High-resolution (ca. 5 k.y.) and data from Ocean Drilling Program Hole 929A near the Oligocene/Miocene boundary document Milankovitch-scale climate variability from ca. 24.8 to 23.1 Ma. Stable isotopic data based on Cibicidoides mundulus better define the timing and magnitude of a maximum at ca. 23.9 Ma (event Mi1), and its association with the maximum near the Oligocene/Miocene boundary. Mi1 is marked by a gradual increase of 1.2 over about 250 k.y. from 23.9 to 23.6 Ma to a maximum of 2.15, followed by two rapid (<20 k.y. each) decreases of 0.6 at 23.62 and 23.57 Ma. The maximum of 1.6 is reached at the end of three cycles of ca. 400-k.y. period, culminating at 23.65 Ma (coincident with the maximum). Oxygen and carbon isotopic data covary through the sequence studied, suggesting a strong relation between organic carbon burial and global climates. However, leads by about 40 k.y.
Spectral analysis confirms Milankovitch-scale variability in , , and percent sand content (>63 Ám/g), with a dominant period of about 41 k.y., especially during the 24.0-24.8 Ma interval. Concentration of variance at this period strongly suggests a high-latitude control, probably East Antarctic ice sheet variability.
1Shackleton, N.J., Curry, W.B., Richter, C., and Bralower, T.J. (Eds.), 1997. Proc. ODP, Sci. Results, 154: College Station, TX (Ocean Drilling Program).
2Institute of Marine Sciences and Earth Sciences Board, University of California, Santa Cruz, CA 95064, U.S.A. firstname.lastname@example.org