, high-resolution (∼5 kyr) benthic foraminiferal carbon and oxygen isotope and percent coarse fraction time series were constructed for Ocean Drilling Program site 929 on Ceara Rise in the western equatorial Atlantic. These time series exhibit pervasive low- to high-frequency variability across a 5-Myr interval (20.5–25.4 Ma). The records also reveal several large-scale secular variations including two positive (∼1.6‰) oxygen isotope excursions at 22.95 and 21.1 Ma, suggestive of large but brief glacial maxima (Mi-1 and Mi-1a events of Miller et al. [1991]), and a long-term cyclical increase in the carbon isotopic composition of seawater (shift of ∼1.52‰) that reaches a maximum coincident with peak δ18O values at 22.95 Ma. Lower-resolution (∼25 kyr) records constructed from benthic and planktonic foraminifera as well as bulk carbonate at a shallower site on Ceara Rise (site 926) for the period 21.7–24.9 Ma covary with site 929 δ18O values reflecting changes in Antarctic ice-volume. Likewise, covariance among carbon isotopic records of bulk sediment, benthic, and planktonic foraminifera suggest that the low-frequency cycles (∼400 kyr) and long-term increase in δ13C values represent changes in the mean carbon composition of seawater ΣCO2. The time series presented here constitute the longest, most continuous, and highest-resolution records of pre-Pliocene climate and oceanography to date. The site 929 carbon and oxygen isotope power spectra show significant concentrations of variance at ∼400, 100, and 41 kyr, demonstrating that orbitally induced oscillations have been a normal characteristic of the global climate system since at least the Oligocene, including periods of equable climate and times with no apparent Northern Component Water production.

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