July 26, 2011
One of the big-picture studies we do here at Olorgesailie focuses on how the landscape changed over time, all the way back to the beginning of the sediment record more than 1 million years ago. This area of the southern Kenya Rift Valley has the most precisely dated record of archeological and fossil remains in the world for the past 1 million years. There is only one major gap in the time sequence, between the Olorgesailie and Oltulelei Formations – a gap that resulted from widespread erosion in the region between about 490,000 and 340,000 years. But other than that, Olorgesailie presents a pretty continuous record from the handaxe era up to and beyond the time of the African origin of our species, Homo sapiens.
My geologist colleague, Kay Behrensmeyer, and I have been studying landscape change in the region throughout the entire time period recorded in the Olorgesailie sedimentary layers. Kay’s short visit draws to an end tomorrow, as she heads back to Nairobi and flies home to her family. So we spent the morning going over the evidence for landscape change in the younger beds, the Oltulelei Formation, which includes the oldest Middle Stone Age excavations at BOK-2 and nearby sites. Let me provide a couple of images that can help clarify what I mean by ‘landscape change’.
For example, in the first image, you can see thick white bands and gray bands – where the upper layers were laid down on top of (and thus after) the ones below. The white layer just above the line of rocks and trees is a thick layer of diatomite (look forward to me talking about diatoms tomorrow!). The diatoms in the diatomite show that a large lake once existed at that point in time in the Olorgesailie area. The gray band above it is a thick volcanic sand, which show two things – that there was a volcanic eruption in the vicinity, and that the debris from the eruption filled up wide stream channels that had cut across the region after the lake dried up. So, count ‘em up: that’s three major landscape changes. First, the lake was present and then dried up. Second, the dried-out landscape was eroded and water channels crossed the area. Third, after a nearby volcano erupted, the ash and pumice and gravel was carried into the Olorgesailie region and it filled up the channels to form yet another, different kind of landscape.
One thing I’m especially interested in is how early humans responded to these big transitions in the environment. Stone tools were the early humans’ version of a business card – they left the tools behind as if to say ‘We were here!’ Thus we use the stone tools to indicate whether our early ancestors were very successful or not in surviving the major shifts in the landscape, or perhaps repopulating the landscape after an especially difficult time. Just think, 10 centimeters of volcanic ash blanketing the ancient terrain would have killed all the grass, forcing the abundance of grazing (grass-eating) animals to depart. That’s a big drop in opportunities for obtaining meat from those animals. So every landscape change affected the plants and animals – in fact, the entire ecosystem – and thus the early human toolmakers as well.
Now consider this next image:
White and gray layers reflect the gradual build-up of the Olorgesailie Formation layers. They indicate many shifts between lake and land from about 1.2 million to 490,000 years ago. The darker brown sediments are ancient river sediments. The narrow brown bands across the present-day landscape represent the erosion of large river channels, which were eventually filled up with silt and sand of the younger Oltulelei Formation. A big question: What caused the repeated cutting of river valleys, followed by silting up of those river and stream channels?
After 490,000 years ago, the largest landscape changes we see were caused as river valleys were cut by erosion into the underlying Olorgesailie Formation. These valleys were then filled up with brown silt and sand. The filling was followed my more erosion as a new river valley formed. This process happened at least 3 major times, and several minor times.
Kay and I discussed two main causes: Earthquakes or climate? Earthquakes could have caused uplift of the Olorgesailie region, with rain and wind then carving out channels and creating valleys in the high ground of the uplifted landscape. The other possibility is climate. That is, during dry times, the water levels across the region would have dropped, causing the occasional rain storm to carve the landscape down further and further over many years to reach the lower levels. During wet times, as the regional water levels rose, the rivers and streams would have backed up and slowed, which caused the silt and sand in those flowing waters to be dumped within the channels. In other words, cycles of erosion and deposition, over and over again as climate shifted over time.
At present, Kay and I are pretty certain that these big fluctuations of the landscape in the younger geological beds reflect strong climate cycles – that is, big shifts between dry times and wet.
The work continues as we keep seeking better clues to see whether we are right or not...