Shipboard investigation of magnetostratigraphy and shore-based investigation of diatoms and calcareous nannofossils were used to identify datum events in sedimentary successions collected at Ocean Drilling Program (ODP) Leg 201 Site 1225. The goal was to extend the magnetic record previously studied at the same site, ODP Leg 138 Site 851, and provide a comprehensive age model for Site 1225. Two high–magnetic intensity zones at 0–70 and 200–255 meters below seafloor (mbsf) were correlated with lithologic Subunits IA and IC in Hole 1225A. Subunit IA (0–70 mbsf) contains the magnetic reversal record until the Cochiti Subchronozone (3.8 Ma) and has a sedimentation rate of 1.7 cm/k.y. This agrees with previous work done at Site 851. Subunit IC (200–255 mbsf) was not sampled at Site 851. Diatom and nannofossil biostratigraphy constrained this subunit, and we found it to contain the magnetic reversal record between Subchrons C4n.2r and C5n.2n (8.6–9.7 Ma), yielding a sedimentation rate of 2.7 cm/k.y. Biostratigraphy was used to establish the sedimentation rates within Subunits IB and ID (70–200 mbsf and 255–300 mbsf, respectively). These subunits had higher sedimentation rates (~3.4 cm/k.y.) and coincide with the late Miocene–early Pliocene biogenic bloom event (4.5–7 Ma) and the Miocene global cooling trend (10–15 Ma). High biogenic productivity associated with these subunits resulted in the pyritization of the magnetic signal. In lithologic Subunit ID, basement flow is another factor that may be altering the magnetic signal; however, the good correlation between the biostratigraphy and magnetostratigraphy indicates that the magnetic record was locked-in near the seafloor and suggests the age model is robust.
1Niitsuma, S., Ford, K.H., Iwai, M., Chiyonobu, S., and Sato, T., 2006. Data report: Magnetostratigraphy and biostratigraphy correlation in pelagic sediments, ODP Site 1225, eastern equatorial Pacific. In Jørgensen, B.B., D'Hondt, S.L., and Miller, D.J. (Eds.), Proc. ODP, Sci. Results, 201 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/201_SR/110/110.htm>. [Cited YYYY-MM-DD]
2Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. greigite@ganko.tohoku.ac.jp
3Graduate School of Oceanography, University of Rhode Island, South Ferry Road, Narragansett RI 02882, USA.
4Department of Natural Environmental Science, Kochi University, Kochi 780-8520, Japan.
5Faculty of Engineering and Resource Science, Akita University, Akita 010-0851, Japan.
Initial receipt: 3 January 2005
Acceptance: 1 February 2006
Web publication:
18 May 2006
Ms 201SR-110