Paleoatmospheric CO2 oscillations through a cool middle/Late Cretaceous recorded from pedogenic carbonates in Africa

Abstract

This study presents the first African pedogenic carbonate estimated atmospheric CO2 concentrations for the Early to Late Cretaceous transition. Pedogenic carbonates have been used extensively as proxies to reconstruct past climates, yet few studies have focused on the climatic conditions of Africa during the Mesozoic. 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. Paleosol carbonate nodules were sampled from Bk horizons of seven particularly well-exposed and well-characterized paleosols through the Cretaceous Galula Formation, including four from the lower Mtuka Member (Aptian–Cenomanian) and three from the upper Namba Member (Campanian). The Cretaceous Galula Formation is a fossiliferous continental sedimentary succession of braided fluvial deposits that are well exposed in recently incised river drainages of the Rukwa Rift Basin, Tanzania. 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 1860 ppmv was estimated for the Aptian–Cenomanian, before declining to an average of 520 ppmv in the Namba Member, during the Campanian. Atmospheric pCO2 values fluctuated, but remained above 1000 ppmv through the middle Cretaceous, and correspond to the “cool” greenhouse period that spanned the Aptian–Albian. The gradual decline in pCO2 (550–502 ppmv) recorded in the Namba Member paleosols occurred during Late Cretaceous cooling following the Cretaceous Thermal Maximum.