18. BULK ¹⁸O AND ¹³C RECORDS FROM SITE 999, COLOMBIAN BASIN, AND SITE 1000, NICARAGUAN RISE (LATEST OLIGOCENE TO MIDDLE MIOCENE): DIAGENESIS, LINK TO SEDIMENT PARAMETERS, AND PALEOCEANOGRAPHY¹

ABSTRACT

Stable isotope records spanning from the latest Oligocene through the middle Miocene have been produced at Sites 999 and 1000 to address major changes in climate associated with the transition from relative global warmth of the late Oligocene-early Miocene to the Neogene "ice-house" world.

The isotopic records of Sites 999 and 1000 clearly show the middle Miocene oxygen-isotope increase from ~15.5 to 13 Ma, and all Miocene oxygen-isotope events "Mi" can be recognized. Additional ¹⁸O increases occur at ~24.5, ~23, 20.5, and 17.8 Ma. In addition, the long-term ¹⁸O increase between ~15.5 and 13 Ma shows internal quasi-cyclic structures, reflecting a periodicity of ~400 k.y., the orbital eccentricity, which is superimposed on the longer term cooling trend related to the major ice-sheet growth in Antarctica. The long-term trends in carbon isotopes are marked by two positive excursions: the ¹³C increase near the Oligocene/Miocene boundary, from ~24.5 to 23 Ma, and the Monterey Excursion between ~17 and 13.5 Ma. The increase in ¹³C at 17 Ma leads the first major cooling step in Antarctica by 1.5 m.y. However, within the Monterey Excursion several ¹³C maxima occur with cyclic periods of ~400 k.y. in parallel to climate excursions as indicated by oxygen isotopes.

Long-term isotope shifts, coinciding with lithologic changes indicating the changes in sediment physical properties, are linked to changes in climate.

¹Leckie, R.M., Sigurdsson, H., Acton, G.D., and Draper, G. (Eds.), 2000. Proc. ODP, Sci. Results, 165 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/165_SR/165TOC.HTM>. [Cited YYYY-MM-DD]

²Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, U.S.A. mmutti@earth.usc.edu (Present address: Institute of Geology and Paleontology, University of Stuttgart, Herdweg 51, D-70174 Stuttgart, Germany. maria.mutti@geologie.uni-stuttgart.de)

Date of initial receipt: 23 June 1998
Date of acceptance: 21 May 1999
Ms 165SR-016