FLORISPHAERA-PROFUNDA, A POSSIBLE NANNOPLANKTON INDICATOR OF LATE QUATERNARY CHANGES IN SEA-WATER TURBIDITY AT THE NORTHWESTERN MARGIN OF THE PACIFIC

Three piston cores from the Ryukyu Trench slope, northwestern margin of the Pacific Ocean, contain nearly continuous paleoceanographic records of the last 320, 165 and 105 kyr. Ages of the cores are constrained by planktic deltaO-18 stratigraphy and roughly substantiated by the presence of two calcareous nannofossil events. Spectral analysis of the changes in relative abundance of Florisphaera profunda, a nannoplankton species presently living in the lower euphotic zone, reveals a maximum power at approximately 0.01 cycles/kyr, corresponding to the approximately 100 kyr/cycle of orbital eccentricity. Much lower power is observed at the approximately 23 kyr precessional band. This differs from the equatorial Atlantic Ocean variations, which show dominant power in the approximately 23 kyr precessional band in the changes of F. profunda abundances (Molfino and McIntyre, 1990a). Besides, our cores were taken outside the equatorial divergence zone. We also show a rather close relationship between F. profunda abundance and sea-water transparency (Secchi depth) in the present northwest Pacific, based on data from 204 sites of surface sediment samples. Because sea-water transparency is primarily affected by bio-productivity change in open ocean, stratigraphic changes of F. profunda abundances in our cores were to have been caused by productivity change as well pointed out by Molfino and McIntyre (1990a,b). A mechanism proposed by them for its change, however, is not applied to our cases due to differences in spectral analysis results and in geographic situation. Alternatively, we suggest that geographic change of the northwestern margin of the Pacific during the late Quaternary caused F. profunda abundance change through the sea-water turbidity change which has been affected not only by bio-productivity but also by land-origin particles supply.