Oligocene climate forcing and palaeoceanography of the equatorial Pacific (paper presented at: AGU Fall Meeting, San Francisco, 2004)

Abstract

: A planktonic and benthic foraminiferal stable isotope stratigraphy of the Oligocene equatorial Pacific (Ocean Drilling Program, Site 1218) was generated at 6 kyr resolution between magnetochrons C9n and C11n.2n ($\sim$26.4-30 Ma on a newly developed astronomically calibrated time scale). Our data allow a detailed examination of Oligocene paleoceanography, the evolution of the early cryosphere and the influence of orbital forcing on glacioeustatic sea level variations. The Oligocene climate and ice sheet dynamics were strongly influenced by orbital forcing. Spectral analysis reveals power and coherency for obliquity (40 kyr period) and eccentricity ($\sim$110 kyr, 405 kyr) orbital bands, with an additional strong imprint of the eccentricity and 1.2 Myr obliquity amplitude cycle, which drove ice sheet oscillations in the southern hemisphere. Heavy oxygen isotope intervals correspond to maxima in carbon isotopes, indicating substantial changes in the carbon cycle that accompanied the glacial events. Planktonic and benthic foraminifera $\delta^{18}$O are used to constrain the magnitude and timing of major fluctuations in ice volume and global sea level change. Glacial episodes, related to obliquity and eccentricity variations, occurred at 29.16, 27.91 and 26.76 Ma. These correspond to glacioeustatic sea level fluctuations of 50 to 65 meters. High amplitude cyclic variations are recorded in the carbon isotope signal of planktonic and benthic foraminifera, the water column carbon isotope gradient and estimated percent carbonate of bulk sediment. Maxima in $\delta^{13}$C and the increased $\Delta\delta^{13}$C values are associated with each of the glacial events. Alteration of high latitude temperatures and Antarctic ice volume thus had a significant impact on the global carbon burial and equatorial productivity. We investigate the implications of a close correspondence between oxygen and carbon isotope events and long term amplitude envelope extrema in astronomical calculations during the Oligocene, and develop a new naming scheme for stable isotope events, based on the 405 kyr eccentricity cycle count.

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