A Paleogene calcareous microfossil Konservat-Lagerstatte from the Kilwa Group of coastal Tanzania

Abstract

Microfossil assemblages and their shell geochemistry are widely used in paleoceanography, but they can be signifi cantly altered by subtle variations in preservation state. Clay-rich, hemipelagic sediments of the Paleogene Kilwa Group of coastal Tanzania host calcareous microfossils that are exceptionally preserved, as evidenced by morphological, taxonomic, and geochemical data. The planktonic foraminifera are preserved as glassy, translucent tests with original microgranular wall textures that resemble well-preserved modern specimens, and they arguably yield geochemical values that are relatively unaffected by recrystallization. The calcareous nannofossils are extraordinarily diverse and represented by unique assemblage compositions that include dissolution-susceptible taxa, especially holococcoliths and rhabdoliths, and fragile and very small (<3-µm) heterococcoliths, many of which are new taxa. Notably, the extant, deep-photic-zone taxon Gladiolithus is documented for the fi rst time in the pre-Quaternary fossil record. 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 lithifi ed, carbonate-rich sediments. Most of the lost diversity, however, is confi ned 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.

Major shifts in calcareous phytoplankton assemblages through the Eocene‐Oligocene transition of Tanzania and their implications for low‐latitude primary production

“…This is, to our knowledge, the first documentation of long-term productivity changes between the haploid and diploid coccolithophore life cycle phases in response to pronounced paleoceanographic disturbance and we interpret this as representing increased nutrient availability in the low-latitude surface ocean. Many of these fragile and dissolution-susceptible holococcolith taxa are not usually preserved in typical carbonate-rich pelagic sediments, which is one of several preservational filters that can limit our understanding of past ecological change from an analysis of moderately to poorly preserved microfossil assemblages [Bown et al, 2008a;Young et al, 2005].…”

“…The deposition of these sediments in an outer shelf to upper slope environment, well above the paleolysocline, means they were unaffected by the dramatic variations in CCD depth through the EOT. Samples taken across the EOT consistently yield a highly diverse and exceptionally well preserved calcareous nannofossil assemblage (Figure 1) [Bown et al, 2008a;Dunkley Jones et al, 2008] and can be correlated directly to the EoceneOligocene boundary by planktonic foraminifera biostratigraphy and the onset of the EOGM by stable isotope stratigraphy .…”

“…[6] The recent recovery of a latest Eocene to earliest Oligocene sequence of hemipelagic clays from coastal Tanzania [Nicholas et al, 2006], which host exceptionally well preserved calcareous microfossils [Bown et al, 2008a;Pearson et al, 2008], provides a unique opportunity to examine the coccolithophore response through this critical climatic transition in a low-latitude setting. In this study we present a high-resolution record of coccolithophore assemblages through this succession and discuss their implications for changes in surface water productivity linked to major climate change.…”

Warm ocean processes and carbon cycling in the Eocene

“…However, the deeper limit of this estimate is uncertain, and given the narrow shelf and steep slope that is typical of the East Africa margin, it is quite possible that palaeodepths were considerably greater than this. Both the foraminifera and nanofossil assemblages indicate a fairly constant, open, deep and relatively oligotrophic environment; there are no restricted assemblages or conspicuous shelf-restricted or eutrophic taxa [52]. Coring of modern hemipelagic sediments offshore of Tanzania has revealed that silty clays of similar facies to the Kilwa Group are currently being deposited at depths of 500-1800 m within 50 km of the shoreline [58]; hence, a more conservative depth estimate for the Eocene sediments would be 300-1800 m. For both modern and Eocene environments, it is likely that onshore currents brought gyre water onto the continental slope, lending an oceanic rather than coastal character to the water column.…”

“…The significance of the carbon isotope data has not previously been discussed. These Tanzanian and Mexican data are particularly valuable, as they are derived from exceptionally well-preserved assemblages with no evidence of micrometre-scale recrystallization [17,51] (see also [52]). This is important, because such diagenesis can have a large effect on the stable isotope composition of foraminiferal tests, including δ 13 C [17,53], despite claims to the contrary [54].…”