STRONTIUM ISOTOPE AND CARBONATE CLUMPED ISOTOPE MEASUREMENTS OF SHARK TEETH, CORALS, OYSTERS AND STROMATOLITES FROM UPPER CRETACEOUS TO QUATERNARY COASTAL MARINE SEQUENCES FLANKING SOUTHERN AFRICA

Sea level along the coast of southern Africa has fluctuated by several hundreds of meters over the last 100 million years, as evidenced by dated marine sequences. However, in current models the amplitude and timing of global sea-level changes vary considerably. Together with imprecise biostratigraphic ages from the marine fossils, this prevents differentiation between the rates of tectonic uplift and subsidence along this continental margin.

Here we used strontium isotope measurements to study the stratigraphy of Upper Cretaceous to Quaternary marine terraces preserved at different elevations (0-350m) along the southern coast of South Africa, between Cape Town and Durban (>1000km). Measured ⁸⁷Sr/⁸⁶Sr in shark teeth, corals and oyster fossils confirm at least 2 episodes of marine transgression during: the Eocene-Oligocene at 36-33 Ma; and the Miocene-Pleistocene at 10-1 Ma. We tested both the high-resolution laser ablation and high-precision dissolution methods to reduce age ranges. However, some of the variability may be due to the influence of radiogenic strontium from freshwater and/or the effect of diagenesis. The samples were analyzed further using ¹³C-¹⁸O based clumped isotope measurements. First, we calibrated local modern stromatolites; isotopic temperatures are statistically within error of modern values of 20-22°C. Measured paleo-temperatures range between 17 and 29°C in the Upper Cenozoic (Messinian-Zanclean) oyster shells, and 20-21°C in Upper Cretaceous (Campanian-Maastrichtian) and Eocene calcarenites. We will discuss this new data. Ultimately, our aim is to contribute to global models of sea-level changes and to better quantify vertical movements of southern Africa during plate tectonic expansion of the South Atlantic.