July 27, 2011
Once upon a time, this arid and dusty terrain was once the home of an ancient lake. Observations of the sediments show that water once filled the Olorgesailie basin. The paleo-lake was a major hotspot, attractive to wildlife and hominins. Therefore, reconstructing the lake and its environment is paramount in our understanding of the life that once lived here.
Actually, we should use the plural ‘environments’ because the lake changed considerably over its history. The person who has helped us understand this visited camp today. Bernie Owen is an old friend to our project, and he’s spent many summers with us at Olorgesailie as our diatom specialist. He was joined by colleagues Robin Renaut, Tim Lowenstein, and a graduate student Ginette Felske.
Diatoms are a type of phytoplankton – one-celled algae that live in either marine or lacustrine (lake) environments. A unique characteristic of diatoms is their silica shell. They are very microscopic, of course; in fact, many of them are less than 0.01mm in size. They are beautiful to view under a microscope because their structure is composed of various intricate geometric shapes, much like what you might see through a kaleidoscope. Let me show you what several of them look like under a special microscope called an SEM (scanning electron microscrope). Each of these photos gives the species name, and the kind of lake environment it favors.
Diatoms are valuable in to our research because different species of diatoms are found in different environments. Some lived in deep water, while some lived in shallow water. Some diatoms lived only in marine environments, while others prefer the marshy fresh water of lakes.
As diatoms die, their shells and skeletons sink to the bottom of the lake, forming a layer on the lake floor. This layer is compressed over time and forms a unique sediment called diatomite. As the lake dries up and refills over thousands of years, the layers of diatomite become stacked on top of each other. A useful tool for the geologist!
Bernie collects samples of the diatomite sediment to create a record of environmental change within the ancient lake over hundreds of thousands of years. Thus far, he has determined the water pH, conductivity, and botany and vegetation of the lake. Bernie’s work suggests that the paleo-lake fluctuated between a freshwater and alkaline lake in the oldest member (Member 1), eventually forming a very deep freshwater lake during most of Member 2 times. But at the base of Member 2, there was a period of extreme alkalinity. The lake was also saline, as evidenced by a thin layer of halides (salt crystals). His research further suggests that 1 meter of diatomite took about 7000 years to accumulate, and that the lake could have emptied and refilled rapidly, perhaps in a period of 100-500 years! The lake returned to fresher conditions every so often, but with lots of fluctuation in the chemistry and size. It eventually dried up for good around 490,000 years ago. Although some of the younger sediments show that ponds formed on occasion, none of them was extensive enough to think of it as ‘the return of Lake Olorgesailie’.
This year, Bernie is just passing through. He and his team are on their way to Lake Magadi, a fascinating lake located further south, closer to the border of Tanzania. Lake Magadi is a saline, alkaline lake. It’s mostly covered by soda (sodium bicarbonate) and therefore attracts beautiful brine-feeding flamingos. It has also attracted a big soda mining company, movie producers (The Constant Gardener was filmed there), and of course, geologists!
We had great discussions over lunch with Bernie, Robin, Tim, and Ginette, and we learned more about their research at Magadi. They’ll be in the field for about 5 days before heading back to Nairobi. We look forward to doing more research together in the future.