Image: Modern Malawi, a sunny day on the south end.
A break from current research, and a step back in time to a project I worked on in Minnesota— scientific projects often continue for years, sometime decades: new projects pop up, occasionally a project gets neatly wrapped up, and others seem to always need one more analyses (great cartoon).
In this case, the elusive data was an age model. We had nearly 400 m of sediment (that’s a lot of mud!) from Lake Malawi in East Africa but didn’t know the age of the mud at the bottom. Dating lake sediments is often a challenging endeavour, but that’s a story for another time. This was an ongoing struggle as interpreting any record from the core means a lot more in context when we can compare it to what was going on in the rest of the world. Learn more about the drilling efforts to collect the sediments here
With the combined efforts of many individuals (14 of us co-authored the paper, many others assisted along the way!) we were able to determine the age of the sediments, and reconstruct the temperature and precipitation history of Lake Malawi. Hot and dry is a very different environment (and has very different implications) than hot and wet.
So now we take a much larger step back into time—about 1.3 million years.
Image: Modern Malawi, the rift valley is beautiful in January.
So what did we do?
My contribution to the project was determining the lake surface temperature through time using biomarkers in the sediment. Biomarkers, put simply, are anything left in the sediment from a living organism that can tell us about the environment when that organism was alive. In this case, I was looking for left over lipids (includes fat) from very tiny organisms (single celled archaea). We classify the lipids by their shape; and the ratio of one shape to another is indicative of the temperature of the water the tiny organism was living in. Fortunately for us, these critters were living in Lake Malawi for the entire 1.3 million years we could collect sediment from so we can have a temperature record that extends that entire time period.
What’s significant about this time period?
By going back 1.3 million years, we can see how the climate around Lake Malawi responded during what is called the “Mid Pleistocene Transition,” or MPT for short. The ‘T’ (transition) referred to is a shift in the orbital (Milankovitch cycles) forcing controlling the climate of Earth. Put simply, the Milankovitch cycles describe Earth’s relation to the sun and include obliquity (changes on ~40,000 yr time scales), precession (changes on ~23,000 yr time scales), and eccentricity (changes on ~100,000 yr time scales). Obliquity (or tilt) refers to the angle of Earth’s axis, this influences seasonality! Precession refers to the orientation of that axis, for example in modern times our axis points towards Polaris, thus it is our north star. Finally, eccentricity refers to the shape of earth’s orbit around the sun (is it more circular or more elliptical?). About 900,000 years ago our planet changed from climate responding mainly to obliquity (40,000 yr cycles) to a climate responding mainly to eccentricity (100,000 yr cycle); thus the ‘transition.’ The Pleistocene is the name of the geological epoch extending from ~2.6 million years ago until ~0.01 million years ago (the beginning of the Holocene). Since the transition happened in the middle of that epoch, we get “Mid Pleistocene Transition.”
What did we find?
Overall, we see that the Lake Malawi region shifted from a mainly arid (dry!) but highly variable environment prior to 900 ka to a more humid environment after the Mid-Pleistocene Transition. Since the transition, the climate of Lake Malawi has been dominated by 100 ky cycles of warm, wet interglacials and cool, dry glacials. The amplitude of the temperature change between glacials and interglacials increases from ~500,000 yrs ago until ~125,000 years ago. These amplitude variations do not match ice volume records, however they are in good agreement with atmospheric carbon dioxide levels.
Why is this exciting?
Our findings demonstrate the complex nature of the climate of the African continent. Over the same period, records from North Africa show the opposite trend with the climate there progressing to more and more arid conditions. In South Africa, the same shift to 100 ky year cycles is evident but there is no long term trend towards more humid or more arid conditions. These changes would have had many implications on the habitability of each region.
The full paper available here