1. PLANKTONIC FORAMINIFER BIOSTRATIGRAPHY AT SITE 925: MIDDLE MIOCENE–PLEISTOCENE1

W.P. Chaisson2 and P.N. Pearson3

ABSTRACT

Ocean Drilling Program Site 925 is situated on Ceara Rise in the western tropical Atlantic at 412N, 4330W and in 3041 m water depth. The section contains well-preserved planktonic microfossils through the late Miocene, with only moderate dissolution of carbonate in the middle Miocene. No unconformities are apparent.

Datums used to establish the biozonation, and to further refine the age model, were constrained shipboard to within 1.5 m. In general, one sample per core has been re-examined to identify all species present. Globorotalia menardii, Globorotalia tumida, Globorotalia truncatulinoides, Pulleniatina obliquiloculata, and Neogloboquadrina dutertrei vary considerably in abundance, and each is occasionally absent in the Pleistocene, which suggests significant oceanographic change through this interval. The published ages assigned to planktonic foraminifer datums are compared to those suggested by the astrochronologic time scale. Several significant discrepancies (>0.25 m.y.) are found. Some are due to taxonomic problems (e.g., first occurrence [FO] of Pulleniatina finalis, FO and last occurrence [LO] of Hirsutella cibaoensis, LO Neogloboquadrina acostaensis), but many may identify real diachrons (e.g., LO Hirsutella margaritae and FO Truncorotalia crassaformis). Discrepancies that involve zonal boundary markers (FO Globoturborotalita nepenthes and LO Fohsella fohsi) should be further studied, as revisions to the Neogene zonation may be in order.

Global rates of planktonic foraminiferal taxonomic evolution are compared to values measured at Site 925. Significant discrepancies are found, which suggests that the closing of the Central American Seaway decoupled patterns of evolution in the tropical Atlantic and Pacific.

An assessment of planktonic foraminiferal dissolution at all five sites in the Ceara Rise depth transect reveals three apparent “transgressions” of corrosive bottom water to shallower depths on the rise between 10 and 12 Ma, at 7 Ma, and at 1 Ma.

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).
2Paleontological Research Institution, 1259 Trumansburg Road, Ithaca, New York 14850, U.S.A. Present address: Institute of Marine Sciences, University of California, Santa Cruz, California 95064, U.S.A. 71024.617@compuserve.com
3Department of Geology, University of Bristol, Wills Memorial Building, Queens Road, Bristol, United Kingdom.