During Ocean Drilling Program (ODP) Leg 162, a continuous sequence of carbonate-rich sediments was drilled at Site 982 in a bathymetric depression on the Rockall Plateau (Fig. 1), a platform lying at ~1000 m water depth (WD) situated in the North Atlantic roughly between Iceland and Ireland. At Site 982, four holes were cored: in Holes 982A and 982C, the drilling stopped in upper Miocene sediments, Hole 982B extended to the lower Miocene, and Hole 982D only penetrated Holocene sediments. Of these, Holes 982A (57º30.992'N, 15º52.001'W, 1135.30 m WD) and 982B (57º31.002'N, 15º51.993'W, 1134 m WD) were investigated for Bolboforma studies. Preliminary biostratigraphic data on Bolboforma assemblages identified at Site 982 are included in the Leg 162 Initial Reports volume of the Proceedings of the Ocean Drilling Program (Jansen, Raymo, Blum, et al., 1996). The main characteristics of Bolboforma assemblages and the stratigraphic distribution of the taxa are described here in more detail.
Site 982 is located close to Deep Sea Drilling Project (DSDP) Hole 116 (57º29.76'N, 15º55.46'W, 1115 m WD), rotary drilled in 1970 and only spot cored. Nevertheless, Spiegler and Müller (1992) investigated the calcareous nannoplankton and the Bolboforma assemblages of this hole and presented a correlation of Bolboforma zonation and nannoplankton stratigraphy in the Neogene at that site. The redrilled continuous-cored sedimentary sequence at Site 982 gives the opportunity to review and complete the biostratigraphic results at Site 116.
Bolboforma are useful index fossils that supplement the standard microfossil zones based on planktonic foraminifers and calcareous nannofossils. They are marine calcareous microfossils of uncertain origin. Nearly 100 different species are known, ranging from the early Eocene to late Pliocene in age. They have not been recorded in Holocene material. Bolboforma taxa are characteristic of predominantly temperate to cool regions in both the Northern and Southern Hemispheres. Qvale and Spiegler (1989) and Spiegler and von Daniels (1991) defined Bolboforma zones. Spiegler and Müller (1992) correlated Neogene Bolboforma zones with nannoplankton stratigraphy.
Bolboforma specimens are Lagena-shaped and measure 50-250 µm in diameter. Most of the specimens are 150 µm in size or smaller. However, in contrast to Lagena (benthic foraminifer), several Bolboforma species are known to encapsulate a smaller chamber within the exterior test (Spiegler, 1987). The wall of the outer test as well as the inner cyst are of monocrystalline structure. The wall texture of the hollow spheroidal to subspheroidal test ranges from smooth to strongly ornamented with different arrangements and combinations of spines, ribs, reticulations, and flanges. The tests display a simple aperture often situated at the end of a distinct neck. The specimens are usually single chambered, but double-chambered forms are also known.