Greening the Sahara: Stalagmites, isotopes and mankind’s route out of Africa
This is a Physical Geography Seminar by Dr Mike Rogerson (University of Hull)
Multiple lines of evidence indicate that the spatial extent of the Sahara is variable, and reaches minima when northern hemisphere insolation is high. Rainfall in North Africa is therefore linked to the same Precession and Obliquity cycles that drive variabilities in high latitude climate on timescales ~10^5 years. These changes are best recorded in time-series of aeolian dust blown off North Africa into the Atlantic, and in the occurrence of “sapropel” records in the Mediterranean. However, these records record indirect consequences of the rainfall changes in the Sahara, namely dust transport through the atmosphere and freshwater deficit in the Mediterranean Basin. In both cases, some influence from beyond the Saharan region is possible. Indeed, direct evidence of hydrological change in central North Africa has proven difficult to establish.
Isotopic “fingerprinting” of freshwater entering the Mediterranean during humid phases, using neodymium and strontium isotopes, demonstrates a predominantly southern source, and implicates significant runoff from Libya. This is compelling and direct evidence of hydrological change in the region of central North Africa, but is a signal integrated over a drainage basin roughly the combined size of Germany and France and these fingerprinting tools currently cannot be used to create high-resolution time-series. A stalagmite (SC-1) provides means to provide more precise insight into past Libyan rainfall. SC-1 simultaneously records changes in rainfall amount (via growth rate) and moisture source changes (via d18O) for 3 humid periods during Marine Isotope Stage 3. This evidence confirms the link between rainfall in Libya and northern hemisphere insolation, but provides some surprises also. Isotopic changes are dominated by variable balance of Atlantic and Mediterranean moisture sources, and show a high coherence with high-latitude boreal records, further indicating a linkage between Saharan rainfall and North Atlantic temperature.
Increased understanding of rainfall and hydrology in the Quaternary of Libya inspires new questions, not least whether the rivers that flowed across the region in the past could have provided corridors for sporadic human migrations out of southern Africa? Modelling of surface water flow during the last interglacial provides the first constraint on how and where these rivers would have flowed at the time of the first observation of Homo sapiens skeletal remains north of the Sahara.
This event is open to all UoB staff and students.