Abstract

Living and death assemblages of selected benthic, symbiont-bearing foraminiferal species were compared at a NW-Pacific island slope. Two transects with different morphologies were chosen, one demonstrating decreasing, the other slightly increasing, steepness. Intensities of depth transport were estimated by measuring the differences between distribution parameters of living individuals and empty tests. Three factors were shown to induce depth transport: (1) traction caused by offshore bottom currents or the frequent tropical cyclones that cross the area, (2) slope steepness, and (3) differences in test buoyancies. Due to the depth position of living populations, the combination of these three factors leads to varying displacement intensities and mixing of empty foraminiferal tests.

The two investigated larger foraminiferal species with porcelaneous tests living at the shallow slope—the rod-shaped Alveolinella quoyi (d'Orbigny) and the discoid Amphisorus hemprichii Ehrenberg—showed high buoyancy and are transported commonly down slope. Less displacement was found in Heterostegina depressa d'Orbigny, which inhabits the upper slope, and this is due to a lower test buoyancy. The thick-lenticular Nummulites venosus (Fichtel and Moll) and the spherical Baculogypsinoides spinosus Yabe and Hanzawa, both living in deeper regions, are transported less often, while Cycloclypeus carpenteri Brady, with a similar depth distribution, shows a high degree of transport to the deepest zone as the result of high buoyancy. The deepest-living species in this study, Planostegina operculinoides (Hofker), is not transported, but accumulates under conditions of reduced sedimentation.

The complex slope topography, combined with the exposure of the coast to tropical storms, leads to deposition of allochthonous specimens from surrounding 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 and slope inclination, disguise the clear depth dependence of larger foraminifers as shown by living individuals, representative proportions of the deepest-living species within an association of empty tests allows for the approximation of the upper depth limit.

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