Diatoms in Paleocene sediments of the southwestern South Atlantic

Hemiaulus kristoffersenii sp. nov. Hemiaulus nocchiae sp. nov. Hemiaulusl oonkii sp. nov.

Hemiaulusl velatus sp. nov. Triceratium gombosii sp. nov. Trochosira gracillima comb. nov.

Trochosira marginata sp. nov. Trochosira radiata sp. nov. Hole 700B provides one of the most continuous diatomaceous Paleocene profiles known.

Stratigraphic ranges of diatom species from this and other Southern Hemisphere sites are calibrated against calcareous microfossil zones. The first-appearance datums of Triceratium gombosii, Hemiaulus incurvus, and Triceratium mirabile in Paleocene deep-sea sediments are useful for regional stratigraphic correlations. Quantitative analysis of the biosiliceous microfossil groups (diatoms, silicoflagellates, radiolarians, and archaeomonadaceae) shows that preservation of diatoms is confined primarily to the upper Paleocene (planktonic foraminifer Zones P3 and P4 and calcareous nannofossil Zones upper NP5 to lower NP9).

In the lower Paleocene only short intervals in Hole 700B are diatomaceous. A correlation between the degree of silica diagenesis and the calcium carbonate content of the sediment is not obvious. Diatom species analysis reflects changes in the paleoenvironment between island-related upwelling conditions with highly diverse and well-preserved diatom assemblages and less productive periods resulting in less wellpreserved diatom assemblages with a higher content of robust neritic diatoms.