The Earth’s deep mantle, climate change and the dawn of humans
This is an interdisciplinary project together with Lars Werdelin from the Paleobiology Department of the Swedish Natural History Museum and Rodrigo Caballero from the Meteorology Department of Stockholm University.
The East African Rift (EAR) is the most prominent rift system on Earth and transects the high-elevation East African Plateau. It is a biodiversity hotspot and the oldest known human ancestors have been found within its confines, making it a natural laboratory for interdisciplinary research straddling Earth and Life Sciences. Here we propose a truly transdisciplinary project, combining Paleobiology, Meteorology and Geology, that takes advantage of the symbiosis between the Swedish Museum of Natural History and Stockholm University.
We posit that changes in East African climate, biology and geology were driven by superswell-related uplift, with topographic uplift controlling the strengthening of the Asian monsoon and ultimately governing biodiversity evolution, including the emergence of our ancestors. As yet, no study has looked into how those very different processes actually interacted and communicated with each other, however. We aim to address this issue by implementing the geodynamic numerical model, combining it with a fully coupled Global Circulation Model to directly simulate the climate response to deep-mantle-driven topographic uplift. In addition, we will obtain temperature and precipitation data at a sub-regional level by using data from the cheek tooth crowns of herbivorous mammals, following Liu et al. to compare with the results from the modeling. Information on Miocene African biodiversity and its changes will be gleaned from a recent compilation. The data collected there will be entered into the NOW database8 for further analysis of richness, origination, and extinction patterns. Our study will be important because it is the first to address how seemingly unconnected processes at different scales from the Earth’s interior to its atmosphere interacted and controlled the evolution of terrestrial fauna and eventually the human lineage.
East Africa sits above the greatest thermal anomaly of Earth, the African Superswell, which rises from the core-mantle boundary (~2900 km deep). The Superswell is the reason why Africa has the highest mean elevation of all continents and why the EAR formed some 30 Ma ago in a continent that has largely been dormant for the past 500 Ma. It is widely believed that topographic development caused climate change in East Africa, which in turn was important for floral and faunal change in the Middle and Late Miocene, eventually resulting in the chimpanzee/hominin split by 7-6 Ma. Moucha and Forte developed a geodynamic numerical simulation based on flow pattern changes in the mantle that allows for the reconstruction of the amplitude and timing of topographic uplift above the African Superswell since 30 Ma. For East Africa, their results show that geographically extensive (~5 x 106 km2) uplift in excess of 500 m of elevation had begun by 15 Ma and became pronounced after 10 Ma.
Climate controlled biodiversity change is linked to the expansion of grassland (specifically C4 grasses) at the expense of forests due to increased aridification. The latter coincides with a shift in Asian climate toward today’s monsoonal regime. If monsoon winds strengthened, sea-surface temperatures would have dropped if the thermocline was deeper at ~10 Ma than it is today. Colder sea-surface temperatures in the Indian Ocean off the east coast of Africa would have deflected moisture transport away from East Africa towards Asia, which may have contributed to cooling East Africa, causing a shift towards arid conditions.