Olduvai gorge
Olduvai Gorge is named after the oldupai plant (Sansevieria ehrenbergii) pictured here in the bottom left corner.
Olduvai Gorge is well known as a paleoanthropological site excavated by Louis and Mary Leakey in the 1950s and 60s. Several species of hominins found there: 1) Homo habilis, 2) Homo erectus, 3) and Paranthropous bosei, which make it an important location to understand climate and environment. I have worked on two projects at Olduvai Gorge: The Olduvai Paleonthropology and Paleoecology Project (TOPPP) and Ancient Climate and the Authigenic Clay Index of Aridity (ACACIA).
Olduvai Gorge incises into the eastern margin of the Serengeti Plain. Paleo-Lake Olduvai expanded and contracted throughout the Pleistocene and was a saline-alkaline lake unsuitable for drinking. So why were the hominins occupying this landscape? This is the question I've been working on since 2010.
TOPPP
For TOPPP, I focused on the paleosols identified in uppermost Bed I. These deposits are ~1.8 Ma years old and are bracketed by tuffs that allow for reconstruction of slices of a landscape over a 20 kyr period. As paleo-Lake Olduvai expanded and contracted throughout the Pleistocene, lake clay was deposited followed by the formation of soils along a lake margin. Because there are a series of paleosols deposited throughout this 20 kyr time period, changes in climate can be tracked using changes in the bulk geochemistry. The paleosols have increased weathering of mobile elements like Na and Fe up section, likely due to changes in precession. You can read more about these paleosols in Quaternary International here and here.
The combination of paleosol features in outcrop, micromorphology (description of soil features and fabrics identified at the microscopic level), and bulk geochemistry allowed for a reconstruction of the landscape where slight changes in topography and proximity to a freshwater spring along a fault affected abundance of organic matter and degree of pedogenic development
This is thin section of a paleosol where a root pore was first lined with FeMn and then zeolites. The FeMn would have been mobilized during wetter conditions and precipitated during dry periods. The zeolites precipitate during dry climate as evaporative-pumping drove saline-alkaline water upward in the soil
ACACIA Project
The ACACIA Project is focused on the lacustrine authigenic clays deposited by paleo-Lake Olduvai. The lake was shallow, alkaline, and oxidizing and therefore does not preserve organic material like pollen, so a new method to understand changes in the lake was needed. Previous research in the Olduvai Basin has shown that the composition and abundance of authigenic clay mineral phases can record changes in the lake. Because paleo Lake Olduvai was a hydrologically restricted lake and saline-alkaline, Al-rich detrital clays washing into the lake interact with these brines and can be altered. When this occurs in the Olduvai Basin, more Mg is incorporated into the clay mineral. Therefore Al-rich clays are interpreted as freshwater and Mg-rich clays represent elevated salinity that is necessary to alter the clays. We only study very smallest clay-sized particles < 0.2 μm because the smallest sizes are more likely to be authigenic, but this clay mineral alteration in saline-alkaline waters also affects the overall bulk geochemistry, which is much easier to measure. This research was just published in Geology!
A) X-ray Diffractogram of the 060 peak which is key to interpreting whether the clays are Al-rich (freshwater) or Mg-rich (saline). The structure of the clay mineral refracts differently depending on the amounts of Al vs. Mg. B) This change in the <0.2 μm size clay fraction also has an affect on the overall bulk geochemistry of the sediment.
The overall bulk geochemical concentrations of Al and Mg in these lacustrine sediments changes throughout the history of paleo Lake Olduvai during the Pleistocene. Six freshening events (gray vertical bars) are identified that are best correlated to December insolation in the southern Hemisphere (20°S) and today is associated with the shorter monsoon in Tanzania. This adds to the growing body of evidence that precession dominated Sub-Saharan Africa during the Pleistocene.