Changes in seasonality and productivity recorded at low latitudes in Tanzania during the PETM

The Tanzanian Drilling Project (TDP) has been active since 2002 in recovering Cretaceous and Paleogene marine sediments from along a 150km piece of coast in southern Tanzania. Late Cretaceous and Paleogene sediments along this stretch of coast are part of the Kilwa Group (Nicholas et al., 2006). The sediments largely comprise organic-rich marine clays and claystones, with occasional interbedded limestones and sporadic carbonate-rich beds. Calcareous microfossils are generally very well preserved, due to the impermeability of the clays and claystones, and do not show the recrystallised microstructure typical of deep-sea oozes and chalks (Pearson et al., 2001). van Dongen et al. (2006) have shown that the Kilwa Group sediments contain well preserved terrestrial biomarkers, indicating a shallow maximum burial depth. The sediments are interpreted as having been deposited in a bathyal outer shelf to upper slope setting at an estimated depth of 300-500m. TDP Site 14 recovered sediments from the very latest Paleocene and the Paleocene-Eocene Thermal Maximum. These well preserved, organic-rich clays were analysed using three separate techniques designed to complement the standard paleoclimatic analyses of planktonic and benthic foraminifera and nannofossils carried out by other workers. Nitrogen isotope chemostratigraphy is used in this work as a proxy for past productivity and associated nutrient supply. In order to identify any other potential contributors to, or dilution of the nitrogen isotope signal, sedimentary mineralogy and elemental geochemistry techniques were also employed. The results from mineralogy and elemental geochemistry analyses indicate that terrigenous run-off increased during the PETM at this locaton. Kaolinite abundances, meanwhile, show that this region did not experience any major changes in humidity. Mineralogical investigation also found a large reduction in calcium carbonate in these sediments during the PETM interval. Finally, nitrogen isotope chemostratigraphy recorded an increase in its regular cyclicity recorded prior to the PETM. In conclusion, this data suggests that the southern coast of Tanzania was a place of extreme local climate with wetter and more intense precipitation in the rainy season. The rainy season may even have become longer, much like the effects of modern El Niño. The weathering in this region would have been intensified during the PETM, with older and deeper soils being eroded and washed out to sea in swollen rivers, where they were deposited as far out as the mid to outer shelf. These influxes of terrestrial sediment brought nutrients to the lightly acidified upper levels of the ocean allowing what planktonic life remained there to flourish for a time, driving down their nitrogen isotope signal, which was then recorded in the marine sediments as they died. Nicholas et al., 2006, Journal of African Earth Sciences, v. 45, p. 431. Pearson et al.., 2001, Nature, v. 413, p. 481-487. van Dongen, et al., 2006, Organic Geochemistry, v. 37, p. 539-557.