Displacement of larger foraminifera at the western slope of Motobu Peninsula (Okinawa, Japan)

Living and death assemblages of selected benthic, symbiont-bearing foramini feral species were compared at a NW-Pacific island slope. Two transects wit h different morphologies were chosen, one demonstrating decreasing, the oth er slightly increasing, steepness. Intensities of depth transport were esti mated by measuring the differences between distribution parameters of livin g individuals and empty tests. Three factors were shown to induce depth tra nsport: (I) traction caused by offshore bottom currents or the frequent tro pical cyclones that cross the area, (2) slope steepness, and (3) difference s in test buoyancies. Due to the depth position. of living populations, the combination of these three factors leads to varying displacement intensiti es and mixing of empty foraminiferal tests. The two investigated larger foraminiferal species with porcelaneous tests l iving at the shallow slope-the rod-shaped Alveolinella quoyi (d'Orbigny) an d the discoid Amphisorus hemprichii Ehrenberg-showed high buoyancy and are transported commonly down slope. Less displacement was found in Heterostegi na depressa d'Orbigny, which inhabits the upper slope, and this is due to a lower test buoyancy The thick-lenticular Nummulites venosus (Fichtel and M oll) and the spherical Baculogypsinoides spinosus Yabe and Hanzawa, both li ving in deeper regions, are transported less often, while Cycloclypeus carp enteri Brady, with a similar depth distribution, shows a high degree of tra nsport to the deepest zone as the result of high buoyancy. The deepest-livi ng species in this study, Planostegina operculinoides (Hofker), is not tran sported, but accumulates under conditions of reduced sedimentation. The complex slope topography, combined with the exposure of the coast to tr opical storms, leads to deposition of allochthonous specimens from surround ing shallow areas. Specimens from backreef regions are transported into the forereef during waning storms (e.g, Amphisorus hemprichii), while elements of relict sediments are reworked on the deeper slope during these episodic events. Although both factors, in combination with down-slope transport an d slope inclination, disguise the clear depth dependence of larger foramini fers as shown by living individuals, representative proportions of the deep est-living species within an association of empty tests allows for the appr oximation. of the upper depth limit.