28. MILANKOVITCH-SCALE CLIMATE VARIABILITY RECORDED NEAR THE
OLIGOCENE/MIOCENE BOUNDARY1
B.P. Flower,2 J.C. Zachos,2 and H. Paul2
ABSTRACT
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 µm/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.
flower@earthsci.ucsc.edu
