Silicoflagellates were very sparse in the Leg 199 samples studied. No silicoflagellates were found in Holes 1215A, 1216A, 1217A, 1221B, 1221C, or 1222A (Tables T1, T2, T3, T4) and only in one sample from Hole 1221A (Table T4). The three sites found to have silicoflagellates in multiple samples were from the southernmost sites. More specifically, these samples were found in Holes 1218A (Table T2), 1219A (Engel and McCartney, this volume), 1220A, and 1220B (Table T3). The abundance and diversity of silicoflagellates did not provide sufficient information to determine biostratigraphic zonations. The northernmost of these three sites, Site 1220 (10°10.601´N, 142°45.491´W), had a diversity and abundance generally similar to that of Leg 138 Hole 854A (McCartney et al., 1995), which is presently at a similar latitude. This supports the trend found in the Leg 138 study of rapidly diminishing silicoflagellate abundance away from the equator.
For Site 1221, silicoflagellates were only found in Sample 199-1221A-5H-4, 70–71 cm. This site is located south of the four sites that were not found to have silicoflagellates, further demonstrating the trend toward diminishing abundance away from the equator. The three silicoflagellate taxa, Corbisema inermis, Corbisema regina, and Dictyocha spinosa, found in this sample suggest, on the basis of experience gained from the Hole 1219A study, that the sample in question is probably from the Dictyocha hexacantha Zone, although the nominative species for this zone was not found.
The general absence of silicoflagellates can be the result of some combination of low biosilicious productivity and a water column and pore waters that are corrosive to opaline silica (Scherer, 1998). However, this does not entirely explain the frequent presence of radiolarians and occasional presence of complete diatoms in samples where silicoflagellates were not found. Silicoflagellates, with their thin and hollow skeletal elements, may be more susceptible to dissolution than the larger and more massively built radiolarians and diatoms.
The diminishing presence of silicoflagellates away from the equator may suggest that silicoflagellates simply do not thrive away from regions of highest productivity.