Orr, Theresa J1, Roberts, Eric M2, Wurster, Christopher M1, Singleton, Russell E1, Lawrence L2, & Mtelela Cassy3
1Department of Geosciences, James Cook University, Cairns, Australia; 2Department of Geosciences, James Cook University, Townsville, Australia; 3Department of Geosciences, University of Dar es Salaam, Dar es Salaam, Tanzania
Pedogenic carbonates are valuable terrestrial climate archives of past atmospheric, climatic and environmental conditions. The stable isotope composition of pedogenic carbonates preserves a record of ancient atmospheric CO2 concentration and temperature at the time of mineral precipitation. While pedogenic carbonates have been used extensively as proxies to reconstruct past climates, few studies have been conducted on the climate of mid-Late Cretaceous Africa. The paucity of paleoclimate data from the region represents a significant obstacle in interpreting past climate change, and thus our ability to predict future changes. The preservation of pedogenic carbonates in the Galula Formation of southwest Tanzania provides an opportunity to enhance our understanding of the paleoclimatic conditions of the mid-Late Cretaceous. The Cretaceous Galula Formation is a fossiliferous continental sedimentary succession of braided fluvial deposits that are well-exposed in the river drainages of the Rukwa Rift Basin. This study presents the first estimated atmospheric CO2 concentrations for the mid-Late Cretaceous derived from pedogenic carbonates from Africa. Paleosol carbonate nodules were sampled from seven Bk-horizons located throughout ~500m of the Galula Formation stratotype section, including four from the lower Mtuka Member (Aptian-Cenomanian) and three from the upper Namba Member (Cenomanian-Campanian). Oxygen isotope values averaged -5.6 ‰ and -7.2 ‰ VPDB for the Mtuka and Namba members, equating to mean annual temperatures of 14.5°C and 11.2°C, respectively. Using the stable isotope composition of pedogenic carbonates from the Mtuka Member, an average pCO2 of 1390 ppm was estimated for the Aptian-Cenomanian, before declining to an average of 740 ppm in the Namba Member, during the Cenomanian-Campanian. Atmospheric pCO2 fluctuated through the mid-Cretaceous, rising to a peak of 1900 ppm, before falling to the lowest concentration of 990 ppm, corresponding to the cool greenhouse period that spanned the Aptian-Albian. The gradual decline in pCO2 (930ppm to 620ppm) recorded in the paleosols of the Namba Member occurred during Late Cretaceous cooling following the Cretaceous Thermal Maximum. This work indicates that the mid-Late Cretaceous was not a continuous greenhouse climate, and that an episodic climate mode prevailed, rather than a monotonic increase or decrease in pCO2 and temperature throughout the geologic period.
Theresa Orr is a PhD student at James Cook University, Queensland. Her primary research focuses on reconstructing the paleoclimate and paleoenvironment of the late Mesozoic to Early Cenozoic using paleosols. Her work uses morphology, mineralogy, element geochemistry and stable isotope analyses to interpret paleoclimate and paleoenvironmental histories.