3. Magnetostratigraphic Calibration of the Late Oligocene Climate Transition¹

Benjamin P. Flower² and Kathryn E. Chisholm³

ABSTRACT

Magnetic polarity stratigraphy and stable isotope data from Ocean Drilling Program Leg 202 Core 202-1237B-31H allow calibration of the late Oligocene climate transition to the geomagnetic polarity timescale (GTS 2004). Oxygen isotope (¹⁸O) values in the benthic foraminifer Cibicidoides mundulus decrease from ~2.20 in upper Chron 9n (Oi2b) to ~1.90 in upper Chron 8r. The mean decrease is smaller than suggested by a recent compilation of benthic foraminifer ¹⁸O records. The decrease occurs in three steps from 323 to 321 meters composite depth (mcd) within Chron 8r (27.027–26.554 Ma), and is here termed the "Chron 8r ¹⁸O shift." Linear interpolation within Chron 8r estimates an age of 26.35 Ma for the final decrease to lower values marking initiation of the late Oligocene climatic optimum. The initial decrease at ~26.7 Ma is closely associated with the last occurrence of Paragloborotalia opima (P21/P22 planktonic foraminifer zonal boundary) between 322.51 and 322.76 mcd in lowermost Chron 8r. This study is a modest step toward an integrated magnetic polarity and astronomical chronostratigraphy for the late Oligocene based on Site 1237.

¹Flower, B.P., and Chisholm, K.E., 2006. Magnetostratigraphic calibration of the late Oligocene climate transition. In Tiedemann, R., Mix, A.C., Richter, C., and Ruddiman, W.F. (Eds.), Proc. ODP, Sci. Results, 202 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/202_SR/204/204.htm>. [Cited YYYY-MM-DD]

²College of Marine Science, University of South Florida, St. Petersburg FL 33701, USA. bflower@marine.usf.edu

³Collegium of Natural Sciences, Eckerd College, St. Petersburg FL 33711, USA.

Initial receipt: 27 September 2004
Acceptance: 2 November 2005
Web publication: 14 April 2006
Ms 202SR-204