Triple Oxygen Isotopes
In modern waters and paleoclimate records such as ice cores, deuterium-excess (d- excess) or the deviation of δ18O and δD from the global meteoric water line (GMWL) has been used extensively to quantify evaporative effects; however, the application of d-excess to the rock record is limited because of the lack of co-occurring O- and H- mineral phases. Studies of the relationship between 16O, 17O, and 18O (triple oxygen isotopes) have shown that these isotopes are sensitive to evaporation similar to d-excess in water. Studies of the relationship between δ18O and δ17O have resulted in a triple oxygen isotope anomaly or Δ17O (similar to d-excess), which represents the deviation from the slope of the GMWL (0.528) on a δ’18O vs. δ’17O plot (Landais et al., 2010; Luz and Barkan, 2010). Evaporation results in an increase in δ18O and decrease in Δ17O in waters (Fig. 2A), such that soil waters in arid settings which have potential for evaporation should have Δ17O values that are equivalent to or lower than the observed range of global precipitation (Fig. 2B).
A) Schematic where Δ17O represents the deviation of δ17O and δ18O from the equilibrium Global Meteoric Water Line (GMWL; λ = 0.528). B) An evaporative increase in soil water should cause decrease in Δ17O values and increase in δ18O compared to the observed range of global precipitation.
Location of the modern Serengeti Ecosystem, which evidence from paleosols suggests may have expanded during the Late Pleistocene as Lake Victoria dried up.
Reconstructing Paleo-Aridity in east africa
Recent droughts in East Africa in 2011 and again in 2016 left millions in a state of food insecurity and highlight the need for understanding drought in equatorial Africa. Similar periods of drought and extreme climate variability are also considered to be key factors in human migration out of Africa and the emergence of the modern savanna ecosystem during the last 100 thousand years. However, testing this hypothesized connection between extreme climate variability and evolutionary change is difficult without equatorial records in the Lake Victoria region, which is part of the hypothesized Nilotic corridor for human migrations out of Africa.
In blue is the range of modern precipitation in the Lake Victoria Basin. In purple is reconstructed precipitation from paleosol-based bulk geochemical proxies and from paleosol features that indicates an environment similar to the modern Serengeti.
Further south, a megadrought (135-70 ka) is recorded in Lakes Malawi and Tanganyika with prolonged low stands >350 m below modern. In the Lake Victoria region, paleosol-based proxies, stable isotopes, and presence of arid-adapted fossil fauna indicate a grassland environment much drier than modern conditions, consistent with the megadrought identified further south, but aridity likely persists much longer to ~36 ka near Lake Victoria (Read more about it here). Early modern humans dispersals from equatorial East Africa began at ~70 ka by which time Malawi and Tanganyika had already returned to wetter conditions, and so were humans migrating during a wet or dry period? To answer this question, we are using a multi-proxy approach utilizing paleosols with in situ fossils and Middle Stone Age artifacts to reconstruct aridity in the Lake Victoria region.
A new triple oxygen isotope proxy (17O/16O, 18O/16O) will be used to isolate the influence of evaporation in the water balance in combination with more traditional isotopic proxies (δ13C, δ18O, Δ47) and non-isotopic bulk geochemical methods (CIA-K, PPM1.0). Complimentary studies of these proxies in modern Serengeti soils will anchor the paleoenvironmental interpretations and allow for the reconstruction of aridity for landscapes occupied by humans to help evaluate the role of long-term drought on human evolution and the emergence of the modern Serengeti Ecosystem.