A Paleogene Calcareous Microfossil Konservat-Lagerstätte from the Kilwa Group of Coastal Tanzania

The Paleogene Kilwa Group of coastal Tanzania host calcareous microfossils that are exceptionally preserved, as evidenced by morphological, taxonomic and geochemical data. These clay-rich, hemipelagic sediments were initially targeted for paleoclimate study as they contain ‘glassy', exceptionally-preserved planktonic foraminifera that yield stable isotope values which are less-modified by diagenetic recrystallization than pelagic carbonate-rich sediments. These isotopic studies are currently providing a radically different view of tropical temperature changes through this warm climate interval. However, in addition to the exceptional foraminiferal preservation, we have now discovered that these sediments also host extraordinary calcareous nannofossil preservation, which is manifested in very high species diversities, distinct assemblage compositions, and the conservation of small and fragile coccoliths, many of which are new to science. The Kilwa Group calcareous nannofossil diversities are consistently higher than all coeval assemblages and provide a benchmark against which to compare other Paleogene biodiversity data. Highest diversities are preserved in hemipelagic, clay-rich lithologies and the greatest losses occur in lithified, carbonate-rich sediments. Most of the lost diversity, however, is confined to distinct taxonomic groups (holococcoliths and Syracosphaerales), and in general the preservational potential of Paleogene coccolithophores was greater than in the modern oceans because a larger proportion of the biodiversity fell within the larger size fractions. For both foraminifera and coccolithophores, incorporation into impermeable clay-rich sediments that have never been deeply buried appears to have been critical in producing this Konservat-Lagerstätte preservation.