Quantitative Nannofossil Biostratigraphy of Quaternary Sequences from Guyots in the Central and Western Pacific Ocean

Abstract

Quantitative analysis of calcareous nannofossils was performed on Quaternary sediments recovered during Leg 144 at Sites 880 (Takuyo-Daisan Guyot), 871 (Limalok Guyot), 872 (Lo-En Guyot), and 873 (Wodejebato Guyot). For each sample, at least 500 specimens were counted with the light microscope and the relative abundance of taxa was computed as percentages. Calcareous nannofloras are very abundant and well preserved. Assemblages are dominated by the gephyrocapsid group, which was subdivided using a simplified taxonomy based on morphologic and morphometric characters that are easily applicable during light microscope investigations. Quantitative analysis of nannofloras revealed distinctive changes in the abundance patterns, especially within the Gephyrocapsa group, at all sites. These events seem to be coeval on a global scale and were used for high-resolution biostratigraphy. Most nannofossil events and biozones previously proposed for the Quaternary were recognized.The sequence recovered at Site 880 is almost complete, but a hiatus was detected in the upper Pleistocene, where the G. oceanica Zone is extremely short. At Site 871, the Quaternary sequence seems to be complete and all the nannofossil events and biozones were detected. Also at Site 872, a complete sequence of nannofossil events was recognized, but the Quaternary section is very thin and suggests very low sedimentation rates and/or the occurrence of minor hiatuses that cannot be determined with biostratigraphy. The spacing of nannofossil events at Sites 800, 871, and 872 is quite different, suggesting variable sedimentation rates and minor gaps caused by the strong currents on top of these guyots. At Site 873, most of the Quaternary is missing; detection of only the lower Pleistocene nannofossil events suggests that the accumulation of recent sediments on Wodejebato Guyot is strongly affected by current activity.Major changes in the composition of Pleistocene nannofloras appear to be coeval on a global scale across a wide latitudinal range within the tropical to equatorial belts. However, the nannofossil records at Sites 871, 872, 873, and 880 indicate that some taxa were locally and temporarily influenced by the specific oceanographic conditions. Further investigations on the distribution of specific taxa, such as the smallest specimens of Gephyrocapsa (2-2.5 µm) and Florisphaera profunda, may help in separating global vs. local/regional effects.